A weld gun cylinder and piston assembly includes a common center end wall interposed between two cylinders. The common center end wall functions as a manifold with bores extending therethrough. The manifold houses two spool valves. The spool valves control the passage of pneumatic pressure through passages...http://www.google.se/patents/US6375156?utm_source=gb-gplus-sharePatent US6375156 - Welding gun cylinder with control valve
