I think the expander behind the ring is optional, depending on the depth of the ring groove. Measure the thickness of the ring, plus the expander, and be sure there's enough depth to contain the assembly, without stacking solid and binding the ring against the cylinder wall. When were these rings manufactured? It might be wiser to take advantage of more recent technology, rather than use "new old stock" stuff from years ago. I haven't seen single-piece oil rings used anywhere in at least 30 years. You might have a good museum exhibit, rather than a good ring design.
Jerry
