Seal the valves with o-rings

[Enginemaker120]

This is regarding 4 cycle hit and miss engine (.75" bore with valve face dia .25")
I know some people have used o-ring instead of piston ring with good results. I am wondering if the same is possible with o-rings on the valves to seal the leak?
I know the process of sealing the valves and angles required, but just wondering what kind of result is expected if one try to use o-rings on valves?

Thanks
Ron

 

[Brian Rupnow]

I don't think it would work very well. When the Viton o-ring is installed on a piston, its sets in a groove that is static on 3 sides in relationship to the ring. The fourth side which is the cylinder wall is a "sliding" contact, but it is always in contact. If your plan is to use a Viton o-ring as a valve seat, the constant "off and on" contact as the valve opened and closed would quiockly erode the o-ring away. If however, you were planning to use it to simply seal around the valve stem, it would probably work.

 

[Jordan]

If you mean the valve stem, some full size engines use O-rings in an internal groove in the valve guide, because it's not possible to use the usual seal at the top of the guide. I don't think they perform as well as a conventional seal.

Jordan

 

[Enginemaker120]

Thanks all. No I am not referring to the valve stem. The actual sealing between the valve face and the valve seat. Thinking instead of making a 45 degree angle on valve and seat, I make the valve margin a little wider and make a groove on it for the o-ring.

Ron

 

[Mike1]

Have you thought what effect the hot exhaust gas would have on the O-ring as it passes over the ring, my thinking is that the direct heat on the O-ring would ruin the seal, just my own thought on this.

Mike.

 

[Enginemaker120]

yes I was thinking of that also. Some of these o-rings can handle up to 500 F. However I am not sure what would be the tempature inside the chamber on a model hit and miss engine?

 

[dman]

yes I was thinking of that also. Some of these o-rings can handle up to 500 F. However I am not sure what would be the tempature inside the chamber on a model hit and miss engine?

hard to say. the actual gasses will be much hotter unless you run alcohol with a rich mixture, but on a hit and miss there will be plenty of cooling cycles so it may tolerate more. I really wouldn't try it. the soft seal would flex and opening/closing time would be inconsistent or other than designed. also how would you keep the seal from falling out of the head or block? not to mention the seat is where much of the valve cooling happens and metal on metal is the best way to dissipate heat the valve absorbs.


you could use a soft metal like brass or other copper alloy and have a 1deg interference angle. in the full scale world its preferred to have a sealing face on the valve that's wider than that of the seat in the head so the seat lands entirely on the face, the seat has a top and back angle that define the edges and interference angles are common so that the seat od contacts the valve first and there is 1 or 2 degrees difference between the valve and seat. as the engine breaks in the valve will slightly sink in to the seat and create a perfect seal. you could test the seal with a small hand held vacuum pump.

 

[Entropy455]

Exhaust gas temperature will be well over 500 degrees F. Assume 700 degrees for an unloaded engine, and over 1000 degrees F under load. Consider that chrome plated exhaust pipes will "blue" at 900 degrees F, and bluing is pretty common on exhaust pipes.

The combustion temperature (pre-work-extraction) for Propane is 3596 degrees F. Natural Gas is 3562 degrees F. Light Fuel Oil is 3820 degrees F. Thus the best sealing surface for engine valves is metal-to-metal contact.

I'm even skeptical about using rubber o-rings for piston rings. I acknowledge that people are successfully using them, and that it "generally" works for low compression display engines, operating under minimal load. . .

 

[Lakc]

For a hit n miss you might get away with it. Using an o-ring in a groove on the back side of a T shaped valve and close enough tolerances to keep the gas from impinging directly on it. Maybe even make it seal on the outside of the combustion chamber with a desmodromic type action. The question is why would you want to, as the time honored traditional method is simpler and totally reliable. Its ok to try new things and be different, but I would save that for something else. We whine and moan about how hard it is to make valves seal sometimes, but we learn our weaknesses and when finally done right we have accomplished something.

 

[neseng]

VITON 'O' rings and high temperatures are a BIG NO-NO! When viton is exposed to high temperatures, it degrades and one of the nasties produced is 'HYDROFLOURIC ACID' which is a deadly poison.
<<Hydrogen fluoride gas is an acute poison that may immediately and permanently damage lungs and the corneas of the eyes. Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death. By interfering with body calcium metabolism, the concentrated acid may also cause systemic toxicity and eventual cardiac arrest and fatality.>>
I would not even entertain the idea of using 'O' rings, especially Viton, in the applications you have suggested or in any application exposed to high temperatures on any engine.
Stay safe and live long! Cheers, Norman.

 

[MachineTom]

As most other have said the exhaust gas temp's will eat any o ring in seconds.

It is not difficult to get a very good seal between valve and seat, with careful grinding and lapping. Having seen slo-mo videos of internal cylinder functions, the exhaust valve goes to bright red/white in the first couple degrees of opening, and closes at dull white hot. This on full size engines, but the temps will be only a bit lower on models.