Rear left. 3.25" Bolt Circle Diameter; hole to hole distance 48.6mm. 3/8" counter bored mount holes. 56.3mm ID. Mount style ''B''. Machined from billet stainless steel with a center carrier of 7075 billet aluminum in a polished finish. Note: 8-1/2" diameter brake rotors are designed for custom applications. They will not provide the brake efficiency of a stock 11-1/2" diameter brake rotor. All K-Tech 8-1/2" rotors are designed to work with K-Tech 2 and 4-piston calipers or PM 2-p calipers only. Note: Will physically fit both Big Twin (right side mounted) and XL Sportster (left side mounted) models but come in a right & left style for correct matching optical fitment.