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Thread: Wanted:Mark25 oversized pistons and rings

  1. #1
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    Default Wanted:Mark25 oversized pistons and rings

    I'm in need of 2 pistons w/rings(flattop or deflector) for a Mark25.
    It is bored out .040" over(2.4775"), any help would be appreciated.

  2. #2
    Sam Cullis Mark75H's Avatar
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    Nov 2004
    Annapolis, MD USA
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    Neither Merc nor Wiesco made pistons that size. Pistons for .040" over are going to be very very hard to find (I would predict you will be waiting quite a while) and probably cost you at least $150 each and may be semi finished requiring some machine work. Is the motor really worth that much? Wouldn't it be easier & cheaper just to find another std bore Mk25 or a Mk28?

    Another alternate would be to have new sleeves installed (I think Jim Ruck does this for about $110 per cylinder) and go back to std bore .... but I think that would cost more than finding another good block, too.

    If you are really dedicated to that block you might try boring it to 2 9/16 and using Mk28/58/78 pistons. I don't know how much 'meat' there is in the sleeve, it might work, or it might go thru to aluminum.

    Flat pistons will probably drop your Mk25 to about 9 hp, you have to stick with deflector pistons.
    Last edited by Mark75H; 03-15-2006 at 03:42 PM.
    Since 1925, about 150 different racing outboards have been made.

  3. #3
    Team Member MTECHMARINE's Avatar
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    Jun 2008
    Milton Florida
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    Call Mike Lalonde at 856-304-2491 he has them. In Daytona FL

    I have these running in a KG-9 and a 20CID Wizard.

    They need machining for the wrist pin washers tho.

  4. #4
    Team Member A/B Speedliner's Avatar
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    Jun 2005
    New Smyrna Beach, FL
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    I believe Alan van Weele has 0.040 over pistons made by Weisco
    Old Race Boats Still Flip You Out

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