In the Super Astute, if the gearbox is all original, 10.5 maximum, maybe even a 13.5 for safety. Plastics do get brittle with age, and the Super Astute gearbox was never that brilliant originally anyway !
If you just want to run it around the yard, a 10.5 or 13.5 will be fine.
If you want to run it seriously, you need a few mods... New set of gears and the intermediate metal option idlers ( if you can find one ), metal idler holder instead of the plastic one that always breaks, and mod the spur to take 48dp rings. The SA gear diff is ok, as long as both drive wheels are in contact with the ground, but the moment one comes up, you lose all drive instantly....
The basic slipper cries in pain at any serious torque put through it, so dont bother. It cant handle it for long. Another reason for low torque motors...
There is a mod to use a Top Force idler to be able to use the old Astute ball diff in the SA gearbox, if you can make the slipper last
Thorpe did a few options for it, their ball diff was good, if you can find one.
I built a SA to use in our vintage series but couldnt get the gearbox to last very well even with the mods above, so i simply got an old Astute gearbox with no slipper and the old ball diff, and used that instead, much tougher - low speed controller settings and a easy thumb take place of the slipper
