Dar, might I suggest you invest in a somewhat later engineering hand book? Even my 1960 Caxton Engineers' Handbook doesn't list the 1/4x24 anymore. It was used in the early 1900's along with some other weird thread pitches like 27tpi (!) but their use was discouraged as far back as 1949 in the USA. The only exception to the normal 1/4-28 crankshaft that I know of is the old Merco 29/35 which uses a 1/4-26 BSF (Whitworth Fine). I have two of those engines and only one prop nut .

Oops, I see there were a couple more posts while I was typing this....agreed, you'll never find that 1/4-24 in your local hardware shop or probably in any engineering shop either. Have a look [link=http://www.sizes.com/tools/thread_american.htm]here[/link] Dar.