Silicon piston ring

[Mike BrettParticipant @mikebrett89695]

What size to choose

[Mike BrettParticipant @mikebrett89695]

Hi all

I have a cylinder bore of 25.90000 mm. The only silicon steam piston ring I can find on the market near to this size is 25.4000 or 26.9875. What would be best , to buy the smaller one and expand it over the piston or try and squash the larger one in.

Cheers Mike

[JasonBModerator @jasonb]

I have a few 24mm bore engines that I run with 2.4mm section Vitron rings @19.6 ID which gives an OD of 24.4mm so would go with the larger of your two options

[Alain FooteParticipant @alainfoote90915]

Silicone not silicon surely!

[Nigel BennettParticipant @nigelbennett69913]

With an O ring, the important thing to achieve sealing is for the radial gap between the bottom of the piston groove and the cylinder wall to be a little smaller than the O ring section. How much "nip" you decide upon for your O ring is probably best determined by reference to the manufacturer's data sheets. I'd say about 0,13mm or 0,005" would probably be OK.

If you fit a small O ring and stretch it over a groove, then the cross-section of the O ring will reduce slightly; per contra, squashing an oversize O ring into a bore means that the material has to go somewhere, and your "nip" figure may need to be adjusted slightly to cater for either case.

You do need superb surface finish on your bore to accommodate O rings; otherwise you'll end up wearing them out very quickly!

Personally I'd use metallic piston rings – preferably cast iron – but that depends on your cylinder material.

(And yes, Alain's right – Silicone with an E. And Viton not Vitron…)

Edited By Nigel Bennett on 26/10/2019 15:22:31

Edited By Nigel Bennett on 26/10/2019 15:22:45

[Tim StevensParticipant @timstevens64731]

Please – how do you measure your bore so accurately? 25.90000mm indeed?

And it might help to know that internal conbustion engine rings are expanded by the pressure which builds up behind them – with the ring pressed firmly against the bottom of the groove. But these rings are proper Ramsbottom's Metallic Packing, invented by a steam engineer for steam engines.

Cheers, Tim

[Clive Brown 1Participant @clivebrown1]

A few sellers on Ebay offer silicone O-rings of 26mm OD. 3mm & 4mm cross-section. Either red or translucent food grade.

[Mike BrettParticipant @mikebrett89695]

Hi

Actually the bore measured 25.975 but I thought 25.9 would be accurate enough for a rubber ring. Cylinder material is brass and the finish is very good. Its not an engine as such but a water pump, so maybe I could get by with a normal large cross section O-ring. There is no great pressure involved , I just don't want any water leaks.

Mike

[duncan webster 1Participant @duncanwebster1]

if it's cold water then you don't need silicone, nitrile will do and is more easily acquired. For a dynamic application, especially if a completely tight seal is not needed, then 5% squeeze is enough. Any tolerance to increase this

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