Notes On The Assessment Of Pistons And Piston Rings - AH03.10-P-0001-04MM
Engine 111, 112, 113, 133, 137, 139, 152, 155, 156, 157, 159, 166, 176, 177, 178, 200, 254, 256, 260, 264, 266, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 282, 285
Engine 112, 113, 157, 176, 177, 264, 273, 279
Engine 281
Shown on aluminum cast piston
Shown on aluminum forged piston
The following notes should help with the assessment of the piston and should also support with making a professional decision on its condition and further use.
Possible damage may also be identified on the contact pattern of the piston skirt (9).
The following illustrated damage profiles are only a selection of the possible forms of damage on the piston, compression rings (2) and oil scraper ring (3). Not all of the possible damage profiles are shown here. When assessing the forms of damage, always incorporate the surface condition of the cylinder barrel into the damage diagnosis.
Piston damage can occur as a result of the following variables:
- Leaky air intake ducts downstream of the air filter
- Irregular or uncontrolled combustion
- One or more faults in fuel injection system
- Too little or too much clearance between piston skirt (9) and cylinder barrel
- Poor engine oil quality (oil viscosity) or engine oil thinning/contamination, e.g. due to fuel
- Soiled or damaged oil filter
- One or more faults in oil circuit, e.g. oil pressure too low
- Bent or damaged oil spray nozzles for cooling the pistons
Piston without damage
Slight deposits on the piston crown (5), in the combustion chamber recess and in the valve pockets (6) are normal. Sinkage of the spray pattern of the injection valves on the piston crown (5) and in the combustion chamber recess is also normal.
The combustion is OK. Continue to use pistons.
Piston without damage
The piston has slight carbon deposits on the top land (1). Slide coating (4) is present throughout. Compression rings (2) and oil scraper ring (3) move freely in the radial grooves.
Continue to use pistons.
Shown on aluminum cast piston
Piston without damage
Piston skirt (9) does not have any score marks. Compression rings (2) and oil scraper ring (3) move freely in the radial grooves.
Continue to use pistons.
Shown on aluminum forged piston
Shown on aluminum forged piston
Piston crown (5) with carbon deposits
Piston crown (5) has carbon deposits. Compression rings (2) and oil scraper ring (3) move freely.
Carbon deposits on the piston crown (5) are caused by e.g.:
- Defective valve stem seals
- Damage to cylinder barrels
- Short-distance operations
- Damage to exhaust gas turbocharger
- Poor fuel and engine oil quality
- Defective crankcase ventilation system
Continue to use pistons.
Piston crown (5) with manganese and iron deposits
Manganese and iron deposits on the piston crown (5) are caused by high manganese and iron content in the fuel. Manganese and iron are added to fuel in some countries in order to improve knock resistance.
Continue to use pistons.
Shown on aluminum forged piston
Score marks (arrow A) in area of piston skirt (9)
Score marks (arrow A) on piston skirt (9) are caused by e.g.:
- Dirt particles
- Detached oil carbon
Score marks (arrow A) do not lead to piston seizure or oil consumption.
Score marks (arrow A) must not be raised (free of burrs). Continue to use pistons.
Compression rings (2) and oil scraper ring (3) worn and firm in the radial grooves
Compression rings (2) and oil scraper ring (3) are firmly clamped into the radial grooves of the piston. These are clogged due to carbon deposits and can be traced back to excessive oil change intervals or the use of non-approved engine oil.
Do not continue using pistons.
Shown on aluminum cast piston
Shown on aluminum cast piston
Slide coating (4) is worn (arrow B)
Slide coating (4) is partially or extensively worn (arrow B).
Do not continue using pistons.
Scoring marks (arrow C) in the tread area of the piston skirt (9)
Scoring marks (arrow C) are present in the tread area of the piston skirt (9). These extend partly into the piston ring area. The surface of the scoring marks (arrow C) is metallically clean, rough and without high-gloss transition points.
Do not continue using pistons.
Shown on aluminum forged piston
The piston is charred and burnt (arrow D) into the area of the piston skirt (9) Piston crown (5) is thermally overloaded by knocking combustion and is
partially charred and burnt from the top land (1) to the piston skirt (9) (arrow D). As a result of e.g.:
- Uncontrolled combustion
- Incorrect fuel used
Do not continue using pistons.
Piston with broken ring land (7)
The piston has broken out in the area of the ring land (7) extensively over the piston pin boss (8) (arrow E). As a result of e.g.:
- Uncontrolled combustion
- Low fuel grade
- Incorrect fuel used
Do not continue using pistons.
Piston with damage from foreign objects
The piston crown (5) is damaged by fragments (arrow F). As a result of e.g.:
- Broken insulator of spark plug
- Foreign bodies aspirated or introduced into the combustion chamber
Do not continue using pistons.
The piston is cracked and burnt through (arrow G)
Piston crown (5) is centrally cracked and burnt through (arrow G). Structural failure of the piston material caused by e.g.:
Output increase
- Uncontrolled combustion
- Low fuel grade
- Incorrect fuel used
Do not continue using pistons.
Shown on aluminum forged piston
Shown on aluminum cast piston
Piston with siloxane deposit (arrow H) (only on vehicles with code 924 (Bivalent natural gas drive) or with code MZ8 (NGT engine EEV) or with code MZ2 (Natural gas engine M271))
Due to the use of natural gas with a high portion of biogas, siloxane deposits (arrow H) occur on the piston. The siloxane deposits lead to damage on the cylinder barrels.
Do not continue using pistons.
Shown on aluminum cast piston
Intake valves fitted
Intake valves have been fitted into the valve pockets (6).
Consequential damage due to:
- Incorrectly configured timing
- Damage to timing assembly/chain drive
- Anti-twist damage
Do not continue using pistons.
Shown on aluminum cast piston
Exhaust valves fitted
Exhaust valves have been fitted into the valve pockets (6).
Consequential damage due to:
- Incorrectly configured timing
- Damage to timing assembly/chain drive
- Anti-twist damage
Do not continue using pistons.