Hi Craynerd & Da_Kengineer_Meister,
I think i'll be able to answer the both of you with the same reply -
I cheated slightly with the power piston and cylinder, i bought them from a company called Airpot (albeit with great difficulty, living in Ireland is no fun at all when trying to source any type of manufacturing materials, had to get them in Australia

). The cylinder is borosilicate glass (i think) and the piston is graphite. They run very smoothly and need very little force. The cylinder i have is 7mm OD with 5.02 ID and 5mm piston, i currently have the piston set at a 4mm stroke from top dead centre to bottom, i'll see how that works.....or doesn't

.
If you haven't heard of them you should check out their cylinder & piston sets, they're fairl cheap and definitely save a lot of work.