[MrForbes]

The planet carrier plate in the 56mm gearbox appears to be made of something other than tool steel, as it starts out about half as hard as the plate in the 42mm gearbox. It can probably be treated to result in a similar doubling of hardness.

I'm curious to know what you're doing with the planet gear pins, as I expect their strength characteristics, as well as the tightness of their press fit, will be affected by the hardening/annealing process.