Wilwood Caliper Forged Superlite 4 1.88 1.75in Pistons 1.25in Disc eBay Wilwood Caliper Narrow Superlite 4R Black 1.25 1.25in Pistons 1.10in Wilwood Caliper Forged Superlite FSL4R ST Aluminum 1.88 1.75in Pistons Wilwood Caliper Narrow Superlite 4R Black 1.75 1.75in Pistons 1.25in Wilwood Disc Brakes Caliper Part No: 120 13233 N. FSLI4 and FSLI4 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 temperatures in the caliper, fluid, and seals by up to 25% over standard stainless steel pistons. These are the go to calipers for all types sustained hard braking on dirt and asphalt ovals and a wide range of off road, rally and road course applications.