Some important informations regarding drive wheel axle of the 1/6 scale Cromwell:
The shaft consists of an M8 threaded rod.
It is necessary to mill the flats on the threaded rod.
At least one groove on each axis is required.
Even hand milled with a Dremel will suffice.
Secure the screws on the bevel gear with the blue thread locker after final installation.
For the shaft I recommend material "steel 10.9"