All gears are 2 mod i.e. the diameter of the gear (in mm), is 2 times the number of teeth.
Bottom shaft/axle is 15mm dia. Upper shaft is 12mm, same as motor shaft.
All gears from RS - same as HPC's range of budget gears.
The hub isn't fitted to the axle in these shots.

The upper bearing block has oversize holes to allow for adjustment of backlash.
The vertical line of four bolts on the left locate the pivot for the pneumatic cylinder .

Distortion of the metal around the pin drive on the final gear. Gives an idea of the forces involved.