Abstract: An hydraulic cylinder for driving a reciprocating pump at 5000 to 10,000 psi pressures having a separate spool casing which serves as a detachable extension of the cylinder. The detachable spool casing houses in a single grouping seals for the fluid being pumped, seals for the hydraulic fluid driving the piston, a bearing for the piston rod, and lubricating means for the seals and bearings. This construction provides ready access to these parts for repair and assures concentricity between the piston rod and the bearings and seals in which it reciprocates.In one embodiment a wiper ring is provided between seals to scrape from the piston rod abrasive material in the fluid being pumped and discharge it from the cylinder. In another embodiment an external shock absorber decelerates the piston at one end of its extended stroke, the shock absorber also serving to actuate a valve controlling flow of hydraulic fluid to the cylinder.