Abstract: This invention is a structure and process of truing piston cylinders. The structure consists of a main mandrel support means to hold a mandrel member therein including (1) a support base means; (2) a piston cylinder support means mounted on the support base means; and (3) a power supply means connected to the support base means and the piston cylinder support means. The piston cylinder support means includes a stationary headstock and a movable tailstock, both supported on the support base means. The stationary headstock and the movable tailstock are operable to hold a piston cylinder to be trued therebetween plus each having a mandrel receiving member. The power supply means is operable to supply fluid under pressure to the stationary headstock and return fluid to a reservoir tank through the movable tailstock.