"oil-less" compressor? Most have a piston ring that is non metal and gap-less. That will make the ring end gap hard to figure. If you can remove the ring and fit the piston you can check the fit. It should be a free running fit and that was about 0.003 inch. Get a long feeler gauge at 0.002 and 0.005. Put the feeler in the bore and slip the piston in with it. If you can pull the 0.002 and not the 0.005 you are about right. If the piston was rough you might need to hone the cylinder just enough to smooth it out. When you re-install the piston with the ring be careful not to catch or roll the ring as you go in. Then see if you can move the piston in the bore. If you can the motor should be able as well. Air less is what you are trying to fix
