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Thread: Fast Fred's Secrets of the OMC MOD50's Thread #1

  1. #501
    FFX-61
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    .008 for a wiseco, thats the the piston die' plus .008 thousands.
    on a stocker i'll lean a bit on the loose side of the spec, just a bit.

    but mostly usein wisecos my self. In independent testin of a Fast Fred power head, 2% or less leak down, she had wisecos , at that spec.

    guessin at the factory spec, .004-.005 that may not be right on, should be close. i'll look it up.

  2. #502
    Team Member 850cc racer's Avatar
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    Hey Thare Fred, whats going on over there? you building anymore of them mod 50's? ...you been racing>?

    yeh i think wiseco say .004? im going to be running Vertex pistons which are a forged but have a teflon coating i think it is. the block is at the machine shop today getting measured up to see if they go +10 or +20 over. whilst its there anything else that should be done?

    850

    p.s. also are you able to advise jet sizes? it has the FT 2bbl front, was going to pull carbies down and clean etc check a few things while im there

  3. #503
    FFX-61
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    wisecos expand, unlike a cast piston the wiseco IS guna grow .004 when it heats up. if you set a forged piston at cast piston specs it WILL stick.

    gots some jet specs some ware

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    Team Member Danny Pigott's Avatar
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    Question Crankshaft

    Anybody ever seen a Harold Naus 3 cyl omc crankshaft. Harold is gone but some of these have got to still be around.

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    Team Member Sam La Banco's Avatar
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    What's different about it???

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    Team Member sheuninck's Avatar
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    Default .040 cylinder expansion

    Will a .040 over bore cylinder expand more than a stock one? How much more? With wiseco piston needing to be looser, but also with .040 over bore the rattle does not go away when warmed up?? Should I run hotter or cooler to get more compression?

  7. #507
    John (Taylor) Gabrowski
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    Default This thread is sure educational!

    The more I read and re-read this thread the more I am learning something from all of you that serves to be great information before I do anything to that new stock 3 Holer complete block I bought at the garage sale with the its blown up complete engine this block is meant to replace. Its really informative.

  8. #508
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    at.040 over bore, it is common to give the forged piston up to .009 thousands total, thats the piston diameter plus .009 total.

    so when the piston is in the hole, centered, thares a .0045 thousands gap all the way around the piston and the cylinder wall.

    bore taper, if the bore has a lot of taper,( bigger at one end than the other)
    the skirts will talk, this is not good.

    coolin, no matter how you do it, the temp gots to get to 140deg give or take 5deg, and stay thare when you are on it for how ever long that is.

  9. #509
    John (Taylor) Gabrowski
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    Default So to simplify....

    Fast Fred:

    So you go bore plus .009 with a forged piston so at the engine warmed up operating temperature you end up with a .004 to .0045 piston clearance in operation at racing speeds and temperatures?

  10. #510
    Team Member Sam La Banco's Avatar
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    On an OMC 3 cyl. the amount of clearance set up is all about what you want to do with the motor. If you are going to use it on a real race course
    it's all about acceleration, I have won 4 nationals and a worlds with the motor and have not had a speedo in the boat since 2000, we set them up as tight as we can.
    If you just want to go fast, set it up on the loose side, just don't drag race your friends. On previous threads I gave the clearances we use, but that is with OMC pistons.
    I would also suggest you find out what the clearances are at .060 below the bottom ring, thats where they stick, thats where they should be set up at.

    The motor will run fine with up to .001 taper, but none is better.

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