7/8-18 threaded. 5/8" (16mm) reach. Used with 24mm spark plug socket. Fits early IOE Intake Over Exhaust (half OHV and half Flathead) models. Copper-core center electrode. Spark plug comes with the patented UltrasealT coated shell to help keep the spark plug body corrosion free and better looking for a longer period of time. 14-26 F J 22-26 FD JD JDH (NU)