O-ring groove depth--Viton ring---another look.

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Brian Rupnow

Design Engineer
Project of the Month Winner
May 23, 2008
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Barrie, Ontario, Canada
I have built a number of 1" bore internal combustion engines, using Viton 0-rings. The o-rings I use are 1" o.d. x 0.070" cross section, and I use one ring per cylinder. I use a 0.093" parting off tool and cut the groove 0.056" to 0.058" deep. My reasoning was, that if the o-ring is 0.070" cross section and the o-ring groove is 0.058" deep, then that gives a radial "squeeze" of 0.012". My engines run quite well with these numbers, but my hit and miss engines never have very long "miss" cycles. I have recently been following videos posted by Andrew Whale from the United Kingdom, and on his build of the "Hoglet" engine which has a 1" bore, he makes his ring grooves 0.080" deep---and they work fine. This sounds wrong to me, that the depth of the ring groove is 0.010" deeper than the cross section of the actual 0.070" o-ring. However, he gets good compression with his 0.080" deep ring grooves and his engines seem to run fine when completed. Now, if Andrew is right and I am wrong, that could explain why my hit and miss engines have such short "miss" intervals.---I have too much friction between the piston and the cylinder the way I am doing it. I have an extra 1" bore cylinder here, and a 1" piston to go with it. I just set the piston up in my lathe and took the ring groove down to a depth of 0.080", and then put a 1" viton ring in the deepened groove. I assembled it with the cylinder, and yes, it seems to have lots of compression!!! I am shocked!!! I guess the next test for me will be to take one of my existing hit and miss engines, remove the piston and deepen the ring groove to 0.080", then reassemble the engine and run it to see if my "miss" cycles become longer.---Who Knew!!!

I've said it here and on other forums before most published groove sizes are for high pressure applications. For steam and IC engines about 5% compression of the ring is about right so for your 0.070" ring I would have a 0.065" x 0.090" groove

There are a couple of published charts that give the groove sizes for model use such as Reeves paper catalogues or "Model Engineers Handbook"
This is my engine running, and I have modified the piston to have a 0.080" deep x 0.093" wide ring groove for a 1" outside diameter by 0.070" cross section Viton o-ring. It runs like a champ.--Read the write up posted with the video for more information.---Brian Rupnow
And here we have the engine going in and out of hit and miss mode with the modified piston. I would never have believed that an engine would run with such a deep ring groove for an 0.070" cross section Viton o-ring.
I think the theory is that the gasses get behind the ring and push it out to make contact with the cylinder

The hoglet drawings give the depth of groove as 0.070"
Since I have two or three pistons that will fit this engine, I hunted around for one which was a slightly tighter fit into the cylinder and put in an 0.070" deep ring groove. This piston was also cast iron as opposed to yesterdays being made from aluminum. The engine seems to run a bit better with the 0.070" deep ring groove. It is still running a bit faster than I would like, so I can add a bit of weight to my governor balls, or try a weaker spring which holds the governor arms together. Either of these will result in the governor kicking in at a lower rpm, and consequently slowing the engine down.---Brian

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