Wilwood's GP310 Rear motorcycle disc brake caliper has been designed and engineered for use on 1984 - present Harley - Davidson Motorcycles. Built around a 4 - piston, high performance powerhouse, this billet aluminum caliper brings distinctive, bolt - on styling Bracket - mounted calipers with a standard 3.50 mount spacing are available for all most models (except Springer and 4 - speed FL models).