H-D #3, standard heat range. M18 X 1.5 threaded. 1/2" (12.7mm) short reach. Used with a 24mm spark plug socket. This D16 (H-D #3) is the standard recommended spark plug for regular riding. For extensive high speed riding, like extended highway use, a range colder spark plug is available as Champion D14 (H-D #4) (MCS 931601). Spark plug has a copper-core center electrode and is equipped with the patented Ultraseal™ coated shell, to help keep the spark plug corrosion free for a longer period of time.