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Thread: +.085 pistons!

  1. #1
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    +.085 pistons!

    G'day all!
    I've bought a '42 MB with an engine that'd been removed. I found two broken valve springs and one bent int. valve, and #4 pistons that's in rough shape, the bore is ok though, somehow. The engine is +.080 over-bored, according to a rebuilders plate riveted to the valve cover. All four pistons measure 3.21+/-, so it's +.082-5.
    I've found +.080 pistons and rings, but I'll have .008 piston-bore. Too much.
    My question for any experts here: Is there pistons of a larger dia. that were used in a different engine with same crown to pin off-set?
    Any suggestions are welcome.
    I have a friend who is an experienced engine rebuilder, used to have his own shop/business. I also worked in an engine rebuild shop 22 years ago, so I know some things. My block is at his shop. He's asked me to ask the experts, hopefully there are some here.

    Thank you in advance.
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  2. #2
    Super Moderator bmorgil's Avatar
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    Andy, it is surprising that the bore is still good. unfortunately .080" is the max oversize. The piston to bore fit is maximum .005" for fit out of round and taper. So yes, if you are sure the piston to cylinder does exceed .005", it is to big. To determine this though you should have the new piston you intend to install. Its actual measurement for size might be big. Though you usually bore to the size required (i.e. .080" over standard for a .080" piston) On older pistons often the bore had to be opened up a bit to facilitate the pistons actual size. This is why most machine shops want the pistons to measure before they bore the block. You just never know.

    The engine is basically a Continental Industrial Engine. Though it is a Willys Engine, it is based on the Continental design. Most if not all the parts will interchange. The only manufacturer I know of that makes pistons for them is the original manufacturer, SilvO-Lite. https://uempistons.com/series-2167-s...-2-2-2606.html . I have had custom pistons made by https://www.jepistons.com/ and https://ariaspistons.com/ needless to say I was very happy with those. There are a handful of custom piston manufacturers out there. The final and best fix on a motor that is already .080 over and needs to go further, is to sleeve all 4. The blocks are getting rare and the new reproductions are expensive. You may be able to find a good used one.

    Mine is .080" over. I had it sonic tested prior to going that far. There was plenty of meat there. However if your machine shop finds it needs to go to .085" with custom pistons, I would definitely get it sonic tested first. Sleeving all 4 is no big deal for a good shop. It should be much less than a new block.
    Last edited by bmorgil; 10-12-2020 at 06:00 PM.

  3. #3
    Super Moderator LarrBeard's Avatar
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    Even if you go to a custom piston/overbore solution, you have pushed that block further than it was ever intended to go. If the engine is original, sleeve it to keep it. If it's an after production L-134, maybe another used engine would be a solution.

  4. #4
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    Thank you for the info and advice, much appreciated! I passed your response to my machinist friend.

  5. #5
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    Thank you for the info and advice, much appreciated!
    How do I identify original or L-134 please?
    Here's a pic of the block, #'s might not be clear enough though.DSC01256.jpg

  6. #6
    Super Moderator gmwillys's Avatar
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    I would be suspicious that there isn't some cylinder damage from the beating that took place. I agree with Bmorgil in having the block sonic tested. I would be inclined to sleeve the block, just to prevent further issues in the future.

  7. #7
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    Yes, considering the condition of the piston, I immediately looked at the cylinder. Besides a very slight scratch, I've seen much worse with a piston in better condition.
    I'm now looking at the block to see if it's original; #s match the id plate on the glove box. I will know early this afternoon. If not, it may be a post-war replacement. Two local MB collectors say they have engines, where I will source an MB block since my block needs a re-sleeve. May as well put the work and dollars into a period correct engine.

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