Thick vs. Thin O-Rings

[rick41]

Hi all,  I am building a Thompson internal combustion engine from castings by Bob Herder.  It has a cylinder diameter of 1.00 inch and a 1.00 inch stroke.  Plans show a 1/16 inch piston groove.  I have decided to use o-rings in lieu of regular piston rings since I have had good luck with o-rings on several hit and miss engines.  Question?  Is there any advantage in using a thicker o-ring rather than a thin o-ring or visa versa?  My gut tells me to go with the 1/16 diameter with .004 inch compression on the o-ring.

[Jasonb]

I've done several IC engines with a 24mm bore so not far off your 1" and use 2.4mm actual section rings which seem to work well, for 1.5" I would be using 1/8" nominal section. I'll be using the 2.4 section metric Viton ring in mt version of this engine.

How is your's going ? any photos?

[crueby]

I've always had better luck with the 1/16th cross section rings, have tried thinner ones and found they were more prone to tearing and did not last as long. The really thin ones are only okay for sealing. My engines are all steam, no IC, don't know if that makes a difference. You did not mention material, I like the Viton (FKM) ones, hold up the best.

[Vixen]

Hi Rick,

I would go for the 1/16th section O rings in Viton. You can also get square section (four leaf clover) quad rings in most sizes. I have obtained both types for my Westbury Seal engine and intend to do back to back comparisons. I will start with the standard Viton O ring as that is the one most people use.  A 0.004" compression sounds about right, you could even reduce it slightly.

Mike

[crueby]

Hi Rick,

I would go for the 1/16th section O rings in Viton. You can also get square section (four leaf clover) quad rings in most sizes. I have obtained both types for my Westbury Seal engine and intend to do back to back comparisons. I will start with the standard Viton O ring as that is the one most people use.  A 0.004" compression sounds about right, you could even reduce it slightly.

Mike

Mike, that sounds like an interesting comparison - I was not aware of the quad rings.

[rick41]

Thanks for the responses.  Will try to attach a couple photos.  I will go with the 1/16 diameter.  I am thinking the aluminum piston should be several thousandths less than the cast iron sleeve.  Thoughts?

[Vixen]

Hi Rick,

I would try for no more than 0.002" difference between piston diameter and cylinder bore.

BTW   Brian Rupnow is a firm advocate for the use O seals in place of traditional cast iron piston rings in small engines. There was a good write up and discussion in Doubletop's "30cc Flat four (Puma)" blog, Starting at Reply #174 onward. http://www.modelenginemaker.com/index.php/topic,9156.msg207470.html#msg207470.

Mike

[steamer]

Hi Rick,

I would go for the 1/16th section O rings in Viton. You can also get square section (four leaf clover) quad rings in most sizes. I have obtained both types for my Westbury Seal engine and intend to do back to back comparisons. I will start with the standard Viton O ring as that is the one most people use.  A 0.004" compression sounds about right, you could even reduce it slightly.

Mike

.004 with Viton is going to be a bit stiff...probably need to be less than that....
Dave

[derekwarner]

Irrespective of the crush......[eg., 0.004"] any nominated value will be a % on the diameter of the o-ring

Accordingly a 0.004" crush on a 1/16" diameter section is a larger % on the diameter of the ring, than the % of 0.004"crush  on a 1/8" diameter o-ring

Any surface damage [nicking/tearing] is in all practical terms equal between both diameters, however again the same nicking will be a higher % on diameter to the smaller section ring

The same scenario applies to tolerancing of 0-ring cavities [in dynamic application].......tolerancing is a % on section, so the physical tolerance dimension is lower as a % of section

So for 1 x application builds that are not using CNC proven element trials, you are more likely to get a practial better seaing effect if you use a larger diameter section

I could not recomend any rubber elastomer in any format as piston sealing elements in any form of an internal combustion engine. The same applies to the newer cast plastic material

[The above is based after a lifetime of employment and experience in hydraulic engineering from Metals, Marine, Naval to 18 years as a Professional [self employed independent] hydraulic facilitator sponsored [until retirement] by our Australian Steel Industry]

Derek

[steamer]

I understand Derek...also 34 years designing machines to do things they're not supposed to do...like minimum friction without leaking...and most of the oring bibles don't go there...usually if you used the recommend crush they are WAY to tight for "mortal" applications....compressed air at 100 psi or so....so you reduce the squeeze ...sometimes significantly....to get what you want....it's usually a far cry from the 2500 psi hydraulic systems...and include some wierd attributes that the Oring makers don't get into....like crush under pressure messing with your volumetric efficiency...ect.   Its more black art  than first principles I'm afraid.




Dave

[Jasonb]

For the 2.4mm section metric rings that I mentioned I use a 2.3mm deep groove so 0.1mm compression of the ring which is 0.004" and a width of 2.6mm. My engines will do a few turns with a flick of the flywheel so in no way are they tight yet I get very good bounce off the compression.

On a Piston of your size on a slow running engine I would be at a thou or less clearance, I don't actually measure it just go by feel.

We have a couple of sources of ring (groove) sizing available over here that gives much lower compression specifically for model use which can be found in either Reeves paper catalogue or "Model Engineers handbook"

As for quad rings you can get them in some sizes here but the less common particularly imperial ones proved very hard to get hold of when I wanted one for my RMC type-B, I did find them in the US but postage was a killer even on such a small item, runs OK on a round ring though.

My Thompson has slowed down a bit while doing some pattern making but taking shape now.

[steamer]

Sounds great Jason....What durometer? and stretch?


Dave

[Jasonb]

I use the Viton 75 which is 75 Shore hardness

No Stretch, Metric ring for my usual 24mm dia bores is 19.6mm ID x 2.4mm secton which is the way metric rings are specified and bottom of my groove is 19.4

[steamer]

Width of groove?

[Jasonb]

Same as in post#10