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Thread: piston clearance to cylinder wall

  1. #1
    Member JOURNEYMAN
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    Jan 2004
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    illinois
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    piston clearance to cylinder wall

    I have a problem with my merlin 3 block. Not necessarily the block. I purchased a block in a short block form. Wiseco pistons and eagle crank,rods. Well I never thought I should tear it apart and mic everything from a well known builder on the east coast, begins with Shaf. Anyway with only 70 runs on the motor, over the winter I pulled it out to do some work on the chassis. And was getting ready to put it back in and decided to checked the mains and found it had ingested a piece of dirt or something in the #3 main. So I pulled it all apart and had the crank polished and was cleaning up the pistons when I noticed they had alot of wear on the top "anti detonation" or area just above the top ring land. I was wore these groves almost flat, and on the opposite side on the piston skirt. So I checked the dimension on the pistons, as specified by the manufacturer. It is 4.4935 and then the bore of the block is anywhere from 4.5005 to 4.5015. And the suggested clearance is .005. Giving me a .007 to .008 P/W clearance. Not to mention I had two cylinders with .002 taper....Talked to Wiseco and they told me with this clearance and a short skirted piston that this was causing the excessive wear on the pistons. And it would not be very adviseable to run them in this situation. Meaning I would have to scrap these pistons, bore the motor .030 over and rebalance the entire assembly. Or I could get another new block and sell the old one and still spend the same amount of money, but go to a dart block

  2. #2
    Administrator RACING JUNKIE bjuice's Avatar
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    Apr 2003
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    1st off Hey Doug good to see ya and hope ya up for a good racing season.

    There are many more on here much more qualified to answer your question than me on the best route to go...


    but what i would like to say is that a close friend of mine 3 years ago had almost the same identical problem with a 555 he purchased from the same company.

    He ended up having to get a new set of pistons and had Gene Fulton work the block etc......

    so this problem from this company seems to be pretty common from what i gather.

    Brian
    Bjuice..

    "I'M YOUR HUCKLEBERRY ! "

  3. #3
    Senior Member RACING JUNKIE
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    In my opinion there's errors on both parties here. If Wisco recommends .005 clearance then the pistons should have been 4.495 plus or minus .0005. Because the STD,. bore should have been 4.500 for these pistons.
    I never run .005 clearance on any engine unless they are hyper-utectic pistons. Every engine i build has .006 for gas and .007 for alcohol, .008 for blown alcohol. I ask the the person who bores the cylinder for me to leave a .001 or .002 so i can finish the cylinders myself. I once both a rotating assy from shaf. it cost me more than if i would have piece milled it. I'm not saying they are bad but they are busy and i doubt if their inspection is all that good.
    The .002 taper is unforgivable, but the piston could have wore .0015 in 70 passes, but i doubt it.
    The b/blk 476" i just put together, i ask the machine shop to give me .007, to the used pistons i had. It came back with .009/.010 clearance, was i pissed yes, but the owner of the engine sent the block to one of his friends, that he trusted. I showed the owner the dimensions and told him the possible problems, IE: piston rock less sealing etc. He said o'k i f**ked up but didn't want to fix it with another blk. or buy new pistons. I file fit the rings to .024 and put it together. well the engine ran 10.44 the first time out without good tuning, it will run low 10's next time out i'm sure with 3700lbs.

    O'k enough, if it were my engine i'd just put it back together and go racing, not spend another $, it will run atleast as good as it did before when you were happy with it. You're not losing more than a .1/tenth with these problems. But do agree with you they should have never existed.

    JMO

    Zip.

  4. #4
    Senior Member EXPERT BUILDER
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    One other area to look at is the pin bore clearance ith the pin end of the rod and also the pin bores in the toe pistons maybe tight as well causing some un needed rocking.

    We have seen alot of shops that don't check pin bores in the pistons that cause problems.
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  5. #5
    Senior Member RACING JUNKIE
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    Quote Originally Posted by hink
    One other area to look at is the pin bore clearance ith the pin end of the rod and also the pin bores in the toe pistons maybe tight as well causing some un needed rocking.

    We have seen alot of shops that don't check pin bores in the pistons that cause problems.
    Right on Carl,
    a lot of todays pistons run to tight pin bores and donot consider pin distorion which causes excessive piston wear. A lot of them consider .001 clearence enough with thin pins which causes bind and excessive piston wear.

    Zip.


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