5th speed is an overdrive in all 5 speed boxes except for the KAD racing only box. This is also the only box which fits all the gears into the standard gearcase.
All that being an overdrive means is that it drives the FD pinion faster than the input shaft is turning, it's just that this has certain associated inefficiencies which means that gearbox losses will be higher in 5th than in top.
These boxes have a limit on their ability to cope with torque due to the relatively small bearings. 85 Lbft is meant to be the maximum I think but they have been successfully used on meatier engines than that.
I don't think there's a problem with the mounting of the overdrive gear as such, I believe there may have been a problem with the drive to the overdrive laygear in earlier Mini Spares boxes but this should corrected in the Evolution spec.
It's not like a seperate overdrive unit which you switch in and out, it's just that 5th gear is an overdrive ratio.
Edited by Dan, 29 April 2006 - 05:14 PM.