Main Engine piston clamping bolt Breakage

What happened

1) When the main engine LO suction filter was inspected during anchorage at a foreign port, large amounts of metallic fragments were discovered. When the inside of the main engine crankcase was inspected, it was discovered that 1 out of 4 of the No. 6 cylinder piston clamping bolts was broken and had fallen off.
2) During an inspection of piston at intermediate survey No.1, it was discovered that one No. 10 cylinder piston clamping bolt was broken and had fallen off.

Damages

1) One piston clamping bolt had broken at its base. It was pounded upon inside the piston, deformed and fell off.
2) Breakage of the piston skirt bolt clamping counter bore produced shuddering of the bolt and washer and resulted in conspicuous damage in the form of indentations.
3) Crack formation in the piston fitting part of the piston skirt.

Repair

The piston crown, piston skirt and piston pin were replaced with spares.

Causes

As the part of the break hit against other parts and produced indentations in the ruptured phase, it did not retain its original shape so the cause of breakage could not be determined. However, it is assumed that due to some unknown factor, the clamping bolt loosened and due to the piston’s reciprocating mo- tion, repeated stress on the bolt contributed to the breakage.

Countermeasures

After inspecting all piston clamping bolts, clamping was carried out at the specified torque.