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As a side note I made a practice ring just to check and see how it would flex after heat treating but rather than mount it on the fixture, which would have necessitated making a spacer bushing, I used a method that some builder have used. This method is to machine the ring, cleave it open and then mount it on a piece of steel to hold the gap open. The ring is then held in front of a piece of refractory material like a fire brick or soldering pad and then heated until it drops from the steel spacer bar..... It works on plain rings but not on a ring with holes drilled in it, like and oil ring. What happens is the stress on the ring while mounted on the steel spacer bar and then heated distorts the ring at the drilled holes so you end up with a ring that won't fit the bore properly.
I believe his fixture still spreads the rings using only three points. This should create a bending force in the ring and, due to the holes, force them out of "round". The same as a spacer, though not as bad since Trimble's fixture presses at the center of the ring cross section.Hugh