The best possible combination is a cast iron piston and a cast iron cylinder. There is enough free graphite in cast iron to let the two surfaces slide over each other without galling. Never use steel on steel. For 1/2" bore you don't need any rings. Lap the piston into the cylinder to make an air-tight seal. Leave a cylinder wall of 0.060" thick. Make the cylinder first. Make the piston close to size but not quite. Use a sanding strip and work the piston down until it just starts to slid into the cylinder but will not slide in. Coat the piston with 600 grit carborundum paste or diamond paste and with one part turning slowly in the lathe, work the other part into it very slowly until you have a complete fit from end to end.