Hi Keith. You must have bought a 498cc head I suppose but no matter if you have been able to machine it to 596 combustion chamber size. Could you please check on that hose spigot thread pitch again as I think it should be 20tpi and not 26tpi. If it is 20tpi I can lend you a 1″x20tpi tap if you pay the postage. Phone me to discuss. [number in front cover of Yowl] Regards. Brian.