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Blown piston #1 cylinder 1975 horse mercury

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  • Blown piston #1 cylinder 1975 horse mercury

    I was wondering if there is any sites i can find to assist with replacing a piston on my 1975 mercury 65 hp.

    I just blew it yesterday it now only has 25 psi the other two are still at 128- 130.

    I am fairly mechanicly inclined, so i am willing to try the job my self if i can find some kind of help.

    Or does anyone know how much oit would cost to have it repaired?

  • #2
    Re: Blown piston #1 cylinder 1975 horse mercury

    Here is the issue, I am told that some Merc dealers won't even work on that vintage, I know a local guy who they send them to. That motor has no head, therefore the entire crank, rods, piston, rings, all come out together in one piece. Not saying that it can't be done, mine was blown before I bought it and a friend of the original owner did it for him. I would ask myself if it is worth doing the job rather than buying a used motor. If your into it, you're going to want to hone the block, new rings in every cylinder, new reeds, and a gasket set. That plus labor might be cost prohibitive. On the other hand, if you do have it done, at least you know what you have when you're finished.
    '78 Sylvan Sea Master
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    • #3
      Re: Blown piston #1 cylinder 1975 horse mercury

      You will need a new oversize piston and ring set(~$100), a bore on the cylinder (~$50), and some gaskets(~$50) and some time. The other cylinder can be honed or left alone, as is desired. The block must be completely stripped and taken to a mechine shop for boring. Now scrub the intire block inside and out with soap and water, rinse well, oil the bores and reassemble.

      The other more important thing is to find out why it failed and fix the issue. Normally it is lack of cooling, running too lean, spark advanced too much or all three. it makes sense to rebuild the carbs and waterpump, and retime the motor.


      • #4
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        Re: Blown piston #1 cylinder 1975 horse mercury

        First get you a FACTORYservice manual and read,read read... Then you need to figure out what caused the failure (cooling/ignition/mechanical). All inline Mercury engines do not have a head and before you do anything "mike" the cylinders as they will be out of round guaranteed!!!! If you like the engine and performance you have now, go ahead and bore and rebuild it. If you need more torque and speed go to a larger engine. To properly rebuild it you will need pistons, gaskets,carb and fuel line repairs, water pump and idle restrictor mod's, and special ring compressors to reset the crank and pistons as a assembly. Piston rings come with the new piston and reeds need not be replaced unless damaged.....
        When in doubt,Trim it out!!!
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