An auto parts store should be able to provide you with a useable prop nut. Automotive nuts are usually heat treated to various levels of hardness depending on where & how they are used. Also the pitch diameters vary for the same reasons. I believe Insanemoondoggie is correct about #8. ALSO, make certain the thread is correct 1/4-28 versus 1/4-20 or 5/16-24 versus 5/16 - 18, or for that matter not metric, as they are close in size. A hardened nut of incorrect pitch when forced onto a th,d crank shaft will probably have you replacing the engine c/shaft.

Fredsedno