I contacted SJ and the vintage guy didn't know.

I've been spending too much time and money on this. My covers came with o-rings but I haven't installed the covers yet, wanting to find the correct o-ring. 

They were installed with 96mm ID / 2.5mm cross section rings and they worked (apparently) but when I tried to put the o-ring back in i had to stretch it. And when stretching it the o-ring invariably twists and changes shape. 

Next, I tried a 100mm ID and it was better. I had been told by the seller that someone told him that 100mm was the correct size. 

Ultimately I was able to find a 103mm ID and it works the best. It has to be stretched a little. This particular one is spec'd at 2.5mm but seems thicker. But it works. 

I have not been able to find something slightly larger like a 104mm or 105mm in this cross section in the US, thought I have seen them in the UK and India online. It could work slightly better. 

Here are the 96mm and 100mm to see relative size. 

And here is the 103mm.

The 103mm came from Canada. The 100mm came from the US. 

Probably someone here know better than I know about where to get them easily, but big places like Grainger didn't appear to have them. 

Thanks for that info! I just had a look at McMaster-Carr and they have a 3/32" (2.6mm) Viton o-ring at 3.987", which is pretty close to your 103mm size. It's $4 for two rings, so I think I'll give those a shot. McMaster sizes them by length, not diameter so using your length measurement is most helpful. I really hope one of us can find a source for these and give everyone in the Secret Valve Cover Club a well-deserved break, lol. I've attached a screen shot of the page on the McMaster site for reference.

Places like McMaster may also sell O-ring material on a spool. "Make your own" as it were. Place the end gap so that it ends up in the highest position and then help seal the 2 loose ends with a dab of silicone sealer.

Such as....
https://www.mcmaster.com/products/o-rings/o-ring-cord-stock~/oil-resistant-buna-n-o-ring-cord-stock-7/

• McMaster also sells a kit for bonding the ends of the Cord Stock.

Thanks for including that screenshot, Jeff. 

Someone else said that they thought the correct size was 155 so now it all makes sense. (I had originally thought that that was the inside diameter.

Also, the 2.5mm seems a little bit small for the channel. Something with a slightly larger cross section should be better when they are compressed. 

And Richard, thanks for the info about sealing the ends. Never knew that existed!

Thanks, Richard. Had no idea McMaster had that. 

Darren - I ordered 3 sizes so hopefully one will work. I may have to go up to 1/8" if 3/32" is too small. I'll post my findings when I get the rings.

Posted by: @12727

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Also, the 2.5mm seems a little bit small for the channel. Something with a slightly larger cross section should be better when they are compressed. 

Correct compression is only something like 10%. So on a 3/32 (which measures 0.103") that's only 0.010".

Winner winner, chicken dinner! Looks like the McMaster 155 size is the winner. 155 was larger in diameter than the old ring I took off but when I put it on the valve cover, it fit the best. I've attached a pic showing the height of the new installed ring (left) vs the old ring (right). I put the new ring in the slot and tightened it to the head to get it to seat properly. You can see from the pic that the new ring seems to have enough height to seat correctly. I'll need to get two more rings for the other cover and then I'll test them out and report back. McMaster description: Chemical-Resistant Viton® Fluoroelastomer O-Ring, 3/32 Fractional Width, Dash Number 155

jeff,

this is great. 155 it is. 

did you have to stretch it at all to get it in?

what is the approach on which material of o-ring to get? chemical resistant? oil resistant?

darren

Darren,

I had to stretch it a little to get it in, but not much. The old ring I removed was smaller when compared side by side with the new ring. The new ring is Viton which I believe is fuel, oil and chemical resistant.

Jeff

OK, installed the 4 o-rings and took a 25 mile ride in 80F weather yesterday. Covers did not leak! Attached is a photo of the packaging from McMaster-Carr.

One note: I noticed that one of my covers had tiny o-rings at the base of each stud that hold the covers on. One o-ring looked OK and the other was smushed. The other cover had no o-rings. I have no idea if SJ BMW ever intended to also use o-rings on the cover studs.  My covers (at least for now) are not showing any leaks from around the cover studs. Your results may vary!

I'm pretty confident that these McMaster o-rings are the correct size for the valve covers. I will revise this thread if that changes in the near future.