6.7L Cummins Cylinder Block – Piston – Failure Analysis Inspection

Inspect the piston for damage and wear to the skirt, pin bore, top, and ring lands.
Inspect the piston pin for damage and wear.
NOTE: It is important to note that the illustrations relate to early detection of a piston failure. Progressive damage will
make it difficult to categorize the root cause of the failure.
The degree of progressive damage can make a difference as to the extent of the damage, and therefore, the
illustrations only provide a guideline for identification.

Abrasive and/or Debris Wear
Scratching on the piston skirt with material embedded into the piston can be caused by:
• Ingested abrasive material
• Inadequate cleaning during a previous repair
• Particles embedded in the cylinder bore
• Improper maintenance of the lubrication system
• Debris in the lubrication oil system from another failure
• Scuffing and scoring.

Scuffing and scoring can be caused by:
• Engine overheating
• Oil dilution
• Improper maintenance of the lubrication system
• Piston cooling nozzle malfunction
• Injector overfueling/engine running on an alternative fuel source, for example, oil in the intake air system.
NOTE: Scuffing and scoring on the piston rings indicates a breakdown of the oil film on the cylinder bore wall, causing transfer of material from the piston ring face to the cylinder bore wall.

Other Types of Piston Damage
Hydraulic Lock
Hydraulic locks (oil, fuel, or water) can cause the piston to split in half, or the lands can break and the connecting
rods can be bent.
Dustouts
Particular attention to the wear pattern of the intermediate (second) ring is required for diagnosis of a dustout piston
scuff. The ring will have a polished appearance on the full-face ear of the intermediate ring. The polished area starts
at the bottom of the ring and continues upward on the tapered ring. Occasionally, the edges of the rings are
razor-sharp, and the block cylinder bore has indications of ridges approximately 1-1/2 inches below the top deck.
Top of Piston Meltdown
The top of the piston appears to be melted down and the failure resembles an overfueling situation, such as a stuck
injector. A plugged piston cooling nozzle can also resemble this failure.