The brake master cylinder of OEM MC39571 has 0.94″ bore diameter piston. It is made by aluminum material and has a dual reservior which is plastic.There are two ports of outlet: 9/16-18 & 1/2-20. It fits for Trucks of Chevy S-10 Blazer 1983-1994 & S-10 Pickup 1983-1995; GMC Jimmy 1983-1994, Sonoma 1983-1995, Syclone 1991 & Typhoon 1992-1993; Oldsmobile Bravada 1991-1994.