John, the piston on mine is 3/8" thick, and even then the ring groove is not central. The rings I had to make myself, quite a few bits have gone missing over the many years of neglect, imade them a few thou over thick, about .128" and polished the sides to .125", I left theouter diameter as machined, the old brake disc I used turned up very well. I expanded the gaps 1/4", and heated them with the gas torch for a few miniutes. Put the rings in the cylinder, and check the gap, if its only ever to be run on air, the gap can be almost nil, but we don't know what might happen 50 or 100 years from now, and they could well be around much longer than that, so I made the gap about .005", if it leaks air, it leaks air, so what. I used teflon plumbers tape in all glands, including the govenor, on air you could get away without sealing that. Ian S C