Crack in generator connecting rod serrations

What happened

While the ship was docked for Intermediate survey, overhaul inspection of generator diesel revealed crack formation in all connecting rod serrations of the three generators aboard. The cracks were all found at the upper section of the upper serration teeth.
All connecting rods were replaced with new parts.

Root Cause

The following factors are considered as a possible cause.
1) Insufficient material strength.
2) Insufficient processing accuracy.
3) Decrease in tightening torque.
For item 1), test results revealed no problems. The surface roughness of the teeth was 18S for damaged parts as against 6S in specification. The contact surface of the teeth was not even and indication of strong contact was observed in certain local sections. Also, chamfering of the teeth was insufficient. In addition, the measurement of a torque with which a damaged part was tightened to the index mark showed that the torque had fallen to a range of 47 to 59 kg-m. (Specified torque was 60 to 65 kg-m.)

Corrective actions

1) To increase the rod bolt tightening torque.
To change it from 65 - 70 kg-m to 70 - 75kg-m.
2) Perform torque check in 3000 hours.
3) Perform serration collar check in 4000 hours.
4) Perform magnetic inspection of serrations after 4000 hours, on removal of piston.