Briggs Engine Failure Guide

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Major Engine Failure Analysis

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MAJOR ENGINE FAILURE ANALYSIS

CUSTOMER EDUCATION’S MISSION STATEMENT

BRIGGS & STRATTON IS COMMITTED TO PROVIDE ITS SERVICE ORGANIZATION WITH SUPERIOR TECHNICAL TRAINING PROGRAMS THROUGH WHICH PROFESSIONAL COMPETENCE CAN BE ACQUIRED AND MAINTAINED ON ALL BRIGGS & STRATTON PRODUCTS, ASSURING ONLY THE HIGHEST STANDARDS OF SERVICE SUPPORT FOR OUR CUSTOMERS.

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Table of Contents

Introduction

Page 13

Chapter 1

Abrasive Ingestion

Page 16

Chapter 2

Insufficient Lubrication

Page 16

Chapter 3

Overheating

Page 25

Chapter 4

Overspeeding

Page 29

Chapter 5

Breakage

Page 32

Chapter 6

Combination Failures

Page 35

Chapter 7

Cause / Effect Flow Charts

Page 37

Glossary of Terms

Page 43

© 1996 BRIGGS & STRATTON CORPORATION

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Introduction

Introduction

FAILURE ANALYSIS What do we mean by the term Failure Analysis? Let’s look at some of the descriptions given to us by the dictionary:

FAILURE: “A state of inability to perform a normal function, neglect or non-performance”.

ANALYSIS: “Separation of a whole into its component parts, an examination of a complex item, its elements, and their relations”.


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Introduction

The term “Failure Analysis” is often used by technicians when discussing the results of inspecting a failed engine that has been brought to them. Many of these technicians have spent a great deal of time trying to analyze the failed engine without a firm understanding of the dynamics of why certain components can fail. Even more often, some technicians will just plain fail to analyze.

Without having a strong understanding of the cause and effect relationships of many of the components, some of the clues the engine will have will be completely missed or mistaken for something completely different.

As an example, when abrasives are allowed to enter the intake system at the filter element, evidence will be found on all contact surfaces from the filter element to the crankshaft. However, if the problem was a bad gasket at the intake manifold, the evidence will start at the gasket and travel towards the crankshaft. No evidence will be found in the carburetor. An injustice could have been dealt the operator by telling him the engine failed because of dirt ingestion due to lack of maintenance--when in fact the problem was a defect in the gasket.

True, the failure was abrasive ingestion, but the problem was not the operator’s fault. The abrasive and resulting wear were nothing more than an effect; the cause was the bad gasket.

No two engines that have failed under the same circumstances will normally exhibit the exact same degree of damage. There are too many variables in the manufacturing process that make every engine just a little different than the next. Knowing the patterns of the component failure and how combinations of these events occur will be the best technique for understanding how to investigate major failures.


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Introduction

Perhaps the most challenging task the service technician will undertake is that of Major Engine Failure Analysis. An accurate, cost-effective diagnosis is not possible by attempting to memorize visual evidence and applying it to future situations. To help in this process, Briggs & Stratton has developed this comprehensive Failure Analysis Workbook. When used with the companion video tape #CE3019, most technicians will develop a comprehensive understanding of the dynamics of failure as it pertains to individual components and their relationship to the engine as a complete unit.

Engines can fail for a variety of reasons. FIve categories cover 99% of all failures. The most predominant category is abrasive ingestion followed closely by insufficient lubrication. The final three are overheating, overspeeding and breakage. In this workbook, we will cover the five most common areas of major failure and how they can be compounded together. For good measure some unrelated examples of component failure will be added.

FACTOID: For every gallon of gasoline consumed, a block of air approximately 100 ft. x 100 ft. and 10 ft. high will be consumed. If an engine was to run for 1,000 hours at 3600 rpm the engine would complete the following: • The piston will complete 432,000,000 strokes. • The crankshaft will rotate 216,000,000 revolutions. • Each valve will contact its seat 108,000,000 times. • At 40 hours per week, it will take 25 weeks to complete 1,000 hours.


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Abrasive Ingestion

Chapter 1 Abrasive Ingestion

FIG. 1-1

The ingestion of abrasives is all too common in the small engine industry. An abrasive is a particle that is commonly described as a piece of undesirable foreign material that exhibits an exceptional hardness. The most abrasive particle in the small engine industry is silica.

Silica is a compound of the elements silicon (Si) and oxygen (O2) and is commonly found in sand, and to varying degrees, in dirt. Silica (the main component in quartz) exhibits a hardness of 7 on the Moh’s scale of mineral hardness. Only the minerals topaz, sapphire and diamond are rated harder. The degree of hardness of the abrasive particles is chief in understanding the dynamics of an abrasive ingestion failure of a Briggs & Stratton engine.

When discussing abrasive particles, it is important to have a good understanding of the size and type of particle we are dealing with. The silica particles we are concerned about are as small as 1 micron, and are of a crystalline structure, with very sharp edges. Most of the particles that lead to excessive wear are on the average of 25 microns and larger. To give you a perspective, 25 microns are roughly equivalent to .001″ (.024mm). This is about 1/20th the inner diameter of a pilot jet orifice.


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Abrasive Ingestion

When using the term “dirt ingestion”, an image comes to mind of particles the size of beach sand. Nothing could be farther from the truth. Except for rare occasions, the abrasives encountered are very small. Most air cleaner assemblies use some type of cyclonic system that removes the larger particles. What is left are very small powdery abrasive particles that can pass through nearly any opening encountered. The sample silica in the picture varies from about 3 microns to 80 microns, averaging about 20 to 50 microns. SEE FIG. 1-1 FIG. 1-1 Laboratory sample of silica abrasive used for testing air cleaner designs and engine performance.

AIR CLEANERS The function of the air cleaner is to filter as much abrasive material out of the incoming air as possible. As a filter element begins to become obstructed, less and less air can penetrate. When the element works as designed, air will stop flowing at some point in the process. Before air stops flowing, however, the engine will no longer be running properly. No matter how bad the outside components look, the carburetor side will be clean if the air cleaner is functioning properly. SEE FIG. 1-2

When an air filter is not serviced properly, abrasives are allowed to enter the air intake stream. A tear in either a foam or paper element will allow the air to follow the path of least resistance. Evidence will be found when looking on the carburetor side of the element and inside of the air cleaner. Any dirt in these areas is a sure sign of a damaged air cleaner element or sealing problem. SEE FIG. 1-3

FIG. 1-2 No matter how dirty the air cleaner gets, there should not be any sign of abrasives on the carburetor side.

Considering the environment most air cooled equipment functions within, it is not hard to imagine the amount of abrasives the engine could potentially ingest without proper filtration. FIG. 1-3 When the air filter is damaged or installed improperly, abrasives will pass into the carburetor.


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Abrasive Ingestion

The outside of a carburetor will in all probability be very dirty and encrusted with debris. This condition is considered normal and does not pose any great problem. SEE FIG. 1-4 The problem occurs when the air cleaner element becomes clogged. This condition creates a restriction for the incoming air. Remember, air always attempts to follow the path of least resistance. The next place the air will enter is the throttle and/or choke shafts. The air will locate any weakness relating to gaskets or the air cleaner element. FIG. 1-4 The outside of the carburetor will usually be covered with dirt, but will not affect normal performance.

CARBURETORS If the abrasives have gotten past the air filter, they will continue to travel through the carburetor. The air stream will be traveling as much as thirty five miles per hour. At this speed, abrasives will begin to impact on any surface they come in contact with. SEE FIG.1-5 Evidence will be seen in and around the choke shaft, choke plate, any air bleeds and the venturi.This area on the carburetor should never show any signs of foreign material.

Before we leave the carburetor, lets take another look at this process. Remember the air wants to follow the path of least resistance. If the air filter element is clogged, air will start affecting the throttle and choke shafts. SEE FIG.1-6 Since these items are moving and wet with gasoline droplets, any abrasives moving through the carburetor will migrate to these shafts and stick.

When an abrasive is present in the carburetor, it will begin to affect the throttle shaft. The more the shaft rotates within its bearing surface, the greater the wear that will take place. This is why the throttle shaft generally wears more than the choke shaft. SEE FIG.1-7 As the wear increases, more air passes through bringing more abrasives with it. With the increase in air flow through the throttle shaft the air/fuel mixture becomes leaner and causes performance problems. © 1996 BRIGGS & STRATTON CORPORATION

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FIG. 1-5 Abrasive particles will become embedded on any part they come in contact with.

FIG. 1-6 Abrasives will continue to travel through the carburetor to the throttle shaft.

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Abrasive Ingestion

MANIFOLDS As the high velocity air and abrasive mixture continues to flow into the engine, more evidence will be noticeable. As the air bends to follow the path of the intake manifold, the mass of the abrasive will force some of the particles to impact the inside of the manifold. This will cause an etching effect to occur.

VALVES & VALVE GUIDES

FIG. 1-7 When abrasives are present at a moving part such as the throttle shaft, wear will take place.

Next, the particles encounter the intake valve and seat. As the particles travel toward the cylinder, they will be grinding away at the surface of the valve seat. Any particles on the seat as the valve closes will be further ground and crushed. Since this action takes place mostly in the path of the flow, the evidence will be in line between the valve guide and the cylinder. The appearance will be a valve seat with a wider portion towards the cylinder and a narrower portion in the opposite direction. The valve face will show a noticeable impression or “dishing” appearance. SEE FIG. 1-8 This wear will only occur when abrasives are present. The “dishing” will generally be uniform around the face of the valve as the valve rotates randomly during engine operation. Loss of valve tappet clearance can also occur as face wear increases.

When the abrasives are affecting the valve face and seat, they are also affecting the valve guide. SEE FIG. 1-9 Any object in the line of travel of the abrasives will cause some particles to come out of suspension in the air flow and stick. As the valve stem moves up and down, the abrasives will migrate into the valve guide and begin wearing the guide and valve stem. The appearance of the valve stem will be polished and most likely have vertical scratches. The guide, whether it is machined into the cylinder, sintered iron or brass material, will show the effects of the abrasive wear. It will be difficult to identify wear unless we use the valve guide plug guage to determine if service is required. © 1996 BRIGGS & STRATTON CORPORATION

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FIG. 1-8 Dishing occurs on the face of the intake valve when abrasives are present.

FIG. 1-9 Abrasives migrate down the valve stem and work into the valve guide.

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Abrasive Ingestion

CYLINDERS & BORES Following the path of the air flow, the abrasives will travel from the intake port, across the cylinder bore and impact on the cylinder wall opposite the intake valve. Because of the “eddy currents” of air, some of the particles will flow back onto the cylinder wall below the intake valve. Since there is a film of oil on the cylinder wall, abrasives will stick. Some of these particles will become embedded in the cylinder wall, while others will begin wearing the cylinder wall as the rings and piston move up and down in the cylinder bore. SEE FIG. 1-10

When the abrasive particles that are rubbing between the piston, rings and cylinder wall are larger than the oil film separating the two surfaces, wear will take place. As wear takes place, loss of the crosshatching on the cylinder bore will be the first evidence present. The exception to this will be a cylinder with a DIAMOND BORE™, which has no crosshatch. SEE FIG. 1-11 Under normal running conditions, little or no loss of crosshatch will take place. When a deep ridge has formed at the top of ring travel in any bore type, it is a good bet a large quantity of abrasive material has passed through this area.

To better understand what happens when analyzing this kind of wear, one must understand the relationship of the materials we are dealing with. On a KOOL BORE™ engine there are three basic materials. The cylinder is an aluminum alloy, the rings are cast iron or steel, and the abrasive is predominately silica. You can readily see the softest material is the cylinder wall with the silica being the hardest. Pressure is exerted on the silica particles as they are squeezed between the rings and the cylinder wall. Since the rings are harder than the cylinder wall, the silica particles tend to be forced into the aluminum where it is held much like grit on sand paper. SEE FIG. 1-12


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FIG. 1-10 Loss of crosshatch will be one of the first signs of abrasive ingestion.

FIG. 1-11 When properly maintained, the crosshatching will remain on the cylinder wall.

ABRASIVES

PISTON MOVEMENT RING CYLINDER WALL PISTON

FIG. 1-12 Abrasive particles will embed in the cylinder wall of a KOOL BORE™ engine. When the rings move, wear will result.

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FIG. 1-13 Light scratching can be normal. Most will “heal” over time.

PISTON & RINGS Because abrasives tend to get embedded in the cylinder wall, small scratches will start to form from the larger particles while the smaller ones tend to polish the surface of the piston. This is evidenced by the appearance of the two pistons shown. SEE FIG. 1-13 Some very light scratching is normal and occurs from the break-in process. If your fingernail does not catch when rubbing them, there is no problem. As we look closer at the piston, you will notice the light scratching evident on the piston skirt. Let’s look at a piston with a greater amount of wear. The most striking appearance is the coloring of the piston skirt. What has happened is the abrasive qualities of the material embedded in the cylinder wall have worn some of the chrome plating off the piston. SEE FIG. 1-14 If this condition persists, the piston will gradually begin to weld to the cylinder bore. If you look closely, you can see the beginning of this process.


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FIG. 1-14 Wear will take place given time when abrasives are present.

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The oil ring exerts greater unit pressure than the other rings. Because of this, the oil ring will wear faster than the compression rings. As we look at these oil control rings, we can see different levels of wear with the ring on the right being new. SEE FIG. 1-15 As the face of the oil control ring begins to wear, the face becomes wider. The wider the face becomes, the more it tends to ride up on the oil film covering the cylinder wall. Once this occurs, more oil is left for the compression rings to overcome. Since these rings are not designed to control oil, oil consumption begins to increase.

FIG. 1-15 Oil control rings will show wear very quickly when subjected the abrasives.

When we look at compression rings for wear, it is not as noticeable when looking at the face of the rings. Even though the same relative amount of wear takes place. However, if you look at the ring from the top, you will notice that the ring will generally vary in width. SEE FIG. 1-16

FIG. 1-16 Compression rings will wear, with the evidence being scratches in line with piston travel.

So where does the abrasive and material that has worn off of the rings and cylinder wall go? The lower end! As the material enters the crankcase, it mixes with the oil. Once the abrasive has entered the oil, it then will travel to all of the bearing surfaces in the lower end. Most noticeable will be the connecting rod. SEE FIG. 1-17 The bearing surface will have a dull gray polished appearance.

FIG. 1-17 Connecting rod wear will appear as a dull gray look on the bearing surface.


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Abrasive Ingestion

In this view, you can notice the wear on the PTO bearing journal. SEE FIG. 1-18 The damage is not from metal transfer like a lubrication failure and shows no signs of heat.

Everything discussed about the cylinder wall, piston and rings will be true for the main bearings. The bearing surface will have the same scratched appearance as the connecting rod.

SEE FIG. 1-19 Because the wear can be so fine and be mistaken for machining it is necessary to check the size of the main bearing using a main bearing plug gauge. If the bearing is not cleaned properly, damage will continue to occur because of the abrasives embedded in the bearing material. Ball bearings are also commonly overlooked when it comes to ingestion problems. The microstructure of the races and balls will be damaged just as badly.

FIG. 1-18 Under normal conditions, no marks or scratches will appear on the main bearing journals.

FIG. 1-19 Main bearing wear will look very much like the connecting rod. The wear can be very smooth.


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Abrasive Ingestion

As contaminants build up, they start to settle out of the oil when the engine is not running. If the oil is not changed on a routine basis, this buildup will continue until a sludge begins to form. SEE FIG. 1-20 Operators tend to form patterns in the level of maintenance they perform. A operator that does not change the oil on a regular basis probably does not service the air filter very well either. When sludge appears in the crankcase, there is a good chance other routine maintenance procedures have also not been followed. FIG. 1-20 Any buildup in the crankcase will be evidence of improper maintenance.

As an example, the engine you are looking at exhibits massive abrasive ingestion. All the signs are there, but when you look into the crankcase, it is relatively clean. This would most likely have occurred because during the last maintenance, the air cleaner assembly was not installed correctly. This could have been an oversight, but the damage can still be very severe. Abrasive damage can occur very quickly when you consider the piston will complete as many as 432,000 strokes per hour.

Another common source of abrasive ingestion is the oil fill. If the area is not cleaned before opening the cap, debris can fall into the crankcase. External evidence will be debris in the threads of the cap and in the threads of the fill. SEE FIG. 1-21

FIG. 1-21 Not cleaning the oil fill will result in abrasives entering the engine.

Lower end ingestion will be very noticeable when looking at the piston skirt. Looking at the piston shown, notice the deep scratches in the piston skirt. SEE FIG. 1-22 The scratches follow the path of movement and stop at the lower oil control ring. If the debris had come from the upper end, there would be scratches in the ring land area also.

To illustrate how well a KOOL BORE™ cylinder can “hold” abrasives, notice the deep cuts on the face of these rings. In fact, you can line up the cuts indicating that all ring rotation has ceased. SEE FIG. 1-23 This clearly shows how hard the abrasives can be.


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FIG. 1-22 If abrasives enter the engine from the oil fill, they will generally be much larger. The damage will be more severe.

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Abrasive Ingestion

Since all engines have a certain level of crankcase vacuum when running, any path into the crankcase can be a potential source of ingestion.

As an example, let’s look at the crankcase oil seal. SEE FIG. 1-24 Some technicians think the function of the crankcase oil seal is to keep oil in an engine. This is only true of any seal below the oil level and when the engine is not running. The purpose of the seal is to keep air out of the crankcase. If the seal “wears out”, air is allowed to enter through the seal. If air is entering, so are abrasives. Similar to the throttle shaft, abrasives will wear the bearing closest to the seal. This bearing will exhibit the greatest amount of wear when compared to other bearing surfaces. As the abrasives mix in the oil, the failure will look like other lower end ingestion examples.

Whether abrasives enter from the air cleaner, oil fill, or any point in between, the evidence will follow predictable patterns. The abrasives are harder than the materials the engine is made of. The best analogy would be the abrasives are like very sharp cutting tools, and the parts are moving. If you move metal against a cutting tool, metal will be removed. Any abrasives larger than the oil film that separates the metal surfaces will result in wear. SEE FIG. 1-25

FIG. 1-23 Large particles embedded in the cylinder wall were cutting the rings.

FIG. 1-24 Because of crankcase vacuum, a worn oil seal will allow abrasives to enter the engine.

Since the evidence follows these predictable patterns, a technician, following a systematic approach to failure analysis, will be able to determine the cause of most abrasive failures. Most operators do not realize the damage that can be caused by not paying close attention to proper maintenance.

FIG. 1-25 Evidence will show on every part that comes in contact with abrasives.


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Chapter 2 Insufficient Lubrication

FIG. 2-1

Engine lubricating oils are complex hydrocarbons refined from basic crude oil stocks. Finished products are blends of refined crudes, carefully tailored by the addition of additive packages. Well defined standards developed by the petroleum industry, automotive industry and other business partners assure consistency for consumers.

Webster defines a lubricant as “...a substance capable of reducing friction, heat and wear when introduced as a film between solid surfaces”.

In analyzing failures due to insufficient lubrication, the technician is presented with a two-fold problem. Not only will mechanical parts fail, but the lubricant will as well. Though the process is consistent, visual evidence may vary dramatically.


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Viscosity VISCOSITY RELATES TO INTERNAL FRICTION

The main consideration when choosing a lubricating oil for an internal combustion engine is the viscosity. The viscosity of a fluid is its resistance to flow caused by internal friction. It is this property that causes it to resist flowing past a solid surface or other layers of fluid. Resistance to movement, in essence, causes oil to be incompressible.

An oil film between two surfaces adheres to the top and bottom. If one surface is moved, the corresponding film travels at the same velocity. The opposite film remains stationary. The whole picture then, is multiple layers between the two, each moving at a different speed. A higher viscosity oil has more resistance to movement and vice versa. Two components separated by an oil film are essentially prevented from contact as long as there is movement and an adequate supply of lubricant. SEE FIG. 2-2

Asperities

MOVING

STATIONARY

FIG. 2-2 A lubricant, clinging to both surfaces, forms into layers moving at different speeds.

.00008”

FIG. 2-3 Asperities are microscopic peaks and valleys formed by the machining process.

The surface of a component describes a need for separation. Machined surfaces are characterized by asperities, or minute peaks and valleys, left behind by the finish machining process. They serve a definite purpose in that the valleys act as lubricant reservoirs. SEE FIG. 2-3

The peaks are sheared off during the break-in process, forming plateaus. The plateau becomes the actual bearing surface. SEE FIGS. 2-4 and 2-5

FIG. 2-4 During the break-in process, asperities collide and shear off the taller peaks. Plateaus are formed which become the main support surface of the bearing.


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Before failures due to insufficient lubrication are explored further, it must be noted that the lubricant itself experiences a degradation process. This is a direct result of the environment it must function in as well as the time it is allowed to remain there.

Any plain bearing at rest displays metal to metal contact between the plateaus of the journal and the bearing. As plateaus move toward each other in the presence of a lubricant, there is a tendency to push the lubricant out of the way, much like a snowplow.The viscosity of the fluid resists this attempt. The plateaus instead begin to lift and ride up on a film of oil. When rotational speed is sufficient, a complete separation of components is achieved. A lubricant’s viscosity will directly relate to the degree of separation attained. SEE FIG. 2-6

PLAIN BEARING AT REST

FIG. 2-5

PLAIN BEARING WITH OIL SUPPLY AT START-UP OIL SUPPLY

LOAD LOAD

METAL TO METAL CONTACT

LINE OF CONTACT

ADEQUATE AMOUNT OF OIL - IMPROPER VISCOSITY

PLAIN BEARING WITH OIL SUPPLY

OIL SUPPLY

OIL SUPPLY

LOAD

LOAD

MINIMUM FILM THICKNESS

Metal To Metal Contact

FIG. 2-6


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A normal deterioration due to oxidation occurs in the crankcase as a result of agitation with air. Oil will also experience thermal cracking as a result of the high temperatures in and near the combustion chamber.This is basically a continuation of the refining process that formed it in the first place and results in heavy hydrocarbon residues which add to the formation of sludge. Contamination by unburned fuel, soot, dirt and combustion residues add many solids to the oil. Water, resulting from the combustion process, is always present, particularly during cold engine warmup and also adds to sludge formation. SEE FIGS. 2-7 and 2-8

FIG. 2-7 Combustion byproducts, contaminants and oxidized lubricant build up over time to form sludge.

Failure of the lubricating medium then, is a deterioration of the medium and its viscosity by oxidation, heat and contamination over time. Insufficient lubrication should not be confused with lack of oil, particularly when explaining a failure to an operator. Insufficient lubrication is an oil film that is inadequate in preventing premature wear between components. As oil deteriorates, it loses its viscosity. It is the nature of a viscous fluid to separate two moving surfaces. This ability is proportional to its viscosity. All the oil in the world may be surrounding two moving components but if the viscosity level is not sufficient, there will be metal to metal contact. Wear and eventual failure due to insufficient lubrication will be the end result. A lack of oil reduces the distance between moving components. Surface asperities make contact and weld together. Though they normally break as quickly as they form, new asperities are formed causing more damage as movement continues. Heat generated from friction rises dramatically. The oil begins to break down from the high heat, loses its viscosity, and more metal to metal contact occurs. Scoring and/or seizure are usually quick to follow. Though the causes (poor maintenance) and failures (discoloration, scoring, galling and seizure) are usually the same, the process differs. SEE FIG. 2-9


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FIG. 2-8 If sludge is allowed to form, oil could become blocked from the oil pump.

FIG. 2-9 As the asperities break off, heat is produced. An adequate supply of oil will carry this heat away.

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To comprehend what happens to an engine when it fails due to insufficient lubrication, it is first necessary to understand what is expected of the lubricant in the first place. The function of lubricants in an internal combustion engine is to:

1. Prevent Wear a. Prevent Metal to Metal Contact (Lubricate) b. Prevent Corrosion FIG. 2-10 Scratches are the result of metal to metal contact.

2. Cool the Engine a. Transfer Heat from Internal Components to the Cylinder Block b. Prevent Sludge Formation which Insulates the Engine and Retards Heat Transfer

3. Seal the Engine a. Reduce Deposit Formations which Prevent Rings from Free Movement b. Reduce Wear which is Detrimental to Sealing Rings to the Walls c. Provide a Viscous Fluid Film Between Components FIG. 2-11 Closeup of Fig. 2-10.

4. Clean the Engine a. Reduce Deposit Formation on Pistons and Valve Stems b. Suspend Dirt and Debris c. Reduce Sludge Formation which Interferes with Oil Distribution

All engines will wear over time. Premature wear is considered a major engine failure. Premature wear in an engine falls under two categories: abrasive wear and adhesive wear. Though both have different causes, the end results may look the same. FIG. 2-12 A score is a deeper, more pronounced scratch.


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Abrasives may enter an engine and not cause damage. If the abrasive particle is smaller than the thickness of the oil film separating components, it will be suspended in the oil causing little or no damage and can easily be removed during the next maintenance cycle. If the film is thinner than the particle, a corresponding scratch, cut or gouge will occur with movement. SEE FIGS. 2-10 and 2-11

It may be difficult to tell the difference between an abrasive wear failure and an adhesive wear failure. Other evidence must usually be interpreted and used to make a decision.

Adhesive wear failures result from a lubricating film that is too thin, allowing metal to metal contact. Metal welds to metal. More pronounced in aluminum/steel bearing configurations, a piece of aluminum may be pulled from the cylinder wall, main or connecting rod bearing surface, and dragged against the cylinder wall or bearing, creating a score. SEE FIGS. 2-12 and 2-13 Displaced material from the score is rolled out of the groove, creating a furrow higher than the average surface height of the plateaus. The procedure repeats itself on the new furrow, and soon larger pieces of aluminum are ripped away. The damage now is generally referred to as a gall, and can be evidenced by aluminum that appears to be ripped or torn and/or aluminum wiping, or metal transfer to the steel component. SEE FIGS. 2-14 and 2-15

FIG. 2-13 When a component is scored, metal may be lifted above the surface of the material. The oil film may be penetrated and further damage caused.

FIG. 2-14 A gall occurs when a group of asperities weld together at one time. The piece of material that is ripped loose is dragged against the mating surface...

FIG. 2-15 ...causing extreme damage and potential seizure.


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Metal transfer will only occur from insufficient lubrication. SEE FIG. 2-16 High frictional temperatures are created that cause the aluminum to become very near a molten state. If the surfaces are driven with enough force, they wipe the aluminum between them, much like a paint roller moves paint in front of it.

Cast iron and steel have higher melting points and will not transfer metal like an aluminum surface. Instead, scuffing will be present, in particular on the ring faces. SEE FIG. 2-17 A corresponding score mark will follow down the cylinder. SEE FIG. 2-18

All insufficient lubrication failures will follow this same pattern. Unfortunately, the evidence provided for the technician to analyze may vary in appearance. High localized heat will be present which is the cause for the typical discoloration of and around aluminum bearing surfaces. SEE FIG. 2-19 The discoloration is actually failed lubricant that has carbonized on the surface of the component. SEE FIG. 2-20 More often than not, this appears on the connecting rod at the bearing that has failed. The constant wiping of the piston rings prevents this evidence from being overly apparent on the cylinder walls. If the cylinder is deeply scored, carbonized oil may be seen in the valleys away from ring contact. A new engine, however, run without any lubricant, has no lubricant to carbonize and will not exhibit discoloration.

FIG. 2-16 Metal transfer will only occur from insufficient lubrication.

FIG. 2-17 Cast iron and steel will not melt as readily as aluminum. Insufficient lubrication may cause areas of “scuffing”’ on the ring faces.

FIG. 2-18 A failed cylinder and piston with evidence of scratches, scoring and galling.


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Discoloration Discoloration due to localized heat, particularly at the connecting rod bearing in conjunction with scoring and/or galling, is a key factor and indicator of failures due to insufficient lubrication.

Several areas of the engine function under conditions of boundary lubrication. These are areas where a full oil film is not always present to separate components. Of note is the top piston ring at top dead center. SEE FIG. 2-21 The oil supply for the top ring is the amount of oil that has been squeegeed up the cylinder walls by the rings themselves. The supply must be carefully controlled. Too much oil and the excess will be burned in the combustion chamber causing high emission output and oil consumption along with the risk of causing deposit buildup on the ring itself. Any deposit formation on the rings may cause a lack of sealing ability against the cylinder walls which hinders engine performance. Too little oil and the ring loses the film which separates it from the walls themselves.

Frequent contact does occur between the rings and cylinder. This is remedied by anti-wear and extreme pressure additives that are added during the blending process. In essence, these are chemicals that bond to the surfaces of the materials and form a protective chemical layer to prevent excessive wear. It should be noted that all additives blended in the oil package will either be consumed as they perform their respective functions or deteriorate over time. There is also a limit to their functionality. In the case of the top ring, once the extreme pressure additives are worn away, they must be replenished by fresh oil carrying fresh additives. If this is not the case, scoring and galling will eventually occur.

FIG. 2-19 Discoloration on or around a plain bearing is a signature mark of insufficient lubrication.

FIG. 2-20 Friction due to metal to metal contact causes high localized heat. Any residual lubricant will burn and carbonize on the components.

Boundary Lubrication

RING CYLINDER WALL PISTON

FIG. 2-21 Several areas of the engine function under conditions of boundary lubrication.


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The dynamics of failures due to insufficient lubrication are fairly clear cut. They should, then, be fairly easy to determine. Unfortunately, this is not always the case. Variables such as engine load, operating conditions, maintenance schedule and tolerance stack-ups may change the severity of the failure. A light engine load will exert less pressure against the rings and reduce the force against the cylinder walls potentially increasing their survivability against boundary lubrication even though the oil level may be dangerously low. A PTO bearing with heavy belt loading and a low oil supply in the crankcase may exhibit more defined evidence of failure than the connecting rod bearing. It cannot be predicted which part will fail first. It is this seeming randomness that tends to throw the less disciplined technician off track. All moveable internal surfaces of the engine must be thoroughly inspected before an intelligent decision may be reached.

FIG. 2-22

In addition, knowledge of the environment the equipment is used in, the load conditions it functions under and the maintenance habits of the operator will all help to put the pieces of the puzzle together.


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Overheating

Chapter 3 Overheating

FIG. 3-1

An internal combustion engine converts energy from a chemical reaction into a mechanical rotating force. By far, the greatest amount of energy produced is in the form of waste heat. Without methods in place to remove this heat, the engine’s expected life span would be measured in hours as opposed to years.

The forced air cooling system used on practically all Briggs & Stratton engines does an excellent job of removing waste heat. Air cooled engines must deal with extreme temperatures and pressures. There is a direct correlation between expected life and any increase or decrease of either of these. An engine is in a state of overheating when it lacks the ability to maintain its internal and external temperature within designed parameters. The main cooling system process is the transfer of heat created by combustion to the cylinder block and ultimately to the moving air stream. A secondary process is the transfer of heat created by internal engine friction to the engine oil which also transfers to the block. SEE FIG. 3-1 Cooling fins are located around the cylinder bore and head to increase the surface area thereby increasing heat transfer to the moving air. An engine may overheat when anything serves to retard this process.


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Overheating

Overheating can be caused by a number of factors, several of which are not even engine related. Blocked cooling fins are typically the biggest factor. Anything that would impede the continuous flow of air across the cooling fins will retard heat transfer. Chaff and debris are perhaps the most common. However, wax dust from floor buffers and airborne tar debris from roof cutters or even dirt buildup from tillers will have the same effect by forming an insulation barrier.

HEAT GENERATED from COMBUSTION of FUEL

USEFUL WORK, ENERGY

OUT COOLING AIR

OUT EXHAUST SYSTEM

COOLING

OTHER

FIG. 3-2 Heat is a form of energy. Proper measures have to be taken to handle the energy developed from burning fuel.

As the fins become insulated by debris buildup, the temperature of the engine will increase dramatically. Nearly all metals expand when heated and return to their original size and shape when cooled. Different materials will expand and contract at different rates. Consider steel head bolts torqued against an aluminum cylinder head. As the aluminum expands, it increases the clamping force of the bolt. Add the pressure created by the combustion process and each head bolt can be subjected to stresses equal to the weight of a full size pickup truck. If temperatures are great enough, the bolts may stretch. The same effect may occur to the threads in the aluminum block. In both cases, the aluminum material of the block and the steel of the bolt have exceeded their thermal yield point. This is the point at which a material will expand and be unable to return to its original shape and size. SEE FIG. 3-3

FIG. 3-3 Cylinders can be warped by excessive heat.

Blown head gaskets and warped cylinder heads can result. Once the gasket is blown, outside air can be drawn into the cylinder on the intake stroke, leaning out the mixture. Engine temperature increase is imminent. SEE FIG. 3-4

FIG. 3-4 Head gasket failure is one of the first signs of overheating.


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Overheating

A cylinder that experiences thermal expansion past its yield point may permanently deform. As the engine cools, the deformed surface will not always return to its original configuration. In effect, this deformation may appear to be a depression along the cylinder wall. The pistons rings will no longer make contact with the cylinder wall and oil will burn onto the surface. The localized discoloration that occurs is called hot spots. Increased oil consumption and a loss of power may result. SEE FIG. 3-5 FIG. 3-5 Oil burned into cylinder wall leaves “hot spots”.

VALVES & SEATS Extreme cases of overheating may cause exhaust valve seats to loosen. Repeated overheating can cause the steel seat to compress the aluminum material of the block. This results in a loss of clamping force around the seat. The seat may loosen or even fall out. Temperatures high enough to cause valve seats to loosen may also warp the head gasket surface of the cylinder block itself. Once this occurs, major repairs are usually necessary. SEE FIG. 3-6

A loose intake valve seat, on the other hand, is rarely caused by an overheating condition. Because of the cooling effect of the incoming fuel/air mixture, extreme temperatures will damage exhaust valve seats before the intake valve seat will fail. SEE FIG. 3-7

Discoloration of components is often a signature of engine overheating. In effect, the discoloration is a residue left from vaporized lubricant. When oil is exposed to extreme heat, it experiences thermal cracking where the lighter ends vaporize and leave the heavier ends of the oil blend. Composed mostly of a tar-like material, this residue burns and adheres to the hot surface. Commonly, piston pins show dark bands on the exposed surface of the pin between the connecting rod and piston body. The inside of the piston dome may be badly discolored as well. Exhaust valve stems may even show signs of discoloration. SEE FIG. 3-8 © 1996 BRIGGS & STRATTON CORPORATION

Form CE8034

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FIG. 3-6 Exhaust valve seat failure will usually be caused by overheating.

FIG. 3-7 A loose intake valve seat is usually a manufacturing defect.

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Overheating

LUBRICATION If an engine continues to run in an overheated condition, the oil will begin to lose its viscosity and serious damage may occur due to insufficient lubrication. Eventual thermal cracking of the oil will result in an extremely viscous material, much like tar, that has little or no lubricating ability. An additional increase in heat will be exhibited due to the increase in friction. SEE FIG. 3-9 FIG. 3-8 A sure sign of excess heat will be discolored wrist pin and piston.

Although burned valves can occur, it is typically not the normal outcome of an overheated engine. A burned valve is more often a contributor to an overheating condition. Once a valve fails to seal, there is a loss of compression. Keep in mind that if compression gases can leak past the valve, outside air can also enter. This will cause a lean air/fuel mixture in the combustion chamber, further increasing the heat the engine must endure.

As mentioned earlier, there are other external factors that can contribute to engine overheating. Equipment modifications can be responsible if enough ventilation is not provided, or access to outside air is restricted. A damaged or mis-directed exhaust system outlet may direct exhaust gases toward the carburetor intake or directly into the engine cooling system. SEE FIG. 3-10

FIG. 3-9 Viscosity breakdown will appear as a sludge in the crankcase.

Nearly all conditions of engine overheating are avoidable if proper maintenance techniques are followed. In almost every case, it is abuse and neglect that cause failures due to overheating.

FIG. 3-10 This example of proper application design will insure proper engine performance. Note separate intake and exhaust air ducts; the exhaust is routed outside the enclosure.


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Overspeeding

Chapter 4 Overspeeding

FIG. 4-1

It is likely to have overspeeding failures show signs of lubrication and breakage problems at the same time. This is because the rod journal and main bearings will have problems receiving enough lubrication to maintain clearance between the bearing surfaces at excessive speeds. The loads placed upon the materials that the components are made of will overstress them and cause breakage. Breakage can occur to external components also. Example: For whatever reason, the governor system fails to control engine speed on a genset. The rotor, bearings and housings are designed to turn at 3600 rpm. If the speed exceeds this design limit, these components could also fail and create some very expensive damage. If broken parts should become airborne, personal injury could result. SEE FIG. 4-1


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Overspeeding

Failures caused by overspeeding, while much less prevalent than ingesting abrasives or insufficient lubrication, can be catastrophic in nature.

STRESS

Briggs & Stratton engines are designed to operate at a top no-load range of up to 4000 rpms. The loads experienced by the internal components of the engine are within acceptable ranges when the engine is operated within certain speed limits. SEE FIG. 4-2

RPM

REASONABLE STRESS

EXCESSIVE STRESS

RECOMMENDED TOP NO LOAD GOVERNED SPEED

FIG. 4-2

When an engine experiences an overspeeding event, the loads on both ends of the connecting rod are increased dramatically. With each increase of 500 rpms above the normal engine speed, the forces on the large end of the connecting rod, as well as the wrist pin of the piston, increase by 44%.

The typical result of an overspeeding event is a broken connecting rod. The connecting rod will normally break at the thinnest part of the beam. In most connecting rod designs, the thinnest part of the connecting rod is about 1 inch from the wrist pin. The reason that the connecting rod will fracture at that specific spot has more to do with momentum and mass than increased loads. The typical evidence of an overspeeding event consists of a fracture of the connecting rod about 1 inch below the wrist pin, as well as finding the piston at TDC. SEE FIG. 4-3

FIG. 4-3 Connecting rod failure at the smallest point is common with overspeeding conditions.


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Overspeeding

During high speed operation, the mass of the piston is instantaneously accelerated in a reciprocal manner. As the piston moves up the cylinder bore toward the combustion chamber, it gains momentum. With the dramatic increase in speed and direction change, the momentum of the piston places an ever increasing load on the connecting rod alternately stretching and compressing the aluminum beam. After a short period of time the connecting rod will fatigue and fracture at the point of maximum deflection. The rod will almost always break at the direction change from TDC moving toward BDC for reasons not completely understood. SEE FIG. 4-4

FIG. 4-4 As the rod stretches and compresses, fatigue will result at the smallest segment of the rod.

Overspeeding fractures in multiple cylinder engines can be difficult to determine. The additional cylinder will continue to power the crankshaft, frequently pulverizing the remains of the broken connecting rod. This will also happen in a single cylinder engine but usually to a lesser degree. Suffice to say, it is rare to find an overspeeding failure where the connecting rod is not broken into many pieces.

Overspeeding can cause other damage. An example could be an increase in vibration or change in the resonance of the vibration within the engine. This could result in metal fatigue in the cylinder, causing a fissure to form in the casting. The vibrations can also affect the equipment the engine is powering.SEE FIG. 45

The key to determining the probability of an overspeeding event in a single or multiple cylinder engine is to look at all of the available evidence. Connecting rod breakage near the wrist pin is a definite indicator. The condition of the governor linkage, governor gear, and flyweights may shed light as to the cause of failure. Has the governor arm clamping bolt lost torque and allowed the shaft to spin? Is the engine mounted on a go-kart, mini-bike or ATV? The application may offer clues. If firm evidence is not available, eliminate other failures that may cause connecting rod breakage such as insufficient lubrication. The answer will be there, it is just a matter of gathering all the evidence. SEE FIG. 4-6 © 1996 BRIGGS & STRATTON CORPORATION

Form CE8034

10/96

FIG. 4-5 Excessive speed can change the vibration frequency and cause breakage of components.

FIG. 4-6 Look for all the signs evident of the cause of the failure.

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Breakage

Chapter 5 Breakage

FIG. 5-1

Breakage can be one of the more elusive types of failures commonly experienced on small gasoline engines. The fracturing of engine or application component parts can occur in just a few hours or over a period of months. The cause of breakage is almost exclusively vibration. It is inherent in all internal combustion engines to exhibit some degree of vibration. Combinations and intensity of vibration exhibited by the mating of an engine and piece of equipment can cause subtle or dramatic breakage problems.

Many times comments are made by inexperienced technicians that the engine is “out of balance”. When we examine the manufacturing process, it is evident that the possibility of this occurring is very remote. The casting, machining and inspection equipment used to make the component parts provide vast numbers of nearly identical parts and assemblies. If any of these parts were to be out of balance, this would translate to large numbers of engines displaying excessive vibration conditions. We know from experience that excessive vibration does not happen on engines very frequently. Excessive vibration circumstances will invariably be related to the application and mounting conditions.


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Breakage

Other than breakage caused by external forces, such as dropping an engine or a blow delivered by running the engine into a stationary rigid object, most breakage problems are either directly related to or compounded by vibration.

The engine cylinder assembly and crankcase cover comprise the main means of support for the crankshaft. Secured to each other by the crankcase cover screws, they support any side or end loading placed on the crankshaft as well as the cylinder combustion pressures and reciprocating vibrations. If properly assembled and torqued, the load is distributed throughout the assembly, much like an eggshell. If the assembly is compromised by a loss of torque on one or more of the attaching screws, the load forces may become concentrated on a specific area. On an opposed twin cylinder engine, the combustion loading against the piston is partially shared by the crankcase cover. If bolt torque is lost, it is not uncommon for a crack to begin at the crankcase cover gasket surface extending to the base of the number two cylinder. In severe cases, the cylinder may separate from the block entirely. SEE FIG. 5-2

The base of the cylinder assembly on horizontal crankshaft engines and the sump of vertical crankshaft engines are thick and rigid, functioning as a pedestal for the activities above them. In effect, they transfer vibrations and other forces to the surface and hence to the equipment where they are usually easily absorbed.

FIG. 5-2 In severe cases, the cylinder may separate from the block entirely.

FIG. 5-3 Loose mounting bolts can allow a cylinder to flex. A crack can form in the structure as a result.

FIG. 5-4 Loose mounting will show up as a polished cylinder bottom and corresponding impression on the mounting surface.


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Breakage

If not tightly secured to the mounting surface, the cylinder assembly must handle this load. Loose mounting bolts may result in a vertical or diagonal crack in the cylinder block emanating from near an engine mounting foot on horizontal crankshaft engines SEE FIG. 5-3 or a broken mounting pad on vertical crankshaft engines. Telltale signs of loose mounting bolts may be a wallowed out bolt hole complete with thread impression from the loose bolt, a polished engine mounting surface and a polished equipment mounting surface. SEE FIG. 5-4 If encountered, always check for trueness of the equipment mounting surface before attempting repairs. If untrue, the new engine or shortblock will not have a flat surface to mount to. Many a repair job has been returned with an expensive repeat failure due to negligence in this area.

When dealing with the issue of breakage, we must remember breakage as a term can describe any component breakage that has been previously covered. SEE FIG. 5-5 If the engine has insufficient lubrication, rod breakage is likely to occur. As we change any condition from the standard design, some kind of effect will result. An example would be an inexperienced technician that places a steel key in the flywheel. The engine runs fine until the operator finds that steel pipe buried in the lawn. The function of the flywheel key is to allow the flywheel to continue to spin and absorb the shock load. When this could not occur, the energy had to go somewhere. The flywheel absorbed some of the energy, with the PTO main bearing taking the rest. SEE FIGS. 5-6 and 5-7

FIG. 5-5 The term breakage can mean any type of breakage covered in this workbook.

FIG. 5-6 Improper service can lead to costly repairs.

FIG. 5-7


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Chapter 6

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Combination and Other Failures


Chapter 6 Combination and Other Failures

FIG. 6-1

Combination failures are probably the most common type of major failure the service technician is likely to encounter. If we think about the circumstances most failures occur under, we can begin to recognize certain patterns. For example, an operator of an engine who neglects to check and change the oil on his equipment more than likely is not maintaining the air filter either. Vice versa, the operator not servicing the air cleaner on a regular basis is probably not changing oil regularly either. When equipment comes in for service or major analysis, paying close attention to the appearance of the equipment can pay dividends in the long run. Because people are creatures of habit, the appearance of the equipment can be one of the first signs that the engine may or may not have been neglected. Recognizing all the evidence, and using a cause and effect systematic approach to major engine failure analysis is the only way to find the answer. Try it; it works.


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Chapter 6

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Page 36

Combination and Other Failures


Other Failures When dealing with combination failures, it is sometimes hard to decide which of the “pure” failures was the direct cause of the overall failure. An example would be an engine with a major problem with the air cleaner element. This will cause abrasive ingestion, and will wear on virtually all of the moving parts. Because the piston rings and cylinder bore are no longer able to control oil consumption, the oil level in the engine goes down. If the engine oil level is not checked, it will be certain that another major failure will occur. More likely, the connecting rod will seize to the crankshaft. But since the stack tolerance for each engine is different, the main bearings or the piston and cylinder wall could just as likely be the next failures to occur. In fact, it is not uncommon to find evidence of two or more of the major failure subjects in one engine. SEE FIG. 6-3 Another example of a failure that can fool some technicians would be an engine with a single bearing seizure. The technician knows that single bearing seizures are most likely to be a defect in material and workmanship from the factory. The major clue the technician missed, is the engine was two years old. A rod journal seizure failure is caused by a lubrication problem. On an engine this old, the cause has to be an insufficient lubrication issue. If the problem had been a factory defect, the failure would have occurred very quickly after the engine was first started. A bearing clearance will not get closer as time goes on, it can only get larger. As the asperities are sheared off, the two bearing surfaces become suspended by the film of oil present. As long as the oil is there, separation will occur. If the oil is not present, the bearings will touch, creating friction and heat, resulting in seizure. By thinking clearly and following a systematic approach to analysis, the cause of most major failures can be determined. Operators will rarely admit to causing a problem, but the evidence will be there if looked at close enough.


Form CE8034

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FIG. 6-2 Abrasive ingestion can cause bearings to weaken and hammer. Compounded with overspeeding, major damage will occur.

FIG. 6-3 Abrasive ingestion will result in higher oil consumption. This condition could result in low oil failure.

FIG. 6-4 A wrist pin failure can sometimes be missed as a problem. The operator cannot cause this failure.

36


Form CE8034

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AIR CLEANER ADAPTER

AIR CLEANER STUD

CARBURETOR BACKING PLATE

AIR CLEANER ELEMENT (INSIDE)

RIDGE AT TOP OF BORE

PISTON SKIRT/ RING WEAR

INTAKE SEAT

INTAKE VALVE GUIDE

SHAFT BEARING SURFACES

WEAR IN BORE HEAVY

INTAKE VALVE

CARBURETOR MANIFOLD

THROTTLE SHAFT

CHOKE SHAFT

LOSS OF CROSSHATCH

CYLINDER BORE

INTAKE SYSTEM

AIR CLEANER

MAIN BEARING WEAR

CRANKSHAFT JOURNAL WEAR

CRANKPIN JOURNAL WEAR

CONNECTING ROD WEAR

LOWER END

10:14 AM

CARBURETOR

EVIDENCE WILL BE FOUND:


4/28/05

ABRASIVE ENTRY

Abrasive Ingestion, Upper End

CE803 BODY 4-05 Page 37

37


Form CE8034

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CRANKSHAFT JOURNAL WEAR

PISTON SKIRT

MAIN BEARING WEAR

CYLINDER BORE

THREADS SCREW ON CAP

RINGS

CONNECTING ROD WEAR

VALVE STEMS/ GUIDES

SAME AS OIL FILL

SAME AS OIL FILL

GASKETS

10:14 AM

OIL SEAL



4/28/05

OIL FILL

ABRASIVE ENTRY

Abrasive Ingestion, Lower End


38


Form CE8034

10/96

MAIN BEARINGS

PISTON SKIRT/ CYLINDER

MAIN BEARING JOURNALS

CAM GEAR

CYLINDER

CRANKPIN JOURNAL

PISTON

WRIST PIN

CONNECTING ROD

CONNECTING ROD

BREAKAGE

CONNECTING ROD

METAL TRANSFER

BURNT OIL SMELL

MAG BEARING

PISTON SKIRT/ CYLINDER

PTO BEARING

SLUDGE

LOWER END

CRANKPIN JOURNAL

SCORING

10:14 AM

DISCOLORATION



4/28/05

EFFECT



39


Form CE8034

10/96

BLOWN HEAD GASKET

SHORTENED VALVE LIFE

CYLINDER BORE

MAIN BEARINGS

EXTERIOR OF CYLINDER

HOT SPOT IN BORE

PISTON

WRIST PIN

CRANKSHAFT BEARINGS

PISTON/ RINGS

BURNED EX VALVE

WARPED HEAD

LOOSE VALVE SEAT

WARPED CYLINDER

VISCOSITY BREAKDOWN

OIL

10:14 AM

DAMAGE



4/28/05

DISCOLORATION

EFFECT

Overheating


40


Form CE8034

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CAM GEAR

CYLINDER

PISTON

CONNECTING ROD

10:15 AM

BREAKAGE



4/28/05

EFFECTS

Overspeeding


41


Form CE8034

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INJURY TO CUSTOMER

BAD BEARINGS

MISSING BRACKETS

BROKEN WELDS

WORN MOUNTING HOLES

POLISHED BASE

CRACKED CYLINDER

LAWSUIT

POSSIBLE DAMAGED FLYWHEEL

PULLEYS OUT OF BALANCE

LOOSE BOLTS VIBRATION

ENGINE SPEED

10:15 AM

EQUIPMENT



4/28/05

MOUNTING

EFFECTS/ CAUSES

Breakage, Vibration


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Glossary of Terms

A B API ASTM Ambient Temperature Asperities BTU (British Thermal Unit) Ball Bearing

Bearing, Plain

Blow-By

Bushing

American Petroleum Institute. API classifies the intended duty cycle of packaged oil. American Society for Testing and Materials The temperature of air surrounding or encompassing (in this case) a piece of equipment, or engine. Roughness or unevenness created during the machining process of metal by the tool bit. Minute peaks and valleys. The amount of heat necessary to change the temperature of 1 pound of water 1 degree F. A ring-shaped track containing rotating balls against which a rotating shaft applies radial or axial loads as well as rotates. Consisting of an inner ring/race and an outer ring/race separated by steel balls. The purpose is to reduce friction between rotating components. Sleeve or Journal Bearing. Supports a shaft or journal against which a radial or axial load is applied. Typical clearance of .001” per inch of journal diameter. Plain bearings will be of different materials than the shaft or seizure is likely to occur. When the shaft is rotated, the journal is lifted on a film of lubricant. Name given to combustion gases that escape the combustion chamber, past the piston rings into the crankcase. An inserted and usually removable sleeve designed to supply a wear surface for a moving shaft or arm.

C Cam Gear

Cam Lobe

Cam Shaft Carbon Dioxide Carbon Monoxide Carburetor


Form CE8034

The gear portion of the “cam gear” on a one piece assembly. The gear that attaches to the camshaft of a separate assembly. A gear and shaft that is timed with the crankshaft that actuates the intake and exhaust valves. The offset, egg-shaped protrusion machined, pressed or welded to the rotating camshaft used to provide a repetitive straight line or back and forth motion against the tappet or cam follower. The rotating shaft with integral cam lobes. Does not include a gear. A colorless, odorless, incombustible gas created by respiration, combustion or decay of organic materials. A colorless, odorless and highly poisonous gas created during the imcomplete combustion of fossil fuels. A device used to mix liquid gasoline and air into a combustible vapor to power internal combustion engines. 10/96

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Glossary of Terms

Centrifugal Force

Centripetal Force

Compression

Connecting Rod

Crankcase Crankcase Cover

Crankpin

Crankshaft

Crosshatch

Cylinder Assembly

Cylinder Bore Cylinder Head

A fictitious force, present only when a system is examined from an accelerating frame of reference. Centrifugal force is in reality a property of matter called inertia. The force that is necessary to keep an object moving in a circular path and that is directed inward toward the center of rotation. Generally refers to a measurement of pressure in the combustion chamber of an engine at the end of the compression stroke. A fuel and air mix compressed between the piston and cylinder head during compression stroke. A component that connects the wrist pin and piston to the crankshaft and transmits combustion forces to the crankshaft journal. Generally refers to the cylinder assembly with the upper cylinder and lower “crankcase” as an assembly. The side cover that attaches to the cylinder and supports and provides a bearing surface for the crankshaft on horizontal crankshaft engines. Connecting Rod Journal, Rod Journal. A machined, offset bearing surface machined into the crankshaft against which the connecting rod bearing attaches. A shaft that changes reciprocating motion into rotary motion. Force transferred through the connecting rod is multiplied by the crankpin offset exhibiting an increase in torque. Power is commonly taken from the portion of the crankshaft that extends outside the engine block through various “power takeoff” devices. A diamond shaped pattern of shallow scratches in cylinder bore resulting from the use of a rigid carborundum or diamond hone after machining. The main cast housing of an engine that refers to the assembly that most internal and external engine parts are mounted to. An accurately machined hole in the cylinder assembly in which the piston travels. A cast component that seals the upper part of the combustion chamber. The cylinder head contains cooling fins and in OHV designed engines, includes valve train components.

D E F DU™ Bearing


Form CE8034

DU™ is a term used to describe a shell bearing featuring a “PTFE” compound impregnated surface which enhances the lubrication qualities of the bearing. (See PTFE)

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Glossary of Terms

DIAMOND BORE™

Dipper

Dipstick Eccentric

Eddy Currents Flywheel Fan

Cylinder bore that is finished machined versus honed in the manufacturing process; no crosshatch will be present. Refers to the part of the connecting rod assembly that, through rotary motion, “dips” into the oil in the reservoir resulting in the splashing of oil throughout the crankcase. Automotive style oil level monitoring device. The center-line offset bearing that the synchro-balance counter weight rides on. A part used in the synchro-balance system to assure smoother operation of the engine. A whirl or backward-circling current of water or air; a whirlpool. The fins cast into or attached to the flywheel. The fan provides a moving volume of air across the cooling fins.

G H L Governed Idle

Governor (air vane - pneumatic)

Governor (mechanical)

Hone KOOL BORE™ Lapping Compound Leakdown Test

A governor system feature that allows an engine to accept a moderate load at governed idle speeds. Usually incorporates a second “smaller” spring within the control components. A governor system that utilizes the force of moving air from the flywheel to counteract an opposite force applied by the governor spring to control the throttle plate position regardless of load. A system that utilizes the motion and force of rotating counter weights to counteract an opposite force applied by the governor spring to control throttle plate position regardless of load. A tool featuring carburundum stones used to oversize the cylinder during a rebuild. Trademarked name given to aluminum alloy cylinder manufactured by Briggs & Stratton. Used for lapping valves to seats to improve the sealing properties of the valve seat/face mating surface. A test which utilizes air pressure differences to determine the condition of internal parts of an engine such as the mating of rings to the bore or valve sealing integrity.

M N O Main Bearing

Manifold (intake)


Form CE8034

A machined part of the cylinder or ball bearing that provides support, locates and allows rotation of the crankshaft. A component that connects the carburetor to the intake port of the engine.

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Glossary of Terms

Negative Crankcase Pressure Oil Dipper

Oil Pump Oil Ring Oil Slinger

Pressure that is measurably less than the ambient atmospheric pressure. Component connected to the connecting rod on horizontal crankshaft engines. The major component of a splash lubrication system. Mechanical device used to deliver oil under pressure to some or all of the bearing surfaces in the engine. Normally the bottom ring on the piston. Used to control oil lubrication on the cylinder wall. A toothed gear exhibiting a series of projections or “paddles” which “fling” oil throughout the crankcase of the engine providing lubrication for all moving internal engine parts. Usually found on vertical shaft engines.

P R PTFE Piston Rings

Pneumatic Governor Port, Intake Positive Crankcase Pressure Pressure Lubrication

RPM Ring, Compression

Ring, End Gap

Ring, Oil Control

Ring, Wiper


Form CE8034

Polytetrafluoroethylene. A teflon-like lubricating compound. The components of the piston assembly that are used to control compression, oil control and cylinder wall/piston lubrication. (See Rings) See Governor (air vane) The area of the cylinder that the carburetor/intake manifold mounts to. Measurable pressure in the crankcase greater than ambient atmospheric pressure. A lubrication system using an oil pump that supplies pressurized oil to major bearing surfaces in the engine. Will commonly incorporate an oil filter in the system. Revolutions per minute (used to measure engine speed) The ring furthest from the wrist pin of the piston. Uses combustion pressure to seal against the cylinder wall and piston, maintaining combustion pressure. The measurement of the separation of the ends of a given ring when placed in the cylinder bore. Usually measured in millimeters or thousandths of an inch. Usually the ring closest to the wrist pin of the piston and is the lubrication ring for the cylinder wall and piston. Through a series of oil outlets, meters the correct amount of oil for the other components. Center ring. Used to control lubrication and as a backup compression ring.

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Glossary of Terms

S T Sump

Tappet Throttle Plate Throttle Shaft Torque

The crankcase cover on a vertical crankshaft engine. Functions as a support for the cylinder and crankshaft, as a mounting platform in attaching the engine to an application and as an oil reservoir. The component that rides on the offset lobe of the cam gear/shaft to initiate the movment of the valves. The valve or plate that is connected to the throttle shaft of the carburetor. Shaft that operates the throttle plate when the governor link is in movement. A turning or twisting force. The most accurate measurement of the power an engine can produce.

V W Valve Clearance

Valve Guide Valve Seat

Valve Stem Viscosity

Wallered/Wallowed Out

Wrist Pin Wrist Pin Clip


Form CE8034

Distance from the end of valve stem to the surface of the valve tappet (at rest). Usually measured in millimeters or thousandths of an inch. A component or integrally machined cylinder which provides a bearing surface for the valve stem. A metal component that provides a machined mating or sealing surface for a valve. In the case of the exhaust valve, excess heat is transferred to the cylinder through the seat. The part of the valve that the spring and keeper are attached to. The property of a fluid that tends to prevent it from flowing when subjected to an applied force. Viscosity is measured in a viscometer, a container with a standardsized orifice in the bottom. The rate at which the fluid flows through the orifice compared to an arbitrary standard results in a numerical value. The higher the numerical value, the more resistance to flow the liquid exhibits. The dictionary defines “wallow” as “to move with heavy, rolling motion, as a ship in a storm”. The small engine industry uses this term to mean a hole that has been worn irregularly and/or one-sided. The component that connects the piston to the connecting rod. A retainer that keeps the wrist pin in the piston.

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Major Engine Failure Analysis NOTES


Form CE8034

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Produced by the Customer Education Department Briggs & Stratton Corporation

Form CE8034-3/02


PRINTED IN U.S.A.

Briggs Engine Failure Guide

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