Cracks on the exhaust valve casings of the Main Engine

What happened

Immediately after leaving port ( sailing at approximately 100rpm,) a crack was discovered on the collar attached to the cylinder cover of the main engine #4 cylinder exhaust valve (C valve.) The crack extended approximately 70mm from the top edge toward the center of the collar and approximately 50mm in a downward direction. As the crack on the top surface extended to the base of the valve casing’s center cylinder, the situation was reported to the bridge and request was made for speed reduction of the main engine and for the engine to be stopped at the next possible location. 40 minutes later, the main engine was stopped and anchor was dropped.
Other cylinder exhaust valves were examined for cracks. Approximately 7~70mm long cracks were discovered on a total of 15 exhaust valve casings which included spare exhaust valve casings.
Among the 15 exhaust valve casings including spare valves with cracks, 13 of them exhibited cracks occurring between the cam-side collar attachment bolt hole and the base of the valve cylinder. The cracks cut across the collar from the edge of the flange toward the valve cylinder. Although the worst cases exhibited cracks extending right to the base of the cylinder and cracks emanating from opposite edges almost connecting, there was no deformation etc. of the concerned areas.

Repair

Based on the condition of the cracks, for the time being, only the #4 cylinder B valve and C valve which had the worst cracks were replaced and voyage was recommenced towards the next port of call (Nagoya.) Decisions for repairing the cracks were based on manufacturer recommendation. Slight cracks were ground off, metaloc adhesive was applied toward those cracks which could be repaired (9 units,) and those which were extremely damaged (4 units) were unloaded and scrapped.

Root causes

Although it is unknown when the cracks on the edges of the exhaust valve casings formed, it is assumed that they did not form recently. As such, although a determination of cause is difficult, the over- tightening of the clamping bolts on the exhaust valve casing is conceivable as one cause.
Assuming that the attachment of the exhaust valve and tightening of the attachment bolts were carried out by the manual book, as the nuts have hitherto been tightened with an impact wrench, it is conceivable that it would be quite easy for over-tightening to occur and it is speculated that the cracks formed and worsened due to repeated over-tightening over the long-run. The location of the cracks tend to be concentrated between the cam-side bolt hole of the exhaust valve casing and the center cylinder, structural and material problems are also conceivable.