[MrForbes]

The shaft is 3/8" hex, with the ends turned to fit into the 3/8" ID bearings. The older FP first stage gear happens to have a 3/8" hex hole in it.

I was thinking we might step it down to 5/16" where it enters the 1/2" shaft, or possibly make a larger diameter coupler to fit over both shafts (for example, by turning the end of the 1/2" shaft to 3/8", and using a short length of 1/2" od 3/8" id steel tube)