Spriro-lock rings work great, but can be a pain to remove.
Remember to bring that question up this weekend and we can look the specs up in my machinist bible.
But, I think you go by the diameter of the bottom of the groove on OD rings and the ID of the groove on ID rings.

Don


1967 GMC 9500 Fire Ladder Truck
"The Flag Pole"
In the Stovebolt Gallery
1946 2-Ton grain truck
1946 3/4-ton pickup,
1953 1-ton pickup
1957 1-ton flatbed dually,
1954 Pontiac Straight Eight
Of all the things I've lost in my life, I miss my mind the most!