Oversizing piston advise

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Oversizing piston advise

Postby Stuartmorrison » Wed Jun 20, 2018 9:27 pm

Good evening, quick question to any of the engine guys, I have a Johnson 40 needing an oversized piston as one of 2 is damaged. is it normal practice to oversize both pistons whilst in there or keep the undamaged to standard size having oversizing as a backup incase it fails.

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Re: Oversizing piston advise

Postby Alacrity » Wed Jun 20, 2018 10:21 pm

I can't speak for OMC motors but I know Mercury say just rebore the damaged bore only.
Mercs are like women, no 2 are exactly alike. That's what testing is about. In general it is safer to test motors and props than women!
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Re: Oversizing piston advise

Postby floater » Wed Jun 20, 2018 10:50 pm

Whenever I've had car or bike engines rebored it has always been all cylinders. I imagine in a high performance motor (none of mine) the difference in mass between an oversized piston and a stock one could create an imbalance. Racers often balance con rods, pistons etc.
I own a couple of Jonnyrude 40s and mean no disrespect when I say you will probably be ok reboring just the one damaged cylinder.
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Re: Oversizing piston advise

Postby Fastjeff » Thu Aug 30, 2018 11:21 pm

It's my understanding that over sized pistons weigh the same as stock sizes.

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