Wilwood's GP310 motorcycle disc brake caliper has been designed and engineered for use on 1984 to 1999 Harley-Davidson? Motorcycles. Built around a 4-piston, high performance powerhouse, this billet aluminum caliper brings distinctive, bolt-on styling: Direct leg-mounted calipers are available for all single and dual disc models (except Springer and 4-speed FL models), as well as the 1983 XR 1000.