Yes Paul - I had realised what I was missing in understanding what gap you were talking about!
Now my query on your method is reduced to what rational governs the size of the uncompressed ring gap you cut in the mill before the final OD and ID are machined.
Since I first read your first post I have caught up with another article on making piston rings without heat, written by Tom Schwartz, I think for Model Engine Builder, quite a while ago. His article goes into quite some detail on the theory behind his method and gives calculations/formula to calculate what sizes to make rings for different size bores and different ring materials and different fuel. Quite comprehensive. He also gives drawings of the fixtures to enable the rings to be made.
Amongst the formula is one for calculating the size of the initial gap to be milled before the ring is finished, which is the total of the compression gap and the final ring bore gap. The major difference in his method to yours, I think, is that he calculates the size of the ring in its expanded form based on bore diameter plus 10 thou and the compression gap plus final bore gap for the OD, and the ID is expanded OD minus the ring thickness . He then compresses the rings and machines 10 thou to give the OD to size as per your method, but not the ID which has been already set.
I hope I set that out clearly!!
No no, I too did calculate the fully expanded gap before milling it. There was a formula for calculating the wide open gap back in the SIC. days. But as I said earlier IMO, with in reason, the wide open gap is not all that important. What is much more important is that the ring's final OD is perfectly round when installed in the cylinder. The closed ring gap should be near zero once in the bore.
I agree that the ID of the ring is not anywhere near as important as the OD. But if using the method I described its not really much more work to turn the ID in the final ops also.
Note that this is the same method that industry use to make commercial piston rings. In other words the industry also machine the wide open gap prior to final OD and ID finishing. The do not split the ring, and they do not heat treat the ring to form the wide open gap.