thanks for the pics Wolverine. I did not start out designing this. I had a complete Wunderlich prerotator on my Mac powered Bensen. The lower unit set screw cracked out and allowed the cable housing to come out of the lower unit. The inner drive cable went through my prop. The prop lost about 3/4 of one blade and was at about 4000 rpm and full throttle. It was a very interesting ride for the next 15 seconds.
After rebuilding the engine mounts and a new prop. I was hand starting the Bensen blades. I realized the upper unit had a 3/8" square drive in it. I inserted a ratchet wrench into it. Hmmm perfect fit. So, I went to Lowes and purchased a 3/8" to hex shape drill adaptor. I used Jeff Tiptons drill motor and could get about 90 RRPM. I then went and purchased me a 18v drill at Lowes. That little drill motor would give me at least 5 rotations to 110 RRPM and when spun up, I just pitch it to the side of the runway or taxiway and go.
After the flight, I would retrieve my drill.
More than once, it raised some eyebrows at a fly-in!
With the upper unit and ring gear in place, you have a rotor brake also.