 Barrel seals use one or two O-rings in grooves around the end of the manifold tube, which seal against the bore of the valve port.They are usually screwed into the valve port with handed thread, and locked with a lock-nut.They are generally slightly less rugged than face seal manifolds, and more vulnerable to thread damage during assembly, but allow a small amount of cylinder centre distance adjustment, and provide a reliable seal even if not completely tight.