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Rubber o-ring head

Rubber o-ring head

Rubber o-ring head

I never looked much into the technical details of high performance automotive 0-ring heads. I always assumed that some of them must actually use rubber o-rings.
In recent research it doesn't appear to be the case. All the documentation and discussion on it seems to be around metal o-rings.
This got me wondering if there was a good reason not to use rubber o-rings. They are very common in dirtbikes and snowmobiles but only two strokes as far as I have found. Some two strokes run very high compression ratios to I wouldn't think it wouldn't work due to cylinder pressure but that is about the only thing I can think of that would potentially be a weakness for a rubber o-ring.
Unless perhaps it's more a matter of longevity considering two strokes get top end rebuilds much more frequently.
Just curious if anyone has seen rubber o-rings on automotive motors or other four strokes and interested if anyone has theories on why this isn't more common.

RE: Rubber o-ring head

O rings made of high temperature elastomers (not rubber) may be OK for sealing water and oil transfers across the head/block interface but the temperature and pressure in the vicinity of the combustion chamber makes them unsuitable for bore-sealing. Even if the bore seal environment was controlled to the point that an elastomer could survive, what happens when a small leak develops? Combustion gases at very high temperatures (1000+*C) begin to flow past the seal and erode it very rapidly.

je suis charlie

RE: Rubber o-ring head

But how would these concerns differ from a two stroke dirtbike or snowmobile head that can be incredibly reliable with whatever high temp elastomer they may use.

RE: Rubber o-ring head

Very thin and rather wide vs. an O-ring?

RE: Rubber o-ring head

Are you sure these gaskets have rubber around the bore? This Link one clearly has a bare metal bore-binder.

je suis charlie

RE: Rubber o-ring head

Maybe the top surface of that cylinder rides right up against the head, and it's mainly a metal-metal seal.
I wonder what the symptoms would be of a missing o-ring?

Jay Maechtlen

RE: Rubber o-ring head


It is definitely designed to rest metal on metal. Without the o-ring or any type of sealant it would definitely leak though.
With that said I did run my old CR250 for a while using only copper coat instead of the metal headgasket it was designed for. If I remember right it worked pretty well.

RE: Rubber o-ring head

" old CR250 "

That would be aircooled, I'm guessing.

RE: Rubber o-ring head

Not quite that old. I think it was an 88 or so. Water cooled

RE: Rubber o-ring head

I think the finishes and flatness must have been darned good if even multiple coats of Coppercoat kept all the fluids in place even on just one cylinder.

RE: Rubber o-ring head

I wouldn't be the least bit surprised if I just got lucky on that one.
We would polish our heads on a thick glass plate with sand paper. I doubt I did more than clean up the top of the cyl.

RE: Rubber o-ring head

Are you sure an o-ring is sealing the combustion chamber and not used to seal the water jacket (or oil passages if it's a 4-cycle) further out from the cylinder bore? I just can't believe a rubber o-ring would last unless the bore and head were sealing metal to metal inside of the gasket anyways.

RE: Rubber o-ring head

You can see it pretty clear in the pics in that thread I linked to above. There is an o-ring inside to seal the combustion chamber from the water jacket and an o-ring outside to seal the water jacket from atmosphere.
There is a groove that the rings sit in so yes it should sit metal on metal plus or minus the flatness of the head and block.

RE: Rubber o-ring head

I think the red o-ring may be silicone rubber. That would buy some temp resistance.

Tech specs for Regular old o-rings say they are capable of sealing 1000 plus psi if a few conditions are met.

I guess an o-ring removed a bit from the fire might be OK.

RE: Rubber o-ring head

Referring to the photo above, the real primary seal is the land between the cylinder bore and the inner o-ring cavity. It's large enough to provide a pressure gradient, and a thermal gradient large enough to ensure survival of the inner o-ring, which is made of silicone, and fairly resistant to high temperatures. In a two-stroke, any imperfections in that metal-to-metal primary seal will be filled with soot during a modest break-in.

The primary pressure seal is held closed by six sturdy studs. Rotational symmetry assures that stud forces will be evenly distributed.

Contrast that with a multicylinder engine, where there are usually four studs, sometimes five, rarely six, and most are shared with adjacent cylinders. Ensuring uniform clamping pressure is pretty much accidental, even ignoring problems associated with differences in thermal expansion between head and block.

Sealing the cylinder head junction of a multicylinder engine is a whole different deal from sealing a one-lunger.

Mike Halloran
Pembroke Pines, FL, USA

RE: Rubber o-ring head

Just thinkin' out loud; if I were looking at that as a designer, I'd consider a labyrinth-type seal scheme, by providing a protruding ring (of native head material) that would extend down into the O-ring groove a relatively small distance (.040"?), with fairly close radial fit to the groove (.002-.003"?). I'd keep the elastomer O-ring for 100% sealing of coolant.

FYI- A metal-sealing-ring approach (but with steel rectangular-section rings) has been employed numerous times to supercharged racing engines (especially dragrace) which had problems with traditional copper-gasket/metal-O-ring sealing.

RE: Rubber o-ring head

As others noted, the inner o-ring is either an elastomer like silicone or kalrez. These elastomer compounds can handle exposure to temps of around 400degF without compression setting. The primary compression seal around the bore is the metal-to-metal contact between the head and block faces. The pressure the o-ring is subject to would be far less than peak combustion pressures. Plus, the duty cycle for this 2T MC engine is probably far less demanding than that of a 4T auto engine.

I recall seeing kalrez o-rings used as seals between the block and exhaust pipe on some 2T MC engines

RE: Rubber o-ring head

2 stroke peak cylinder pressures are lower than 4 strokes plus an MX engine is torn down frequently so the seal does not need to last very long.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Rubber o-ring head

The inner o-ring only serves as seal for the coolant system. It does not see combustion gas pressures. It works because the single cylinder 2T block and head design allow lots of coolant flow around it. It would not be practical with a multi-cylinder 4T engine that does not have the same coolant flow around the entire circumference of the cylinder bores.

Another issue with o-rings used to seal coolant on one side and lube oil on the other side is the elastomer often responds differently to the two fluids. So it often requires two separate o-ring with different properties for sealing these joints.

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