A couple of weeks ago, when I picked up all the free gears at the tool repair place, I also picked up 3 burned out portable compressors. I discovered a new/different method of sealing a piston in a cylinder,which I thought would be very applicable to small air engines, possibly even steam engines. I know that you can use a neoprene o-ring, but I have found that o-rings create a very high frictional drag on the cylinder wall, and any advantage gained by using them to seal "compression" is offset by this high drag that they create. It appears that the compressor manufacturer made the piston, seal, and piston cap all the same diameter, then used the flat head capscrew to put just enough pressure on the neoprene seal plate to make it "squish out" a bit and provide an air tight seal with the cylinder wall. It also appears that one might be able to control this amount of "squish out" (for lack of a better technical term), by how much the screw was tightened. Now of course, this would demand some blue loctite on the flat head capscrew to prevent it from working loose, and if using this design, the cylinder bores would have to be marginally deeper to accomodate the extra piston height. I would still put in the 3 small grooves for oil retention, both to lubricate the piston in its bore, and to help seal it. Maybe I'm posting information here that has already been tried and discarded, but I thought it was a very interesting way of providing a piston seal without having to get into the advanced machining skills required to manufacture piston rings.---Brian