Piston Rings Function and Design Incorrect reconditioning
Maintenance faults
Piston impact on cylinder head If the sealing surface of an engine block is reconditioned and a piston with standard compression height fitted, this may cause mechanical impact by the piston on the cylinder head in the case of diesel engines. The same occurs if cylinder head gaskets with the wrong thickness are fi tted. Due to these hard impacts, the piston rings start to flutter and vibrate, dislodging the lubrication film and as a consequence sealing is no longer effectively.
Worn-out ring grooves The result: High oil consumption and high cylinder wear (see also “Fuel Flooding”). Remedy: Compliance with the correct piston protrusion; use of correct cylinder head gaskets.
Dirt that reaches the combustion chamber settles in the ring grooves where it causes abrasive wear in the ring grooves and at the flanks of the piston ring. This results in excessive ring height clearance and consequently to a deterioration of the guidance of the piston rings in the ring grooves. The rings will become bent during use and start to flutter. If the rings are severely worn at the sides, they can break.
The result: High oil consumption and poor performance.
Piston rings must be able to rotate freely in their ring grooves during operation (exception: two-stroke engines). If the piston rings are blocked by carbon deposits or dirt in the ring grooves they are no longer able to seal properly and will not wear uniformly. If the rings are clamped in the grooves, the sealing effect is no longer warranted. This causes a blow-by of combustion gases on compression rings, and results in the penetration of oil into the combustion chamber on oil rings.
The result: Piston seizure, high wear and high oil consumption.
Remedy: Regular air filter maintenance, especially in very dusty areas.
Ring joint clearance too small The piston ring joint clearance is comparable to the valve clearance. When the parts are warmed up, the components are thermally heated which results in a change in length. As long as the machine is cold, the joint clearance prevents the pistons in the cylinder from being jammed when the machine is hot. If the joint clearance is too small when the machine is cold, there will be an excessive rate of wear on the piston ring, sealing problems and engine failure could occur when the machine is hot.
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The result: Premature piston ring wear, piston seizure and high oil consumption. Remedy: It is essential that the minimum joint clearances are complied with – a reduction of the joint clearance by the repair shop is not permitted.
Jammed piston rings
Asymmetric piston wear pattern pattern Deformed connecting rods, which often occur as a result of engine failure, can cause the piston to have an incorrect attitude in the cylinder. The piston rings consequently take on an elliptical form and no longer rotate in the piston. This results in uneven wear and piston ring fluttering.
The result: High wear, ring fractures and excessive oil consumption.
Remedy: Regular air filter maintenance and the use of engine oil with the prescribed oil specifi cation for the engine.
Remedy: Check the connecting rod for deformation and torsion prior to installing it.
Abnormal combustion combustion Knocking combustion and glow ignition Knocking combustion or glow ignitions result in excessive pressure peaks in the cylinder as well as high mechanical stress of the parts. The piston rings start to flutter and can break. On molybdenumplated rings, the molybdenum layer can break away.
The result: Piston seizure, loss of engine power and/or high oil consumption.
The oil film is washed off due to incomplete combustion of the injected fuel or to excessive injected fuel. This causes mixed friction or even dry friction of the piston in the cylinder. The metal parts rub against each other without a protective oil film.
The result: High ring and cylinder wear, high oil consumption.
Remedy: Use of high-grade fuels.
Installation of worn-out parts Out-of-round cylinders
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When machining the cylinder bores, it is essential to make sure that the geometry is perfect. The piston rings can still seal slightly oval cylinders. However, sealing becomes more di fficult with outof-rounds of the 3rd or 4th order. These are often caused by tension stressing from the cylinder head bolts. The crescent-shaped gap caused by out-of-roundness between the piston ring and the cylinder will result in leakages.
The result: Poor performance, excessive oil consumption and engine failures. Remedy: Observe the tightening specifi cations for the cylinder head or exclude out-of-rounds during cylinder machining.
Fuel flooding Sliding friction
Out-of-rounds: Mixed friction
3rd order
Remedy: Correct function and setting of the fuel system.
4th order
Ring joint clearance too great
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An excessive joint clearance on the piston rings appears after a long running period and high wear on the cylinder and piston rings. The joint clearance is too great if it has doubled its size compared to its new condition. Clearance extensions of up to 0.3 mm are irrelevant. They contribute neither to a loss of engine power nor to excessive oil consumption. Slightly enlarged piston ring joint clearances are often overestimated. On this topic, also see “Ring joint clearance too small”.
Remedy: Replace worn-out pistons and cylinders.
Piston Rings for Combustion Engines d
S E O C F I N I V & R S E P I S T
Details on this subject can be found in our brochure „Piston Rings for Combustion Engines“. Further information can be obtained directly from your local Motor Service partner or at www.ms-motor-service.com
Worn-out cylinders If new pistons and piston rings are fitted into a worn-out cylinder, the piston rings often hit the upper wear edge of the cylinder. As a result, the piston rings start to flutter and can no longer seal effectively. Also see “Piston impact on cylinder head” and “Worn-out ring grooves”.
The result: Higher result: Higher oil consumption and premature wear. Remedy: Replacement Remedy: Replacement of the cylinder liner or remachine the cylinder bore (drilling, honing).
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