Orange peel on top of piston

Discussion in 'Outboards' started by robbyt8165, Sep 30, 2008.

  1. robbyt8165
    Joined: Dec 2007
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    Location: Saint Louis, MO

    robbyt8165 Junior Member

    My 1987 Force 125hp engine just blew. 3 cylinders fine but fourth has problems. 70 psi compression (150 for other three). Took off head and found metal shavings in the bad cylinder, cylinder wall scored, carbon deposits on head, and the top of the piston is incredibly rough and pocked - kind of an orange peel like surface. I am assuming that the shavings are a disintegrated ring. Spark plug for this cylinder was kind of black and oily.

    Question: What caused all of this? If I fix the the obvious (replace the piston and rings, hone the cylinder), will it just happen again? Is there an underlying carburation or ignition problem? What destroyed the top of the piston?

    Thanks
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I am not famillar with that engine but it sounds like it might have been pre-ignition in that cylinder. Pre-ignition will eventually melt the piston top, damage rings, head gaskets, burn and melt the exhaust valve, etc. Either that cyl. is running lean or it is running hotter than the others, or something causes pre-ignition in that cylinder (bad spark-plug, carbon build-up). Make sure the manifold does not have a leak or crack in the part that leads to that cyl., check that coolant is circulating properly in that part of the head (sometimes corrosion build-up will block water flow), check for a crack in that part of the head (have the manifold and head crack-checked in a machine shop), make sure the spark plug and wire is good, etc.

    Are you sure there are no signs of this in the other cyl.? If so, it just may have shown up sooner in the hotter/lean cyl. Check your spark timing, check your mixture, and your temp.

    If you replace the parts you should balance the new piston to match the others within a gram of weight or it might vibrate, hone the cyl. walls (consider replacing all of the rings while you are at it), and I would replace the exhaust valve too (and check the guide for damage from an overheated valve).

    Good luck.
     
  3. robbyt8165
    Joined: Dec 2007
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    robbyt8165 Junior Member

    Engine recently had a blown exhaust cover gasket adjacent to this piston (it is the bottom piston of a four-in-line engine). Water was squirting out, into the engine compartment. I replaced the exhaust gaskets but don't know how long the engine was running that way. Had about 3-4 hours on the engine after the gasket replacement and before the crisis. Do you think the leak led to less/no cooling? Peculiar, though, how well the engine ran just prior to blowing up.

    Engine also made a rythmic clanking sound at the end. Suppose I need to look at the piston rod and camshaft to see what is going at that end to.

    Thanks.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yep, that's pre-ignition damage, likely from an over heated hole (cylinder) and the noise you had was probably piston slap or worse. You'll find out when the main and rod bearings come off.
     
  5. BHOFM
    Joined: Jun 2008
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    BHOFM Senior Member

    Something as simple as a vacuum leak can cause one or
    more cylinders to run lean and cause this kind of problem.
    especially with engines that run under constant load. Marine,
    aircraft, industrial; automobile engines run under a ever
    changing load and not as likely to have this problem.

    When you get the damage repaired, change all the hoses.
    Cheap, as well a close inspection of the intake parts.
     
  6. Petros
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    Petros Senior Member

    a leaky cooling system could very well cause this. If pressure can not be maintained in the water jacket the coolant will boil away, creating a hot spot. It can warp and crack engine parts because very large temperature gradients can develop across the head or block. have the head and block checked for cracks and warpage.

    If they are only warped it can be resurfaced to bring it back true. There are no reliable way to fix cracks in the head or block. Sometimes the crack can be stop drilled and ground out, and then welded. After the part has to be heat treated to normalize the weld. This can work but unless the machine shop is very skilled at this, it will only crack again at the weld. It is usually best just to replace the cracked part.

    hopefully the block and head are not that fragile and you only have to replace the piston and rings.

    Good luck.
     
  7. robbyt8165
    Joined: Dec 2007
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    robbyt8165 Junior Member

    Of course, there is a little more to the story. The engine has been rebuilt once before and was over-bored .030. So this scored cylinder will need a sleeve installed. Local machine shop quoted $250 labor and $100 parts. Should I do it or give up on this engine? If I do, does the sleeve need to be bored .030 over to match the other cylinders?

    Thanks.
     
  8. BHOFM
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    BHOFM Senior Member

    Yes. The parts must be the same size, weight, or they
    can destroy the boat. Shake it to pieces in short order.

    The price seems a little high, check a few more shops.

    Check the price of a new engine before going on with
    any repairs, it might be the way to go if you plan on
    keeping the boat for a long time. Used engines are
    just "used" engine, last resort only.
     
  9. Petros
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    Petros Senior Member

    You likely can just hone the cylinder, a small amount of scoring, as long as it is honed smooth, will not affect the life or performance much. Just make sure it is not too badly warped, that is what requires over boring. Use a bore mounted dial indicator.

    Also most engines can be over bored to .040" but that means all new over sized pistons. Yes, they all must match weight and displacement.

    Get it apart and check it out, this is all guess work until you know for sure. let us know what you find.
     
  10. jonesg
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    jonesg Junior Member

    I've talked to many marine engine rebuilders and its common to only bore out 1 cylinder, a very common fix.
    The over-bore piston usually weighs the same as the original size for this very reason.

    I wouldn't begin to guess at what happened but I would find out why it happened or it will again if rebuilt.
     
  11. Petros
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    Petros Senior Member

    it is not just the weight of the piston that is the issue, though that is part of it. Over-boring one cylinder means that one will displace more volume than the others, and it usually means slightly higher compression ratio in that cylinder (more volume displaced into the same combustion chamber volume). So that one cylinder will put out slightly higher power than the others with each power stroke, making the power pulses from each cylinder to the crankshaft uneven, causing the vibration.

    If the over-boring is small, like 0.005-.010 inch, and it is a relatively low RPM engine (2000-3000 rpm max speed), and the piston weight is matched, there would likely be no noticeable difference. Marine diesels are much slower than this and it would not surprise me if over boring one cylinder is routine with diesels.

    On racing or performance engines the engine builder not only balances the weight of the all of the moving parts, they also make sure the combustion chamber volume is the same in each cylinder just so there is a smooth power flow with little vibrations, especially at full throttle.

    Though again if the piston bore can just be honed, and not over-bored, you can just buy a standard size piston and replace it without going to the oversize piston. You still have to make sure the piston is matched weight to the others. I have seen supposed matched sets of pistons be out of balance as much as 8 grams within the set, that would cause a noticeable vibration above about 3k rpms. I like mine to be within a half a gram when I rebuild an engine.
     
  12. robbyt8165
    Joined: Dec 2007
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    robbyt8165 Junior Member

    Have the engine apart now

    Finally got time to break down the engine. Rings were fine but roller bearing cage (at connecting rod/crankshaft) was totally melted/burned up. Crankshaft is scored where roller bearings ride. So, at a minimum, I need a new piston, rings, and roller bearing cage for this cylinder.

    Questions:
    1a. Since rings were okay but compression low, does this mean cylinder wall scoring is bad enough to lose compression? Bad roller cage bearing would not have anything to do with compression , would it?
    1b. Will honing be good enough to restore compression? Is this one of those try-it-and-see repairs? Kind of a pain to reassemble entire engine to find out that honing wasn't enough.
    2a. Does scoring on crankshaft need to be ground out by a machine shop? Would NOT doing this lead to a new roller cage bearing quickly disintegrating?
    2b. If crankshaft is reground, is there such a thing as a "over-sized" roller cage bearing, is over-sizing needed, and how is the fit measured/adjusted/etc.?

    Thanks for any input.
     
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  13. pistnbroke
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    pistnbroke I try

    If crankshaft /rod bearing has failed then piston will not move up bore as much as usual hence the low compression...Usually the crankshaft would be re ground under size and a new roller bearing fitted to match. OR crank can be built up and re ground original size..not so common unless its vintage and not replaceable
     
  14. Frosty

    Frosty Previous Member

    You are looking straight into the face of a new engine, a short motor anyway.
     

  15. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Since piston failures in large outboards concern almost always the lowest cylinder, I assume the buildup of sediment in the cooling channels is a contributing factor. Part of the lower cylinder wall reaches a much higher temperature causing a change in shape, resulting in compression loss, hot spots, cracks etc. It was the main cause of death for the old-fashioned 4 and 6 inline Mercury outboards.
     
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