NGK makes a special spark plug that has a rare earth ground electrode that has a short top on it. We had to use them in the RS125, the 2 cylinder RS250 Hondas and some of the other highly developed 500cc-4 cylinder road racing engines that had detonation problems and or had detonation sensors under the spark plug. It also requires a special spark plug cap for these short top spark plugs. The spark plug we used was a NGK R7282 and has a heat range around "10 to 11" on the NGK scale. I have not bought any of these spark plugs in 15 to 20 years, but I would guess that the price today is probably around $60.00 to $80.00 each with the spark plug cap costing around $50.00.
I do not know if NGK makes any other heat ranges in the 7 to 9 range with the short tops. Our single cylinder 250s to 500cc recreational engines will foul a spark plug like the R7282 because they do not produce the high combustion temperatures that these 55 to 60HP per cylinder (125cc) road racing engines produce.
