Wilwood ST FSL4R Caliper 1.88/1.75 1.25 RotorFSL4R ST calipers provide the pad wear performance advantages of differential bore pistons with the option for ultimate heat protection using Thermlock pistons in the ST models. Differential piston bores compensate for leading edge mechanical loading and temperature variances across the pad face to reduce pad taper wear in sustained high temperature applications. Thermlock pistons in the ST models block heat transfer from the pads and reduce