using o-ring as a piston ring

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Tom Jamboretz

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Does anyone have any information on dimensions for ring groove depth and width while using an o-ring for a piston ring. I've done it before but it was a hit and miss operation. I thought I had seen a formula but cannot find it now. All help would be appreciated. TJ
 

Brian Rupnow

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Use an o-ring made from Viton, which is a heat resistant compound. For pistons less than 1.25" diameter, use an o-ring with a nominal 1/16" cross section (they actually measure .070"). Make the groove in the piston 0.093" wide x 0.057" deep, and use only one ring per piston. I have 8 home built model gasoline engines running these rings.----Brian
 

Tom Jamboretz

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Use an o-ring made from Viton, which is a heat resistant compound. For pistons less than 1.25" diameter, use an o-ring with a nominal 1/16" cross section (they actually measure .070"). Make the groove in the piston 0.093" wide x 0.057" deep, and use only one ring per piston. I have 8 home built model gasoline engines running these rings.----Brian
I appreciate the great replies. All good information. As the piston I was working with already had a ring groove cut in it. It was over 0.093" I went to the next cross section o-ring (#117)and groove. The charts called for .140"groove width. As I am using the larger o- ring six I do believe I have in creased the friction substantially. I may need to make a new piston. Also need to find the Viton ring.
 

BobsModels

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Tom

Just a slight variation on what Brian suggests. Vitron O ring 1/16 by all means, you can get them at McMaster, you need to buy them by the 100. Send me a PM I might have what you need.

I tried an O ring for the first time on a 1/4 scale Gade. I had the opportunity to actually take apart two Gade's that use O rings built by two different people (both of whom made many of them for sale). What I found was an interesting consistency of dimensions. About six months ago I pulled apart a York with O ring and found the same dimension to within .002 of the my original analysis.

The two Gade's had a wall pressure or compression of the O-Ring of .0045, and .0048. The York was .0062. Let me define wall pressure as I use it. It is the total amount of compression the O-Ring is under when squeezed by the wall of the cylinder and the groove in the piston. It will likely be different for every cylinder you make.

First I bore and hone the cylinder, measure it and you now have the cylinder ID. I can now calculate the OD of the groove. Assuming a 1/16 O ring sized for your basic piston ID, I would recommend the 1/16 for low friction, the equation is (using a .875 nominal O-Ring):

OD of the groove = Cylinder ID - ((.070 *2) - .005))

So for my Gade the Cylinder Id was .8743:

.8743 - ((.070 * 2) - .005) = .7393

Wall compression = .8743 - .7393 - .140 = -.005 (ie compressed)

I also lap the piston so when the cylinder is blocked the piston stays and when you remove the block it drops through ( assumes cylinder and piston cast iron).

So in my scenario the groove is independent of Piston Diameter but dependent on Cylinder ID.

One other Dimension consistency, all the groove widths were .080 to .084. One of the builders of the Gade's mentioned to me, as well as someone at NAMES that you did not want the O-Ring to roll as in a normal O - Ring application. You just wanted enough room for it to expand when crushed and a little more for expansion when the fuel ignited. Again just some addition information I gathered. Based on Brian's experience with a .093 width roll is not really an issue.

As I said my approach is just a variation on Brians approach. My Gade and the other two run at shows all day, stopping for fuel only.

Anyone else out there have any other variation's, nice thread to get experiences together in one place with some actual dimension information, which is what I had the hardest time finding.

Good Luck

Bob
 
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abby

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I use Viton rings in my steam loco's , actually it is not recommended but I have never had any problems.
Used as piston rings on 1/2" pistons I measure the bore then cut the piston groove to give just 2-3 thou compression on a 1/16" ring , I use 0.064" width for the groove.
I lap the bore first and sometimes lap again until the piston and ring "feel" right.
Using the published data will often give too much friction , leading to stick-slip . The data is usually for very high pressure hydraulic systems.
You might also check out the quadra-lip version of O rings.
 
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Herbiev

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Those quad- lip seals look interesting. Thanks Abby. Never knew such things existed
 

steve-de24

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A quick question about using O-rings on pistons if I may. Do you need to polish the cylinder as well as lapping it?
Thanks in advance, Steve
 

kvom

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For air, PTFE might be a good choice. Viton for steam.

I used viton rings as for piston/valve/throttle seals on my loco.
 

vascon2196

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I used whatever cut-off tool I had, made a groove, and stuffed a regular Buna-N O-ring in the groove until it fit nicely between the cylinder and piston. I've been using the same rubber o-ring for a few years now in my Ford plumbing engine and it still runs strong.

I suppose a Viton O-ring would last longer but if its not broke don't fix it. My engine runs great so I'm not going to mess with it until it stops working.

The Parker O-ring catalog always worked for me when I was in industry if you need hard numbers. McMaster has a great O-ring selection plus a lot of great information about groove sizes.

http://www.mcmaster.com/
 

Tom Jamboretz

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I have started making a new piston so I get can get the groove for a 1/16" o-ring correct. I to would like to know what kind of surface I need in the cylinder and how to achieve it. Is it as simple as a wood dowel wrapped with crocus cloth? All help is appreciated. TJ
 

vascon2196

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TJ

I bought a Flex-Hone from McMaster Carr...it worked great. I used a little ultra fine grit lapping compound when I did it.
 

vederstein

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This is from an earlier thread:

You may not have ever realized this, but typically a 10% o-ring compression is what's used for dynamic o-ring sealing. That's why a 1/8" thick o-ring isn't an 1/8" thick. The depth of the seal gland. Look at the following chart:

Seal Nominal Actual Gland Percent
Series Thickness Thickness Depth compression

000 1/16" .070" .063 90%
100 3/32" .103" .093 90%
200 1/8" .139" .125 90%
300 3/16" .210" .188 94%
400 1/4" .270" .250 93%

Ok, the larger thickness seals isn't as exact, but pretty cool though.
 

Engine maker

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As for O-Ring groove size, Jerry Howell uses a 1' OD x .094" Dia. "O" ring in a .110" deep x .105" wide piston groove on the Farm Boy engine. The piston OD for the 1.00" bore is .995" ahead of the ring and .998" behind the ring (for the piston skirt.) It seems like as the engine runs the "O" ring swells an makes even a better seal. These sizes seen to work fine with very minimal to nil drag. I use these same groove size on all my piston that use 3/32 O rings from 3/4 inch to 2 inch pistons and all have worked perfect.
 

sunworksco

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I have used Viton seals in full size thermal solar systems, for many years, with excellent results...
 
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