I’ve used these size charts by finding the appropriate OEM number, verifying the correct dimensions as shown in the chart, then finding the matching size at my local Ace Hardware store where they have drawers full of O-rings.
Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1957 Evinrude Fastwin 18
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."