Lewis Ultimate Lightest Flat Mount Road Bike Caliper BB2 SL/SLR High-precision CNC machinine Cut from 7075 T6 Aluminum bar stock 2 x 21mm-piston-design for strong braking power Brake mount: flat mount FM size is 34mm Oil: Mineral oil or DOT 5.1 Fluid SL weight: 82.5g Stainless steel screw (without brake pads and fitting) SLR Weight: 68.5g Titanium screw (without brake pads and fitting) Increase the thickness of the inner wall of the caliper, high strength, adapt to high-intensity environment Large piston floating clearance for better compatibility with most brake levers, reducing compatibility issues due to caliper replacement.