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Piston fit with o rings

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astroud:
James Walker design notes for a 0.125 cross section O ring suggest a groove diameter of cylinder bore minus  2 x 0.110 max   minus 2 x 0.114 min

Jasonb:
Mike, an 1/8" nominal O ring as the OP says he has (BS210) actually has a Cross section of 0.139" so not too deep to give a little compression of the ring. These are the figures I use on both my IC and steam engines with Viton rings and are taken from Tubal Cain's "Model Engineers Handbook" and based on tests done by Arnold Throp. They can also be found in a Reeves paper catalogue

If it's good enough for those two then it's good enough for me :ThumbsUp:

These figures compress the o ring far less than what you will see in the manufacturer's data sheets but we are not trying to hold back several hundread PSI with the resulting high friction that amount of ring compression produces.

Like Crhis I typically use a smaller section ring for my steam engines around the 1" bore. For the many 24mm bore ones I go with metric 2.4mm actual cross section rings  I use a 2.3mm deep groove and those that have built to my published drawings have also found that works fine on both <5psi air for display as well as running on steam.

Jasonb:
Go and look up any chart for BS imperial O rings, the nominal sizes are all smaller than the actual cross section of the cord and have always been that way.

Jasonb:
From ME Handbook

Vixen:

--- Quote from: Jasonb on March 19, 2025, 06:25:17 PM ---Go and look up any chart for BS imperial O rings, the nominal sizes are all smaller than the actual cross section of the cord and have always been that way.



--- End quote ---

Thank you for the explanation Jason. I was not familiar with BS imperial O rings nominal sizes being smaller than the actual cross section of the cord. Something learned today.

Mike

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