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PostPosted: Thu Jun 11, 2015 6:05 pm 
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Joined: Tue Jan 17, 2012 10:40 pm
Posts: 12
Do any of you wise owls know how far above the top of the block of a 7.7 the piston crown can protrude at TDC please?

The reason for the question is that I have encountered one where some of its pistons seem to be just kissing the heads - and letting us know it!

The manual indicates there used to be shims available to lift the block off the crank-case or to lift the heads off the block - in the case that any of the mating surfaces have been flashed. I wonder if this has been done to this engine, but no shims applied?

Cheers,
Phil


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PostPosted: Fri Jun 12, 2015 8:21 am 
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Joined: Sat Jan 14, 2012 10:24 pm
Posts: 115
Very quickly: SOME of the pistons. All under one head check the thickness of the head and the gasket. SOME Under both heads check for worn big ends which will allow contact. After that assume block to crankcase issue. Check the n/s side face of the block between 3 and 4 near to the engine number for a stamped .10, .15, .20, .30 etc which will indicate oversize head gasket required.

More likely to be bottom end than gaskets since it should have made this noise immediately after the last reassembly.


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PostPosted: Fri Jun 12, 2015 3:27 pm 
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Joined: Sat Jan 14, 2012 10:24 pm
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If the noise and piston marking indicate that there is possible piston/ valve contact, check for a stretched timing chain.


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PostPosted: Fri Jun 12, 2015 3:45 pm 
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Joined: Tue Jan 17, 2012 10:40 pm
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Thanks for the advice. It is not valve contact and did not do it right after recent rebuild which included new big ends and crank grind. Only makes noise now once the engine warms up. Looking through the injector holes with a miniature camera it seems nos 2,4 and 5 are the offenders. I will tell the owner to look for the oversize gasket markings.
Back to my question, should the pistons be proud of the block face?


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PostPosted: Fri Jun 12, 2015 10:07 pm 
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Joined: Sat Jan 14, 2012 10:24 pm
Posts: 115
I don't have any more relevant technical on the 7.7. The only 7.7 engine block I have occasional access to is 12' up in the air on racking and I'm not sure whether it has rods and pistons in it to check. It is unlikely that I will be visiting it in the near future either.

I am not disputing what you are able to see with the video camera, however it does seem odd that only some cylinders are affected. This leads to the only possible inconsistency I can imagine which would be that there is a difference in the pistons themselves.

If you don't want to take the head off you could try dropping a valve from each cylinder onto the piston and checking protrusion with a dti. at tdc. If you come up with the same answers and indications that the others are noticeably lower you will be some where nearer. However you won't know if all the valve stems and seat recesses are the same unless this was checked during the rebuild.


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