O Ring for piston rings

[Gordon]

What are the specifications for using O rings for piston rings? In the past I have seen specifications for sizing the width and depth of the ring groove. If I remember correctly the groove was actually wider and deeper than the actual O ring and depended on compression to force the O ring against the cylinder wall and against the side of the groove.

 

[Brian Rupnow]

Use a Viton o-ring with an o.d. equal to the cylinder bore, with a nominal cross section of 1/16". The slot in your piston should be 0.058" deep x 0.93" wide. You only need one ring per piston. Run a little oil with your gasoline to keep the ring lubricated. Even though these o-rings are sold as nominal 1/16" cross section, they measure about 0.070" in reality. Just use them as they are.---Brian

 

[Engine maker]

I've built a lot of engines with "O" rings for piston rings. Any where from 5/8" to over 2" and use the same proportions. On a 3/32"(.093") "O" ring I make the groove .105" wide and .110" deep. For a 2" "O" ring I use .210" wide and .220" deep. I also reduce the piston diameter in front of the ring (the head end) by .003" per inch of piston diameter i.e. for a 1" bore, .998" for the piston, and piston head .995". You don't need to press it against the wall. A 1" x3/32" "O" ring is manufactured at 1.005" od and .103 thick, so no need to make it under size to try and "pinch or squeeze) it.

The trick with using an "O" ring is not to pinch it, it has to float in the groove I've attaches some info on "O" sizes, what the size is and what the size they are manufactured as. I also use just standard hardware store "O" rings (the Buna brand) on my hit and miss engines with no problems. I have one engine that runs about 100 hr a year at several different shows and it has the original "O" ring from 2009 when it was built. Like Brian said you only need 1 ring. I built an Associated and put the 3 ring grooves on the piston as shown, just to be accurate to the plans. I put 2 "O" rings on the piston and couldn't pull it through a compression stroke! Had to take one out, and still can just get it through compression.

Remember, you have to do 3 things to make it work.
1 The bore has to be SMOOTH.
2 You have to keep it lubricated
3 You have to keep it cool

Jim G

 

[Jasonb]

I tend to go for a groove depth that is 5% less than the O ring actual section and 120% wider. This gives very little drag but a good seal.

So for a 3/32" nominal ring which is actually 0.103" cross section I'd be 0.097-0.098 deep and 0.125" wide

Works OK for me on spark and hot tube ignition, Petrol or propane firing 2 & 4 stroke as well as steam and air engines

 

[Engine maker]

I've also done some unofficial test on the different "O" ring types Vitron, Silicon and Buna. I used the same hit and miss engine for all the tests. Put new "O" ring on each time and let the engine run for 20 minutes to warm up each ring. This is what I found

Silicon - engine ran the hottest and coasted the shortest i.e. shortest coasting means more heat. I could get about 40 revolutions between hits.

Vitron - temperature was reduced and revolutions between hits increased to about 56.

Buna - ran the coolest and got about 70 revolutions between hits.

This is just what I found.

Let the arguments Begin!

Jim G

 

[Gordon]

OK Thanks. That is the information that I was looking for. I will try again.

Interesting information from Jim G on the different materials.

 

[Vietti]

I've played with O rings too. I agree with all of Engine Makers comments except my Buna rings swelled and became too tight, no problem with Viton. Why do you think Buna rings give a longer coast? I'd like that.

I think the space behind the ring allows the compression pressure to easily enter, push out the ring and get a good seal. Also allows the ring to recede after firing and makes for a longer coast. I also make the piston a little smaller in dia. above the ring.

I've often wondered about two O rings per piston. Have heard it locks up because of pressure trapped between the rings. However, the plans with my Economy casting set call for two O rings per piston, he claims the lower ring keeps the bore clean for the upper to seal. Also pictures of the disassembled Chinese engines on some youtube videos show 2 O rings. I suppose one could drill a small hole between the rings to release the pressure?

Let the purists with CI rings begin to howl!

 

[Engine maker]

The only time I saw the Buma "O" ring swell was when the person that was running it was running it on gasoline. Coleman fuel doesn't seem to attack the Buna.

Jim G

 

[Vietti]

Jim,

You are correct-I run on gasoline!!

 

[mrehmus]

Tom Stuart who used O-Rings on his big hit and miss engines ran the first set for 7 years before he had to replace them. This is the drawing from his article in Model Engine Builder magazine

 

[Gabe J DiMarino]

Your using rubber O- rings for piston rings in internal combustion engines running gasoline as fuel ? What keeps the O-ring from melting ?




















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[Stefan-K]

First of All, short runtimes! And enough oil in the fuel mixture.

 

[Engine maker]

I don't know about short run times, my engines at the shows run between 8 and 9 hours straight, only stopping to refuel, and then start right up again. Although there is a lot of coasting going on.

JimG

 

[mrehmus]

I don't know about short run times, my engines at the shows run between 8 and 9 hours straight, only stopping to refuel, and then start right up again. Although there is a lot of coasting going on.

JimG

Tom Stuart ran his engine quite a bit at the shows. It has been too long to remember if he ran his continuously.

 

[Gordon]

I am not having much success with the O rings. I have remade the pistons with the proper groove and they give better compression for a little while but the life of the O rings is short. I have honed and lapped the cylinders and I have tried several sets of O rings but when I try to start the engine with my drill motor it soon loses compression within about five minutes. I can put compressed air in the cylinder at 30 PSI and there are no leaks even past the worn O ring. I originally had a problem with sealing the head but I seem to have corrected that problem. I may have to go to a CI ring. I am not sure why I have had such rotten luck with engines lately.

 

[redryder]

I am not having much success with the O rings. I have remade the pistons with the proper groove and they give better compression for a little while but the life of the O rings is short. I have honed and lapped the cylinders and I have tried several sets of O rings but when I try to start the engine with my drill motor it soon loses compression within about five minutes. I can put compressed air in the cylinder at 30 PSI and there are no leaks even past the worn O ring. I originally had a problem with sealing the head but I seem to have corrected that problem. I may have to go to a CI ring. I am not sure why I have had such rotten luck with engines lately.

.
Be sure to mix oil with your fuel. I like to use white gas or yard tool fuel. Never automotive fuel. I mix it 20 :1 with Marvel Mystery Oil. 25/1 is OK too. 50/1 may be too light.

If it is a hit and miss engine not pulling a load, it will remain plenty cool.

Another thought re extreme early wear.... You mentioned honing. Honing leaves a rough surface by design which is intended to help seating cast iron rings as they are worn in.

I have not done this myself but for O-rings, you might try a much more polished bore than used for iron rings. I have several model engines that have highly polished cylinder bores and a good Viton O-ring can last years. One O-ring is usually enough in these applications. 2 well sealed O-rings in your piston can be problematic because once the oil between the O-rings dries up, both O-rings will begin to drag, heat up and self-destruct. (unless you have a way to deliver oil between them)

I hope this is helpful.

Gil

 

[Lloyd-ss]

O-rings can be very forgiving and remember that gland dimensions are only suggestions, especially in dynamic operation. The 005, 105, etc numbering system give nominal dimensions. Metric are in actual dimensions. Always work with actual dimensions, no matter what the system. Try and size the gland such that the ring stretches on the groove and barely touches the cyl wall. If it presses too hard on the cyl wall, the sliding will wear it out. Depending on the size range you are working in, you could try using an o-rings that is one size small to take some pressure off the o.d. That way, the squish is on the static part at the bottom of the gland.
If you have trouble finding the right size, look at the metric sizes also.
I am also wondering if anyone has tried using PTFE or polyurethane o-rings, or back-up rings? I have had good luck with PTFE backup rings on both sides of Vinton or poly o-rings with only a few percent of compression on the o-rings.
Lloyd
Gordon, once you find the right combo, it Will work.

 

[kf2qd]

Not working the engines very hard so there is not much heat.

 

[Brian Rupnow]

Take a bottle of your wifes liquid dishwashing detergent and squirt it all around the cylinder head. Turn your engine over with the drill motor. Any air leaks anywhere will blow bubbles. This is a bit messy, but I have surprised myself a few times when one of my engines had poor compression and I couldn't figure out why.---Brian