Cometic 850 nitrile and aluminum base gasket

Where can I obtain a Cometic aluminum and nitrile base gasket for my 850 Commando?

Call Cometic 800-752-9850. They don't list a lot of Norton stuff on their web site last time I looked, you have to ask. They also have head, valve cover and for the MKIII a good primary gasket. I'm pretty sure that phone number is correct

Thanks for the lead. The telephone number is correct. I called Cometic and ordered base and valve cover gaskets, and they are checking on whether they have the tooling to manufacture the head gasket, and they’ll let me know on that. If they can make it I’ll get one of those too. I was told these are all of steel and nitrile foam construction.

On the base gasket they asked for my desired thickness— they make .010,, .014, .020 and .030. I went out to the garage and measured some others I had, which were .020 so I ordered that. Sound right? I want to stay with stock OEM thickness and compression, since I’m dealing with 90 octane “premium” gas.

And assuming I can get a head gasket, I’m assuming it should be .040 thick, correct?

My base gasket was supplied by J. Comstock when I rebuilt my engine. I never measured it, so I can't answer that question.
I guess it would depend on what compression ratio you are looking for. IIRC every .010" was around 1/2" point, but don't hold me to that. I'm working off of memory.

For some reason I think their head gaskets were .042", I already had two AN gaskets laying around, which were .040" so I didn't follow up on that.

Good luck

Thanks. So, in my INOA Tech Digest, Vol 3, it says that for the 850, the base gasket is .018, and the copper head gasket is .040, and the composition head gasket is .030. It also says early 73s had no base gasket, later ones did have a base gasket. With both gaskets installed (no machining on the head or cylinder), the CR for the copper head gasket is listed as 8.1, and without the base gasket it is 8.3. For the composition head gasket, with the base gasket the CR is listed as 8.2 and without a base gasket it is listed as 8.5. It says these numbers are accurate to plus or minus .1 of CR. Based on those numbers, it looks like .010 in thickness is equal to maybe .1 in CR, although in the table in the Tech Digest these numbers seem to vary a little. From looking at that table, I think it is most accurate to say, .010 in thickness affects the CR a little bit, maybe .1 or .2, without sweating the tenths of CR much.

I am aiming to go back to the stock CR and so I'll use Cometic's .020 MLS base gasket and .040 MLS head gasket (if they'll make me the head gasket). Otherwise I already have both copper and composition AN head gaskets, and an AN base gasket, in those thicknesses mentioned above.

Just got my head back from Jim Comstock and it looks great. In retrospect, I should have sent him my cylinder too, although the shipping from Alaska to Colorado would have been considerable.

Or just use yamabond without a gasket.

If you have not rebored the barrel then stick with the same base gasket thickness as before, the unworn ridge is not what you want the rings to hit.

jseng1 said:

Or just use yamabond without a gasket.

Click to expand...

Thanks. Yes, I built my 750 that way. I guess I could here too, on my 850, but I’m just trying to avoid more compression. It sure simplifies things though.

I was kind of hoping you might have had some input on my question in another thread about hone for gapless rings. I’m sort of committed at this stage to 220, though.

And sorry, yes, rebore.

jseng1 said:

Or just use yamabond without a gasket.

Click to expand...

That's what's on mine and it doesn't leak.

My preference is no gasket twixt barrel and cases whenever possible. My logic being that on an 850 it is not possible to retorque the through bolts if / when the gasket crushes down any after some heat cycles.

When I have had to use something in order to get the squish or CR where I want it to be, I’ve used copper base gaskets, JS does varying thicknesses of copper gaskets allowing us to juggle things like this quite nicely.