Seizure in the piston pin bosses (floating-fit piston pin)

The blue discolouration of the piston pin in the area of the connecting rod bush indicates that the clearance there was insufficient, and that as a result the piston pin was only able to rotate in the connecting rod bush with difficulty or not at all. The only rotation of the piston pin took place in the piston pin bore. However, the clearance of a floating-fit piston pin is too small for this. The increased friction caused overheating in the bearing, as a result of which the oil film became ineffective and the piston pin seizure arose.

• The clearance between the connecting rod bush and the piston pin was not made large enough.
• The clearance in the connecting rod bush was bridged by a misalignment of the connecting rod and the piston pin became seized as a result.
• The piston pin bed was not lubricated when the pistons were installed.

NOTE It is essential to generously lubricate the piston pin bed when installing the pistons to ensure that sufficient lubrication is present during the first few revolutions of the engine and to prevent initial rubbing marks from being formed when the engine is started.