It might help you to know how piston rings actually work. This can help at the machining stage so you can concentrate on getting the sealing surfaces smooth – as long as you know which sides of what does the sealing.
What follows is based in ICE practice, but there are really only two differences between petrol and steam, in piston ring terms: The ICE piston works with pressure always on the same side as the sparking plug. Steam is more normally used on both sides alternately (and without any ‘gaps’ in the flow of power). What happens is this: The pressurised gas goes down between the cylinder head and the piston until it gets to the ring, which it pushes against the bottom of the groove. The pressure then can get behind the ring and acts to force the ring outwards against the cylinder wall. So, the rings rely for sealing on good continuous smooth surfaces on the lower face of the ring and the lowest face of the ring groove. On the way back up in a steam engine, the same process happens, relying on the fit of the opposite face of the ring and the opposite face of the ring groove. And it doesn’t seem to improve matters if you use a complex joint where the two ends of the ring meet up – but it does help to reduce the ring gap in the fitted condition as much as you can, on first build.
Petrol and diesel engines often have some identification etched or marked on their rings, and if you follow the pressure faces argument as described above, you will see why these marks are always on the top surface of the ring (as they are marked on a non-sealing surface). Both sides are critical for double acting (steam) engines, so both sides of the ring and its groove must be as near dead flat and smooth as you can get them.
The proper name of piston rings – just in case anyone else is interested in such details – is Ramsbottom’s Metallic Packing. John Ramsbottom was Chief Engineer of the Manchester & Birmingham Railway, later forming part of the LNWR, and he also invented tamper-proof safety valves and the most wonderful invention for small boys, the water trough between the rails so the speeding train could force water into the boiler at speed, and drench anyone who happened to be looking out of the windows. How do I know?
Tim Stevens