Wilwood 120-13575 FSL4R-ST Forged Superlite 4-Piston Radial Mount Disc Brake Caliper - Type III Anodized

The Wilwood 120-13575 Forged Superlite 4R ST (FSL4R-ST) is a high-performance 4-piston radial mount disc brake caliper featuring differential bore pistons and Wilwood's exclusive Thermlock® piston technology. Designed for sustained hard braking on dirt and asphalt ovals, as well as off-road, rally, and road course applications, this caliper delivers exceptional thermal control and structural rigidity in a lightweight forged package.

Thermlock® ST Piston Technology:

• Thermlock® pistons block heat transfer from the brake pads to the caliper body, fluid, and seals
• Reduces caliper and fluid temperatures by up to 25% compared to standard stainless steel pistons
• Differential bore piston sizing compensates for leading-edge mechanical loading and temperature variances across the pad face
• Reduces pad taper wear in sustained high-temperature braking applications

Internal Fluid Port Design:

• Redesigned with internal fluid ports that eliminate external fluid cross-over tubes
• Transfer tube replaced with two additional bleed screw assemblies
• Differential bore configuration requires left-hand and right-hand orientation — no leading or trailing position configuration necessary

Forged Aluminum Construction:

• Caliper body stress-flow forged from premium-grade aluminum alloy billet
• Body design engineered using computer-generated solid modeling and stress analysis technology
• Closed end bridges feature a radial transition down to the main body and piston bore housings
• Elimination of machined steps and sharp shoulders provides measurable increase in body strength and resistance to deflection under load
• Assembled weight: 4.4 lbs
• Piston bore: 1.25 in.
• Rotor thickness: 0.81 in.
• Finish: Type III Hard Anodize

The Wilwood FSL4R-ST is the go-to caliper for racers and performance drivers who demand reliable, fade-resistant stopping power through grueling sustained braking events. The combination of Thermlock® thermal management, differential bore engineering, and stress-forged construction makes this caliper a benchmark choice across short track, road racing, and off-road disciplines.