Andy wrote:I've seen MC21 grooved barrels too, but there's not a lot of room to machine them around the studs.

I doubt RS O-rings are the answer. You will most likely need a different diameter. Remember the lack of head gasket will need to be accounted for elsewhere to pevent detonation.

Fat base gasket is what you would want for that (.06 or greater). Or a custom head with inserts cut to make up for the lack of head gaskets (VHM).

If your going to go with o-rings, your going to want to replace your o-rings much more then you would your head gasket on the stock NSR setup.
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Charles Gallant