I worked on Big Fours. LightFours. BigTwins, V-4s And a Zephyr. We would check clearance by sticking a piece of hollow core solder through the spark plug hole and then turning the engine over. In the days of 30 weight oil we would polish the inside of the combustion chamber to discourage carbon formation. Most ’40s outboards had a 5 to 1 compression ratio. We would take that up the 8 or 9 to one. We used high octane Amoco Premium. It was the only unleaded high test fuel available at that time. The modified Zephyr put out 8 horse power and plained a 17 foot sail boat @ 20 MPH. I raced it in the 7-1/2 horse class on a small Aluma craft with an Oakland Johnson prop against Mercery KE4s, Scott Atwater 483s,. and Marten 60s. A Mercury usually came in second. . . 😆