Viton O-Ring as piston rings

Many folks have successfully built and run engines using Viton O-rings as piston rings.
My question is:
How much deformation (squeeze) between the bore and the bottom of the piston groove should be applied to have a good seal with the minimum friction?

With reference to
http://www.applerubber.com/oring-gland-calculator/

The compression parameter is given to be between 10% and 30%

What have you found a good compression figure for you engine?

I honestly would make a try bar like a jeweler uses to fit "rings" and push it through the cylinder till it feels right.

On a 1/16" cross section Viton o-ring (which is actually 0.070") make the groove in the piston 0.058" deep and 0.094" wide. I have a dozen engines working using that formula.---Brian

Brian Rupnow said:

On a 1/16" cross section Viton o-ring (which is actually 0.070") make the groove in the piston 0.058" deep and 0.094" wide. I have a dozen engines working using that formula.---Brian

Click to expand...

Thanks that is the info I was looking for and it can be translated to different bores.
Assuming the piston is a few thousand less than the bore the squeeze factor is about 14% which agrees with the Applerubber online calculator.

Seems the friction should be quite high but can not argue with success.

The size of a numbered oring is not a "real" size. It's actually the depth of the oring groove for approximately 10% compression:

A 000 series oring has a width of .070" and a groove depth of .060"
A 100 series oring has a width of .103" and a groove depth of .094"
A 200 series oring has a width of .139" and a groove depth of .125"
A 300 series oring has a width of .203" and groove depth of .188"
I forget the width of the 400 series oring but it has a groove depth of .250"

...Ved.

Hi

Go over to this thread, and scroll down to my reply. There is a PDF which I think could help you out. The Gade I build which is referenced in the post now has over 700 hours on its original Vitron O-Ring. Given it has fuel and a good battery it starts on the first try hot, second or third try cold. From Aug 30 to Sept 9th it ran 5-7 hours a day ( a threshing show and a county fair) except for a travel, setup day, and rain . I did not use the standard tables. It has great compression and bounces nicely.

https://www.homemodelenginemachinist.com/threads/how-to-determine-piston-0-ring-size.27084/

Bob

Here is the full text of an article I wrote with Tom Stuart about using O-rings in engines in Issue # 15 of Model Engine Builder magazine:

I have been using O-Rings as piston rings in my Hit and Miss engines for the past 8 years with few problems. An interesting feature of O-Ring use is the very low friction when the engine is coasting and yet the immediate sealing of the O-Ring to the cylinder wall during a firing cycle. O-Rings do a good job on slow–speed engines. They will probably not survive in a high–speed engine and would quickly convert themselves into a melted plastic ring.
You can modify your engine to use O-Rings with minimal effort (new piston and polishing the cylinder walls) and will probably see an immediate improvement in performance. The improvement I mention is more revolutions between a 'hit' and very little exhaust smoke. Of course O-Rings are easier to 'make' than traditional cast iron rings and you only have to use one.
I've had good success as long as I followed a few simple rules about their application.
Unlike 'normal' O-Ring applications, the dimensions of the piston ring groove are such that the O-Ring is not under compression in the static state and does not effect a aero-pressure seal. It takes pressure above the ring to cause it to press against the sides of the cylinder and the bottom of the ring groove.
For the example shown on the Drawing Page 15, a typical 1.5" bore cylinder in this hit and miss engine example will have 0.002" clearance between the piston and the cylinder. The O-Ring with a 1.5" OD and a 5/32" circular cross-section is made of Buna-N material. The piston ring groove is cut to provide 0.005" clearance top and bottom and 0.005" clearance on the back wall of the groove.
If we need a math formula for the groove it is:
Groove inner diameter = Cylinder I.D. – ((2 x O-Ring circular cross section) + 0.010).
With the values we have, that would be 1.5–((2x0.15625)+0.010) or 1.5-((0.3125) + 0.010) or 1.5" –0.3225" = 1.1775".
Cylinder wall finish should be as smooth as you can make it. Somewhere between 2 (mirror) and 32 (smooth shaft) micro inches is a good value. Some people actually recycle old hydraulic cylinders since they have chrome-plated inner walls. That isn't necessary for this application but a good smooth surface will help the life of the O-Ring.
During use, you want to have a light coat of motor oil on the cylinder walls. During a full day of running, I oil the engine about every hour.
Editor's Note:
Because we cannot adequately show surface finish in print, I use a BAR S-22 Microfinish Comparator which is a calibrated assortment of different finishes all cast into a single rectangle about the size of a candy bar. The finishes range from mirror to very rough (in machining terms). In use, you visually compare your surface finish to the samples and select the closest finish on the Comparator.
These are available from machine tool stores.

I should mention that Tom ran his large hit n miss for 7 years with the same O-Ring set up to these specifications.

Here is the drawing for O-Rings for Pistons:

tornitore45 said:

Many folks have successfully built and run engines using Viton O-rings as piston rings.
My question is:
How much deformation (squeeze) between the bore and the bottom of the piston groove should be applied to have a good seal with the minimum friction?

With reference to
http://www.applerubber.com/oring-gland-calculator/

The compression parameter is given to be between 10% and 30%

What have you found a good compression figure for you engine?

Click to expand...

I have unsuccessfully used O'Rings for small model engines a number of times, the friction is far too great, even with minimal contact. I successfully have used both graphite and bronze as pistons with no rings, just a smooth fit. I machine at least two circumferential V-grooves in the piston sides that makes turbulence of any by-passing air or gas. At 100RPM or higher, the effects of by-passing are far, far less than the friction that an O'Ring would provide.

We have multiple approach's that work equally well. When I was building my Gade in 2013 I saw the above article and planned on using those calculations with a different cross section O-ring (1/16 - actual .070 ). I then had the opportunity to take apart two Gade's that were running with O-rings made by the same person. I discovered a difference in approach. Tom's method leaves clearance ( as shown above) at the back wall of .005 and the Gade's I had provided about .0025 compression. The only reason I did not use Tom's method was I had two Gade's that I had seen running constantly at show's for about 3 or 4 years each. The three of us attended many show's together and my best guess was they ran about 70 -80 hours a year. I just decided to use the second approach with the result very good.

I noticed two other differences from Tom's approach. My Gade has compression just by slowly turning the flywheel, no fast turnover. The second is I do not oil the piston other than the oil mix I use , 5oz of either WD40 or Marvel Mystery oil per gallon of white gas (Coleman Camp Fuel).

We have a good collection of approach's in this thread anyone else have different calculators and some results, it would be good to collect them all in on place.

I need to make a correction to my first post, my Gade only has about 550 hours as of this year, the 700 mentioned was for my 1/8 scale Galloway.

Bob

Hi
I made short video to show the small friction level that can be attained with a Vitron O-Ring. The first few seconds shows the bounce. The last few seconds shows the flywheel moving with just the counter weight.

Bob

A better option for piston rings is to use Quad rings also called X rings they are much better suited for the job and are available in viton they have been in my engine 8 years with no problems