Wilwood Right FSLI4-ST Thermlock Caliper, 1.25-Inch Rotor Width / Differential Bore

Thermlock pistons distinguish this right FSLI4-ST from the standard stainless-piston version. Its differential 1.88/1.75-inch bores supply a 5.18-square-inch effective area in a nominal 1.25-inch rotor-width body. Wilwood Engineering is a Camarillo, California-based designer and manufacturer of disc brake systems, founded by Bill Wood in 1977. Calipers, brake rotors, master cylinders, valves, fittings and brake lines form its range for motorsport, street-performance and specialized applications.

Technical Details

• Manufacturer part number: 120-11331. Component: Forged Superlite Internal 4 ST caliper.
• Supply: one brake caliper, not a complete axle brake kit. Match the mating rotor, pads, bracket and connection hardware to the installation.
• The rotor channel is specified for 1.25-inch nominal width. Rotor outside diameter: 11.75 to 13.06 inches. Specified rotor-width limits: 1.19–1.38 inch.
• Piston dust boots: not fitted.
• 3.50-inch lug mounting pattern, right hand; mounting height 2.85 inches and hole size 0.45 inch.
• Caliper weight: approximately 5.2 lb.
• Type III anodize finish on a forged aluminum caliper body.
• 4 Thermlock pistons with 1.88/1.75-inch bore groups. The specified effective area is 5.18 sq in.
• Hydraulic inlet thread: 1/8-27 NPT. Use a fitting with the correct thread and sealing seat.
• Brake pad profile 7420; select friction material for the actual operating conditions.

Fitment & Compatibility

Preserve the right-hand orientation and match the wider rotor's centerline. The common 7420 pad format and 3.50-inch lugs do not remove the need to verify the specified Thermlock assembly.

PMB Performance has worked with brake components for over 20 years. Share the required piston sizing, rotor measurements and bracket drawing for help checking the match.