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O ring/piston fit

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mick H22/10/2018 09:45:27
701 forum posts
21 photos

I am refurbishing a twin cylinder marine engine with 5/8" pistons fitted with O rings. One of the pistons glides down the bore in an smooth and easy manner. The other piston requires more pressure to push it down. I assume that it is most desirable to have equal resistance in both cylinders for best performance. I thought that I might "measure" the force required to send the "easy" piston down the bore by applying weights. Then I could possibly adjust the O ring groove depth on the second piston until the same amount of weight sends it on its way.

As a master of indecision, I then thought....."perhaps the "easy" piston is too easy and the stiffer piston is correct ?" In other words tighten up the "easy" piston ring.

Just how much force is optimal. Any ideas please.


Neil Wyatt22/10/2018 17:57:58
16738 forum posts
689 photos
76 articles

Have you tried swapping them over?

Boiler Bri22/10/2018 19:37:22
810 forum posts
285 photos

The pistons with no o rings on should move freely. As Little side play as you can get. Then with the orings fitted. Presuming there silicone should has a small amount of resistance. But not enough to have to force them. When the pressure comes on the pushing side by adding steam the o rings will seal more. Even running should ensue.

If you look up EAP on the internet they have a chart that gives you the tolerance for the grooves for o rings


duncan webster22/10/2018 20:22:36
2262 forum posts
32 photos

My current loco will use O ring piston seals, so I've been scouring the web for recommended fits. This looks interesting


it recommends a much lower squeeze than I would use for a static seal, basically 5% on diameter of ring squeeze, and width 30% more. This fits with what other people have told me, but real experience would be re-assuring. I'm hoping someone with real experience will respond

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