A while ago I bought some new pistons and rings for the engine (+0.40). When I originally measured the oil ring set (2 flat outer and the middle rippled one), I measured it at approx 5mm where the slot I bone piston was 4mm. I had thought for a while that the set, although correct part nos from Hastings, had incorrect oil rings.
Thinking I had to buy a new set, I revisited the rings today, partly to look at the oil ones and partly to see what the gap would be when installed. The gap was 15 thou on the upper two rings which is pretty much where GC was looking for according to his book. I also had a good look at the oil rings and realised my mistake. The middle ring has raised locating pieces to align the two outer rings and when you correctly assemble the three the size comes to just under 4mm which is exactly the spec by the piston manufacturer, so purchase avoided!
Peter