Standard thermal range, H-D #3. M18 threaded x 1.5. 12.7mm / 1/2" short reach. in conjunction with a 24mm spark plug socket. The usual suggested spark plug for normal riding is this D16 (H-D #3). The Champion D14 (H-D #4) (MCS 931601) is a range cooler spark plug that is suitable for prolonged high-speed riding, such as lengthy highway use. The innovative UltrasealTM coated shell and copper-core center electrode of the spark plug help to prevent corrosion for a longer length of time.