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johnson 9.9 powerhead repair

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  • johnson 9.9 powerhead repair

    i got used 9.9 johnson made in 1977.in bad shape of paint.compresion 90psi for both cylinders.
    low price motor.
    so i decided to instal new gaskets, rings, head gasket, both seals. cylinders got honing/cros mark patern on the surface.
    made a measure- litle worn cylinders but still can be used.
    instaled new omc rings but ring gap 0.28 mm so i got rings to file.
    now i filed rings a nd get gap 0.10 mm( 0.05")
    used a omc geal while puting together cylinder block.
    while cylinders was oiled i got 120 psi for both cylinders.
    powerhead on ,start motor with 1:20 mix to break in.
    run motor for 10 min and now i got 90 psi for both cylinders.
    i'm stumped whats wrong with this rebuilt.
    motor runs but compresion should be higher

  • #2
    Re: johnson 9.9 powerhead repair

    I always retorq the cylinder head bolts after run in. Ring gap seems confusing . I don't think your engine will run too good with .05" ring gap. I would think .020 ring gap would run ok but be the max. Are you sure the cylinder is not egg shaped? I have seen several engines someone honed and put in some rings when the cylinder needed a rebore.


    • #3
      Re: johnson 9.9 powerhead repair

      cylinders are still be used.measurment shows no egg shape.
      i read that ring end gap should be from 0.005"-0.015"
      i have another powerhead but it has lower cylinder worm(90 psi ), upper is 110 psi.
      while taking measurment this lower cylinder is in the same size as this in rebuilt motor.if i hone this powerhead i will got bigger cylinders then in this rebuilt one.
      for me its imposible to det oversize pistons if i would run in rebore


      • #4
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        Re: johnson 9.9 powerhead repair

        from a 1978 manual...0.38 - 0.13 mm..Max. to Min...seems like 0.28 mm would have been good to go?..10 minute breakin time isn't sufficent to seat the rings..check again after a couple of tanks of breakin mix and retorquing the head