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Replacing Johnson Outboard Piston?

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  • Replacing Johnson Outboard Piston?

    Sorry I haven't posted here in awhile, guys, I have literally been on my boat on the water any spare time I've had...

    Which was great...

    Until a week ago, when I got stuck between Racine and Kenosha, and could only get my boat to idle.

    I took it back to the marina, and did a compression test, my top cylinder has 30PSI, and the bottom two each have 120PSI.

    I took the head off to see what was going on, and a reed got into the top cylinder, and put a hole in my piston.

    The cylinder walls have no scoring...

    I have no experience with 2 cycle motors, how difficult is it for me to replace this piston, and what are the steps?

    Also, do I need to pressure test this motor now that the head has been taken off?

    And do you have any more tips?
    Happiness is directly proportionate to the hour meter.

  • #2
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    Re: Replacing Johnson Outboard Piston?

    You should post this over in the Johnson/Evinrude forums.

    I've been able to replace a piston in an older JohnRude 3-cyl without tearing the powerhead off, you have to pull the reed boxes anyway; if you can get at the rod bolts, you can remove the piston.

    Otherwise you're gonna have to pull the powerhead and split the crankcases. You'll need a flywheel puller (an automotive harmonic balancer puller will do). Use Grade 8 puller bolts or you'll break them.

    Note that OMC replacement pistions require special handling to press off/on the rods, if you do it wrong the piston will break. You'd be better off to have an outboard shop remove the old piston and install the new one on the connecting rod.

    Be sure to use new conn rod & wrist pin bearings.

    You'll also need to install a new head gasket and torque it properly. The conn rod bolts also require the correct torque (and a drop of red Loctite on each rod bolt).

    Check all your reed valves, failure of one set might mean the others aren't far behind.

    The trickiest part is keeping the conn rod bearings from falling out when reinstalling the piston. Coat the rod and bearings with Vaseline and this'll help them stay in place while you're handling them.

    This is just an overview; you'll need to pick up a service manual for all the fine details.

    HTH & G'luck..........ed