Forklift- I learned this type of cyl the hard way. Worked on a hoist cyl on a newer clark which I thought only had packing at the bottom of the rod. Imagine my surprise when upon disassembly I found 2 sets of packing- at the top of the barrel & at the bottom of the rod. I had quoted a simple repacking (the rod was severely scratched & I wound up rechroming the rod). After I lost the customer because he was **** at the rechroming charge- I did alot of research on what the cyl was & how it worked. The simplest explanation is at http://en dot wikipedia dot org/wiki/Hydraulic_cylinder. Hope that helps