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Default Nanni Diesel Engine Problems

I have a Nanni 4.330 TDI 115hp diesel engine on my Resort 35
displacement cruiser. The boat is built to charter specifications in
Australia and is set up for charter in the Sydney area. The number 3
piston has lost compression due to a valve seat falling out and
destroying the piston head. It is too early as yet to get the full
picture of all the damage, as the mechanic has only just pulled it out
of the boat, but I was wondering if anyone else has experienced this
problem and what was the cause of the seat falling out. My mechanic
wanted me to look at the engine, because he had never seen another
similar problem. He later explained to me that the only two causes of
this failure were severe overheating of the engine, as a consequence of
which the seat falls out from the expanded head, or because the head or
the seat have been incorrectly milled and the tolerance was
insufficient to hold them together under normal engine operating
temperatures. My problem appears to be the latter because there is
absolutely no evidence of any overheating of the engine, which is why
my mechanic wanted me to see the condition of the engine, water pumps
head gaskets other pistons and so on, just to understand why he thought
it unusual that the seat fell out.

Does anyone have a similar experience or perhaps can give me another
reason to look in to, as to why the seat fell out. The boat is only 5
years old and the engine has only done about 670 hours of operation. I
have limited mechanical experience and understanding, but several
mechanics, including my father in law all agree that this should not
have happened if there were no overheating of the head and block. Would
you let me know if there are any other reasons I should be aware of
before they repair the engine, only to let it happen again sometime in
the future, because we missed something. Any advice is helpful.