A section inserted between the air drive and the hydraulic body to seperate them a distance greater then the travel(stroke) of the plunger/piston. The distance piece is vented, so that any leakage from the fluid end will not be able to contaminate the air drive, and visa versa. Many times, the fluid pumped is not compatible with the air drive 'O' rings, and could cause swelling which would result in excessive friction and stoppage of the air drive. When critical fluids are used , it also protects the hydraulic section from air drive contaminants such as oil or moisture. The distance piece is also used when the fluids pumped are at temperature that could adversly affect the air drive seals. It provides a distance for heat transfer to minimize ther temperature differential.