hughes the plug heat range i refer to is a half increment heat range ie from hot to cold series goes 8, 85, 9, 95, 10, 105. In australia i have raced twostroke gokarts for about ten years and we only ever used either of the following : B#EGV or B#EV in racing. Worst case you may have to buy a B#ES if that is all available. Sparkplugs rarely ever die just get wet/fouled nothing that a bit of contact clean cant fix. Depending on whether the motor we were using was air cooled or water cooled and how much power we were putting out would determine which heat range we would use. In the case of KX500 i reckon an 8 heat range plug is perfect as the motors don't rev stuffall and often are off the throttle (to get the front wheel down) and i am sure the power output versus the capacity (specific power) is quite low compared to modern 125's and 250's. Anyway you can buy buy ngk plugs that go up in half heatranges, here i haven't seen any bike shops that stock them but nearly allgokart shops do.