Hello Captinjohn, I use a snap gage to measure down in gas cylinder bores like the one shown with multiple ports. I check it is 2-3 places to assure I get the right measurements and all measurements are the same and measure snap gage with OD micrometer. I have a few rings for 36MM bores, 3W 26CC, ST 3250, Moki 210, & Echo 30CC. It is possible that i have the ring, you can email me direct if you wish but I will be gone Saturday until 7 PM MST. Thanks Frank Bowman