Stanley is correct #10 – 30 tpi. AT first I questioned this, thinking perhaps the slightly more common #12 – 28, but no! I checked my A-25, sure enough #10 – 30.
I have posted this in the past, but its worth repeating in the interest of not messing-up a nice motor. Back in the day when the American National Screw Standard was commonly in use there were screw sizes from # 2 to #30. Some of these remain in common usage today, like #6, 8 , 10. Others like #3,7, 9, 14 are now obsolete. As for thread pitch, most of the old number sized had three “standard” pitches associated with a given size. For example the #10 (0.189″) came in 24, 30 and 32 TPI. The #12 (0.216″) which we encounter on our old motors came in 20 and 24TPI. Number 14 (0.242″)- 18-20 and 24TPI. Number 16 (0.268″) – 16, 18, 20 TPI. Where one can get into trouble when encountering these old screws is not only the unusual pitch… like forcing a common hardware #10- 32 into that #10 – 30 hole. Likewise the size, note how dangerously close the old #14 and #16 sizes are to the common 1/4′ (0.250) screws, Want trouble? try a common 1/4 – 20 into a #14 – 20 hole in your prize outboard. When in doubt, the micrometer and thread pitch cage are your friends.
Joe B
