Notes on the assessment of pistons and piston rings - AH03.10-P-0001-04MM
ENGINE 133, 152, 156, 157, 177, 178, 277, 279
ENGINE 157, 279
ENGINE 270, 271, 274, 276, 278
The following information should be helpful in assessing the piston, in order to evaluate the condition of the piston correctly and to decide whether it is still usable.
The contact pattern of the piston skirt can also be used to assess damage.
The following damage profiles are only some examples of the possible damage types to pistons and piston rings. When assessing the forms of damage involved, the surface condition of the cylinder barrel must always be incorporated into the damage diagnosis.
Piston damage is caused by:
- Leaky air intake ducts downstream of the air filter
- Irregular or uncontrolled combustion
No damage to piston
Carbon deposits on the piston crown and in the combustion chamber recess are normal, as are signs of the spray pattern of the fuel injectors on the piston crown and in the combustion chamber recess.
The combustion is OK.
The piston can be cleaned and reused.
- Fault in fuel injection system
- Too little or too much clearance between piston skirt and cylinder barrel.
- Poor engine oil quality (viscosity) or thinning of the engine oil by fuel
- Soiled or damaged oil filter.
- Fault in oil circuit; e.g. oil pressure is too low.
- Bent or damaged oil spray nozzles for cooling the pistons
No damage to piston
Carbon deposits on the piston crown, on the top land (arrows) and in the combustion chamber recess, are normal.
Score marks in the carbon deposits are due to moving pieces of carbon deposit.
The piston can be cleaned and reused.
Piston with dust damage
The contact surfaces on the skirt have a matt (scored) appearance, and the machined grooves within the contact area have been completely worn away. In an advanced state there are already slight seizure marks on the skirt and the piston ring are sharp-edged.
Replace the piston and perform additional work:
- Check cylinder barrel and if necessary replace crankcase
Score marks on the piston skirt
The piston has several score marks of the same type around the piston skirt. The surfaces of the score marks vary from highly glossy pressure marks to dark-colored areas that have been rubbed relatively smooth. The piston clearance between piston skirt and cylinder barrel was either too narrow, or the cylinder bore is warped, or there was a fault in the engine cooling system.
Replace the piston and perform additional work:
- Check cylinder barrel and if necessary replace crankcase
Scoring marks in the contact surfaces area of the piston skirt
There are score marks in the contact area of the piston skirt which extend partly into the piston ring area.
The corresponding score marks appear on the opposite side of the piston skirt. The surface of the score marks is metallically clean, rough, and with no glossy transition points.
There has been insufficient lubrication between the piston contact surface and the cylinder barrel (engine oil thinned by fuel).
Replace piston and perform all additional work:
- Check cylinder barrel and if necessary replace crankcase
- Note classification between piston and cylinder bore
- Observe engine oil quality (oil viscosity)
- Check oil filter, oil circuit in engine
- Check oil spray nozzles for piston cooling
Play in connecting rod or piston
The piston or the connecting rod exhibits clearly perceptible play in the tangential motion of the connecting rod (see arrows in picture), which is then evidence during engine operation in the form of clattering noise. Any deviations to the cylinder shape and the dimensional accuracy of the piston pin and connecting rod small end results in a clear air gap becoming visible in the connecting rod small end when the piston pin is inserted.
Replace piston, piston pin, connecting rod and retaining rings, and conduct additional work:
- Check cylinder barrel and if necessary replace crankcase
Chipping and flaking of coating/surface on piston rings
The piston rings have sharp edges. The coating/surface on the piston rings exhibits flaking (arrow A); the piston rings already exhibit an advanced stage of chipping (arrow B) at the edges.
Install new piston rings and perform all additional work:
- Determine height of groove for piston rings in piston and replace piston, if necessary
- Check cylinder barrel and measure cylinder bore, and then machine or replace crankcase, if necessary.