This is the O.S. T-Series RP7 Medium Heat Range Turbo Glow Plug. T-series glow plugs are ideal for racing car engines that create high RPM For .12, .15 and .21 size turbo head on-road racing engines Ideal plug for any outside temperature O.S. T-Series RP6 Medium Heat Range Turbo Glow Plug Glow Plug Wrench, HCAP2550 This glow plug is different from the conventional glow plug. It is made to fit the inner head of engines with a turbo-type cylinder head design. jxs 2/27/09 ir/jxs