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The idea is to open up the gap and then use the heat treatment to 'lock' this in so that when the rings are compressed back to their nominal size there is a pressure against the cylinder wall to start the sealing process. The gas pressure behind the rings then increases the sealing force.
Brian,I'm not sure about your method of expanding the rings! It seems like you'll be changing the whole diameter. The Trimble fixture is calculated to expand the ring to maintain the proper diameter.