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  1. #1
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    Unhappy Loss of compression

    Hi all
    I'm having an issue with an engine I'm working on and I thought I would see if anyone has any suggestions. First a little bit of background - it's an old 308 being used in a boat and water had entered the engine via a hole in one of the water cooled exhaust manifolds.

    The engine was left in this state and eventually seized (i.e. it could not be turned over). I pulled the heads off and after a bit of work managed to get the engine turning over again by hand.

    I cleaned everything up and put the engine back together. Once back together again, if would not start - it would fire on one bank, but not the other. Testing with a compression tester revealed no compression on the non-firing bank (this was the bank that got water in it).

    In order to figure out where I was losing compression I made up a device to inject compressed air into the cylinder. It revealed that I was losing the seal past the rings.

    The weird thing however is that I only lose the seal at the very top of the stroke - before and after TDC it will seal very well (note rockers have been removed to keep valves closed).

    Does anyone have any idea why this would be?
    Is there a way to correct it?

    Thanks in advance for any/all help.

  2. #2
    been here .......too long Smitty2's Avatar
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    rust has eaten away a bit of the the cylinder wall where the water sat...at the top of the piston stroke
    plus it will have got at the rings too (they are metal too) ...
    a re-bore and new pistons/rings should fix that

    you can test this by pouring in an amount of engine oil into each cylinder on the non-firing bank
    betcha the compression goes up
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  3. #3
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    Quote Originally Posted by Smitty2 View Post
    rust has eaten away a bit of the the cylinder wall where the water sat...at the top of the piston stroke
    plus it will have got at the rings too (they are metal too) ...
    I was thinking the same, however the bore walls looked to be in good shape.

    Quote Originally Posted by Smitty2 View Post
    a re-bore and new pistons/rings should fix that
    Very much hoping to avoid that.


    Quote Originally Posted by Smitty2 View Post
    you can test this by pouring in an amount of engine oil into each cylinder on the non-firing bank
    betcha the compression goes up
    I have tried that and it made no difference.

  4. #4
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    You must have had the heads off, what did the bores look like? Either way. My guess is they are toast. Water getting in and keep running it, not going to end well. Even without rust from sitting, it will screw the bores, specially in a boat.

    The reason you notice it most at TDC is that is the most worn section of the cylinder. It will happen with or without waters help but throw some water into the mix and probably water straight out of the river or ocean with crap and salt in it and there you go. If you measured them you’d probably get a nasty surprise.

    I’d have $50 on a re-bore, pistons and rings. Sorry.

  5. #5
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    Yeah, I had the heads off and the bores looked ok - no bad rust or pitting.

    The problem occurred when the motor was run with a garden hose connected to supply cooling water. Normally with the engine running water wouldn't have entered the bores (well very, very little would have), however once the engine was stopped (and the hose still on) it would have pushed water into the bores. So what I am saying is that I doubt it was run with water in the bores, the water entered later and just sat in there.

    Anyone know a rough cost of a re-bore, pistons and rings?

  6. #6
    Part of the furniture HQ308Ute's Avatar
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    It's a short motor rebuild mate, the whole thing has to come apart.

  7. #7
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    No difference to compression when testing with oil in the cylinders typically points to valves not sealing, so I would also be looking closely at the valves and seats for corrosion. Under normal operating conditions (on the lake), water (from a hole in the manifold) can be pulled into the cylinders past the exhaust valve due to reversion and may have been running like this for a while.

    There is no doubt that corrosion has also occurred to the bores and rings.

  8. #8
    Do you ever leave? hz_350's Avatar
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    did you turn the motor over when you had the head off
    my HJ sedan rebuild http://www.aussiev8.com.au/conversio...ull-build.html

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  9. #9
    Do you ever leave? Mickss355's Avatar
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    To be honest, anything less than a strip down, re hone, re ring, re bearing and a dam good clean """"at the very least"""" is only prolonging the inevitable !

    Personally i would re-bore it or get another block or risk breaking down on the water !

  10. #10
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    Quote Originally Posted by hz_350 View Post
    did you turn the motor over when you had the head off
    Yep. The reason I took the heads off was because when I first got the boat I couldn't turn the motor over. After removing the heads and a little gentle persuasion I managed to get it turning over quite freely. Cleaned everything up and then reassembled.

    I'm still interested in what people think a basic rebuilt would cost.

  11. #11
    shovel built race engines onlimiter's Avatar
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    prolly an open cuircut cooled thing!
    most likely its crap anyway!
    pick up a runner and move on!
    rev the chev kev!

  12. #12
    Do you ever leave? Mickss355's Avatar
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    Quote Originally Posted by Joker View Post
    Yep. The reason I took the heads off was because when I first got the boat I couldn't turn the motor over. After removing the heads and a little gentle persuasion I managed to get it turning over quite freely. Cleaned everything up and then reassembled.

    I'm still interested in what people think a basic rebuilt would cost.
    Knowing that it was rust that stuck the rings to the bore - then you freed the engine up but the rust between the rings is still there plus its got between the rings and the ring grooves which has now jammed them up hence no compression and even if it did start up - imagine all the rust grit between the rings being scrubbed up and down the bore grinding away at the piston skirt and cylinder walls !
    I feel ill just thinking about it hahahaha !

    So anyway, the cost of a rebuild will depend on how much you can do yourself !
    If you can do all the assembly work then it will only cost the price of the parts plus machining which isn't that expensive
    Last edited by Mickss355; 07-07-2012 at 08:35 AM.

  13. #13
    Part of the furniture v8bulldogs's Avatar
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    Quote Originally Posted by Joker View Post
    Yep. The reason I took the heads off was because when I first got the boat I couldn't turn the motor over. After removing the heads and a little gentle persuasion I managed to get it turning over quite freely. Cleaned everything up and then reassembled.

    I'm still interested in what people think a basic rebuilt would cost.
    I don't know what engine reco mobs are in Armidale but there's a good bloke In Inverell
    Dave Holder 67210771. He'd give you a price for parts.

    If you could trailer the boat with the heads off they could measure the top of the bores to give you an idea of how much wear there is .... probly wouldn't even charge.

    The motor could already be 0.060 over? you'd know where you stood.
    Last edited by v8bulldogs; 07-07-2012 at 09:05 AM.

  14. #14
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    Thanks for the contact info. Much appreciated.

  15. #15
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    Quote Originally Posted by v8bulldogs View Post
    The motor could already be 0.060 over? you'd know where you stood.
    I'm pretty sure it's original bore. There are no markings on the pistons and a rough measurement with digital callipers indicated it was just over 100mm (can't remember the exact number, but I remember looking at the time and it was less than 20 thou over standard).

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