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.
Fits:
750(45)/1200(74)/1340(80) Side Valve (Flatheads): 36-47 Knucklehead (61/74) (OHV) models