I am making progress on my Echo 65cc. I want to make the prop hub today. What size nut-stud is used on engines of this size by real manufacturers? IE Zenoah G-62. I was thinking 3/8 NF. 5/16 NF would save the bother of reaming prop holes but, is it big enough? 10mm seems popular.

The bolt on the Zenoah G-45 & 62 is 10mm X 1.25mm X 50mm or about 2" long

That's about half way between 3/8" and 7/16"

5/16-24 is what I have used on Echo 44 & 50cc engines. Same as BME uses also.

I would think the 3/8" would be more like it.

I know the 40's and 45's mostly have 5/16" -24 but that mighty small for a 60.

A lot of this depends on what the nose on the crankshaft has? That would be my judge as to size.

I went the same as the thread on the end of the crank. 10x1.25mm. I did not need to though. The last couple of these I copied the CRRC 40 system. It forces everythng to come out straight if you pay attention.

The internal hub taper section is a near 100% contact fit. The straight section of the hub that the fits over the cranksfaft is a push fit. The prop stud then fits into a push fit cylinderical hole on the other end of the hub. The stud is threaded on the inside and threads over the end of the crankshaft. The hub is sandwiched between the crank and the stud. If your hub blank is true in all dimensions and you dial it in when you turn it around in the lathe, the two sides should be super straight after you bore the holes. It is a lot of trouble but I hate shaky engines.