O-ring for piston ring

[Master]

Decided to try starting my Jerry Howell Farm Boy. I used the recommended vinyl o-ring in place of a cast iron ring. Almost no compression. Has anyone had success with these o-rings?

 

[Engine maker]

Decided to try starting my Jerry Howell Farm Boy. I used the recommended vinyl o-ring in place of a cast iron ring. Almost no compression. Has anyone had success with these o-rings?

I built Sn # 003 (the Yellow one you see everywhere) and never had a problem. Has hundreds of hour's on it. I've seen some people have problems. First is the bore on spec. at 1.00" and smooth and shinny? Second did you make the piston groove to the spec. Most of the ones I've seen changed the specs to what they think is correct and they make it too tight. The groove needs to be 0.105" wide and 0.110" deep. The ring is NOT squashed in the groove, it kind of floats. Next try replacing the "O" ring with a new one with a little light oil on it. I've seen engines that sat for a few days loose compression. For some reason the ring shrank, (haven't figured that one out yet, but I'm working on it). Last are you sure you have good valve and spark plug sealing?

I have a Hired man engine that called for 2 cast iron rings. I put 2 "O" rings on it instead. I couldn't pull it through the compression stroke. Had to remove one "O" ring and now I can pull it through but it still has too much compression. So I know that the "O" rings work great. I put them on all my Hit and Miss engines now. Even went back and changed cast iron rings out on the engines I had already done. They run much slower and cooler.

Jim G

 

[Jasonb]

As said you can't just put a Viton ring in a groove sized for an iron ring you need to size it to suit the O ring and don't use the generally published data as that squashes it too much, Model Engineers Handbook and Reeves catalogue shave the right sizes for out use.

Last 5 IC engines that I have made all have Viton rings and make good compression, that's 2 stroke and 4 stroke, spark and hot tube igniton also use them in steam engines

 

[Noitoen]

You could try a X ring

 

[popnrattle]

In my engines bore is 1-3/8". My piston is .002-.004 slip. I use a Sunnen hone to get the bore straight after boring on lathe, then get a cross-hatch with the 500-grit stone(J-95). I get 1-3/8" "plumber's" O-ring with 3/32" cross-section(measures .100" at local hdwe store. Then, I make my ring groove .110 wide so the ring can rock slightly at TDC and BDC. I make the groove dia. 1.185( so that there is .005/side pressure). Also, be sure to slightly chamfer where you push piston and ring in and careful not to install such that the ring has to go past the hole for the oiler. The sharp edges of the oiler hole could nick the o-ring. I get a year(+) of abuse out of each ring and replace them then when I take off the head to remove carbon deposits anyway. Hope this helps. Later,rt

.https://www.youtube.com/user/popnstart/videos

 

[dsage]

I presume some of your sideways squish distorts the O-ring and removes some of your clearance in the ring groove? Do you think it can still "rock" at TDC and BDC as you state?
I was concerned that without a bit of side pressure in the groove the rings might "roll up" as it slides in the cylinder.

I always wondered about the required side clearance in the groove. I looked at quite a few O-ring suppliers websites and all of them spec'd quite a bit of clearance in the groove for sliding seals. I guess they know what they're doing. Nice to see your confirmation of that.

 

[popnrattle]

I presume some of your sideways squish distorts the O-ring and removes some of your clearance in the ring groove? Do you think it can still "rock" at TDC and BDC as you state?
I was concerned that without a bit of side pressure in the groove the rings might "roll up" as it slides in the cylinder.

I always wondered about the required side clearance in the groove. I looked at quite a few O-ring suppliers websites and all of them spec'd quite a bit of clearance in the groove for sliding seals. I guess they know what they're doing. Nice to see your confirmation of that.

you are correct in assuming they would start "rolling" as I tried a 1/16" cross-section ring with those dimensions I used with the 3/32"(.100) and the engine never seemed to run right and seemed the O-ring never seemed to "break-in"(new O-rings have a little more resistance). The 1/16" O ring had been twisting in the groove so I abandoned that size O-ring. I'm thinkin' with the pressure against the cylinder and inner piston groove may distort the o-ring and close up some of the side clearance with the groove.

 

[Brian Rupnow]

I use a 1/16" cross section viton o-ring (which is actually 0.070"0. I cut the piston groove 0.094" wide, (which is the width of my cut off tool) and make it 0.058" deep. I have made over 10 i.c. engines this way and never had a problem with compression.---Brian Rupnow

 

[Master]

I may have gotten myself stuck. The cylinder of my Farm Boy is .980. Not exactly 1 inch. Removing the cylinder and making a new piston is not a major job. Just don't want to have to remake a new head too. Making a cast iron piston ring is easy. Did want to try the o-ring system tho.

 

[Jasonb]

That's just about 25mm you could use a 2.4mm section x 20.6mm ID metric Viton 75 ring in that

 

[BobsModels]

Here is another thread on the subject, I posted some clearance information in a PDF.

https://www.homemodelenginemachinist.com/threads/which-o-ring-for-piston-ring.31858/
Bob

 

[dsage]

I use a 1/16" cross section viton o-ring (which is actually 0.070"0. I cut the piston groove 0.094" wide, (which is the width of my cut off tool) and make it 0.058" deep. I have made over 10 i.c. engines this way and never had a problem with compression.---Brian Rupnow

Your dimensions seem to be consistent with those from the calculations provided on the Parker Seals (and other) seal company websites for sliding seals. I would have thought the 20thou or so side clearance would lead to "rolling up" of the ring. But the squish in diameter probably removes some of the side clearance. Your experience confirms the theory. I like that approach. I'm a sort of "try it and see" sort of guy.
Thanks

 

[mrehmus]

Here is a PDF of Tom Stuart's article from Model Engine Builder magazine on using O-Rings. He was very successful with them.

 

[coulsea]

It is hard to go wrong with an O ring, take the head off and put your hand over the end to check compression. the problem is probably valves.

 

[Misterg]

The cylinder of my Farm Boy is .980. Not exactly 1 inch

Just a thought - the smaller bore isn't causing the o-ring to kink as it's fed into the bore, is it? (As @Engine maker pointed out the ring groove clearances on the Farm Boy drawing are relatively large (and don't adhere to any of the recommendations above), so there's the opportunity for the o-ring to buckle away from the cylinder wall if the bore is under-sized for the ring.)

Could it just have machining / running in debris on it that is preventing it sealing?

 

[popnrattle]

Your dimensions seem to be consistent with those from the calculations provided on the Parker Seals (and other) seal company websites for sliding seals. I would have thought the 20thou or so side clearance would lead to "rolling up" of the ring. But the squish in diameter probably removes some of the side clearance. Your experience confirms the theory. I like that approach. I'm a sort of "try it and see" sort of guy.
Thanks

Brian, what was the bore ranges of the engines you have had success with the the 1/16"(.070") O-rings? Thanks. Later, rt.

 

[Vietti]

Brian,

I've been trying o rings for the first time. I was told to fit them 'sloppy"

I was interested when you said you put a .070" ring in a groove only .058" deep, is this correct?

I've been trying quad rings with a few thou clearance in both the width and groove. So far seeing modest compression improvement and less oiling.

John

 

[Peter Twissell]

The general rule for o rings is to target 20% compression for static applications (where there is no motion against the o ring) and 10% compression for dynamic applications.
A 0.070" o ring in a 0.058" groove gives about 15% compression. Add a bit of clearance between piston and bore and the compression is probably closer to 10%.

 

[TSutrina]

The general rule for o rings is to target 20% compression for static applications (where there is no motion against the o ring) and 10% compression for dynamic applications.
A 0.070" o ring in a 0.058" groove gives about 15% compression. Add a bit of clearance between piston and bore and the compression is probably closer to 10%.

The manufactures of o-ring have tables Parker is the best guide that can be down loaded. Other companies have better information about calculating the dimensions and can be down loaded. Didn't find a calculator the last time I looked.

 

[Vietti]

Been trying O rings, quad rings really. Having some trouble getting reliable compression.

Seems there is a difference of opinion on how much pressure the o-ring should exert on the cylinder wall.

Tom Stewart calls for .005" clearance between the back wall of the piston and the ring. This is about what others, having success have told me.

Others (Pete Twissell) call for 15% compression of the o-ring. Can both be correct or is there an ideal?

I'm now going to try a regular o-ring as I suspect the quad rings are twisting in the oversize groove and loosing compression.

What a great hobby!!!

John in Wyo