I worked on making rings about half of yesterday, and they are a bit tricky to make if you want a somewhat exact fit.
These are made from gray cast iron, and were made larger than the bore, a gap was cut in them using a dremel.
I made a mandrel, compressed them on the mandrel with a hose clamp, and turned them to final bore size.
I see why people use the other method now where you make the ring to the exact bore size, and them wedge them open and heat treat them.
The method I used (start with an oversized ring, and don't use heat treatment) works well, but takes a great deal of care to get right.
From what I have read, most do not heat treat their rings correctly anyway, and overheating or underheating a ring, as well as letting it cool too fast accomplishes nothing.
The oversized rings do not have to be heat treated, and they work well.
I ruined 6 rings before I finally got the method down.
It is a trick to figure out exactly how much gap to cut out.
I finally started making the rings a tiny bit too large, and then filed the ends of the ring very slightly to get the exact gap clearance in the bore.
I had to rework the mandrel to include a ridge so that the ring would stay centered and I could cut an equal amount off all around. (I make symmetrical rings, some make rings thicker on the side away from the gap).
I use an autmotive ring spreader, and it works well to keep from overexpanding the rings. I have also used the method Steve mentions above, and it also works.
The rings I made are 1.125" dia. (final turned size).
I used a feeler gauge to check the gap with the ring in the bore.
I generally make extra rings, in case I break some, but I have not had much of a problem with breaking rings made from good quality gray cast iron.
I make the ring width to fit the parting tool size that is about the right size for the piston size I am using.
When parting off, I use the carriage dial, and part off a tiny bit larger than the groove in the piston, then use 400 grit wet/dry to clean up the sides of the ring and remove any flash.
Rings for small pistons can be a bit delicate, but it can be done, and is not that difficult once you get the method down.