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Maching O ring grooves

Piston groove dimensions

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John Vickery01/09/2021 12:17:27
16 forum posts
1 photos


I am looking for help in maching a piston groove to suit 1.062" dia bore (Allchin 1 1/2). I have ordered 3/4 ID x 1" OD x 1/8 silicon rings thinking these will fit tightly on the piston. Are these correct?

If so can you please advise groove width and dia.

Thanks in advance for any help and advice offered.

Regards John

BC Prof01/09/2021 12:44:10
165 forum posts

Try can not find the file at the moment but the Blackgates catalogue .It's a free download from the site The chart gives full spec for O rings , max min width, depth


JasonB01/09/2021 13:02:24
22588 forum posts
2641 photos
1 articles

You really want the OD of the ring to be as close as possible to the cylinder bore so a BS211 ring with OD of 1.074 would be the better bet for a 1 1/16" bore cylinder.

Groove sizes for a 0.139" (1/8" nominal) section ring would be 0.160" wide by 0.132" deep. These are for model use and are less than the commercial published depths. Taken from Model Engineers MHandbook or Reeves Paper catalogue

Edited By JasonB on 01/09/2021 13:02:36

Vic01/09/2021 14:41:01
3060 forum posts
8 photos

Any decent O ring manufacturer will have the data required.


JasonB01/09/2021 15:02:59
22588 forum posts
2641 photos
1 articles

As mentioned in my previous post do be wary of manufacturers data tables as they compress the rings far more which leads to excess friction and therfore you won't get a smooth running engine. The data from Reeves and ME Handbook will suit your needs giving approx 5% ring compression compared to 12-15% typical commercial compression.

Edited By JasonB on 01/09/2021 16:52:16

HOWARDT01/09/2021 16:38:34
901 forum posts
39 photos

Piston diameter should be calculated to give a specified clearance between the bore, depending on ring compression and surface speed according to the o-ring hardness and material. Smaller diameter rings can be stretched to achieve a better running fit and the groove calculated to suit. I have modified o ring designs go from impossible to move go to freely move and still do the job they were intended for. Perhaps get a few extra rings and do some experimentation to find what works in practice. Manufacturers data is a catch all set of figures, when you talk to them about a particular application they will come up with a different set of figures to the basic data.

John Vickery03/09/2021 13:09:49
16 forum posts
1 photos

Thanks to everyone for your replies and assistance, it was all very much appreciated. i now have all the information i need to get this 'sorted'!

Best wishes,


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