I'm in the process of converting one of the older Echo 40s that had points type ign in rear. Out of a back-pack blower. This is my largest cc convert to-date and have no idea of what diameter prop would be best match.

I have some 2" Alum bar stock to make the prop hub. What I would like to know is what diameter face should I plan on? I have MAS and APC 18" props so that diameter is not a problem, but it a Echo 40 cc , 3 ring piston. 7/16" diameter output shaft will swing a larger diameter, I need some guidelines for surface diameter contacting the rear of prop.

Thanks