I have a 3.5 hp Briggs and it has a 3/4 crankshaft with a 3/8 thread tapped inside the end of the crank shaft. There is a keyway for 3/16 key, but the length of the shaft is a stubby 1.33 inches. I don't see how other than using a pulley it can be used.
Do all the centrifugal clutches and torque converters need more length to have them assembled onto the shaft than 1.33 inches. I know shaft extenders and are available, but are a bit pricy and they have an unusable area where they clamp over the existing shaft which then makes existing work I've done need to be further spaced from the crankcase wall.
Buying another crankshaft that would fit the engine could be much better idea, but no way would I buy a new part at the cost involved, I could instead sell the engine and just get another used one that is right for the job.
Meanwhile I use another slightly older engine 3hp (less 1/2 hp) also a Briggs and it is 5/8 shaft using belt drive Max Torque clutch with it on my bike. The jackshaft I have and the sheave get the ratio I like, but automatic converter and a bigger engine I have would like to see if can use it instead.
Looks like from this page on 30 series there is a driver that is 3/4" and the driven is 5/8". Since I use 5/8 jackshaft and that is where the driven could go I wanted to try that. See if I think that drawing shows where the 3/4" shaft is to be at least 2 an 7/8 inch from the diagram?
https://www.gokartsupply.com/asymapp.htm
MT