The baler I use to compact drip tape for shipment blew out the seal in the hydraulic cylinder.  The cylinder is seven inches, and in my experience, anything over about four inches can be difficult to get the cap off because it takes a lot of torque to unscrew the cap and holding the cylinder can be a challenge.  I made a knocker wrench for this one and the end cap came out with a minimum of fuss.  The rod diameter is four inches; the cap is about eight.  The pins are just half inch bolts with the threaded portion cut off.  The material for the wrench is some 7/8 plate I had on hand.

(knocker wrench 049.jpg)

knocker wrench 049.jpg (46KB - 54 downloads)