I have an XR600 with the full HRC kit. The recomended spark plug is a NGK R 216-10, kinda hard to find. This plug dosen't extend into the combustion chamber as much as the stock NGK. I haven't found a source for the racing plug and I like to carry a spare plug just cause your suppose to. Anyone know if it's required to use the racing plug because of valve,cam,piston clearance issues and if it is, a source for them?Thanks.