If I recall it is the one they used on the '93 Cobras. I don't recall the bore dimensions. The M/C was originally part of a Baer 12" rear brake kit for the Fox body cars. I am currently using this setup with 4 piston Wilwoods up front and single piston Wilwood GM III calipers in the rear. I road race the car at Sebring and Daytona and punish the brakes with zero problems.