A subject worthy of an entire book to explain the whys and wherefores, and also causes much consternation on the part of the transplanter.

Terminology -
BBU - Big Bore Unit
SBU - Small Bore Unit
FD - Final Drive (diff ratio)

A subject worthy of an entire book to explain the whys and wherefores, and also causes much consternation on the part of the transplanter. Much confusion’s spread over which gearbox has the best ratios, is best to use, and with which FD. For a detailed account on this and covering all gearboxes fitted as standard to the Mini, see the relevant separate articles 'Gearbox - Standard production gearbox types'.

Fortunately, when selecting a complete engine and gearbox unit from a Metro, the gearbox will pretty much suit the engine and be easily transplantable whole into the Mini. It shares exactly the same rod-change linkage as the Mini. Difficulties only arise when fitting a rod-change gearbox into a pre-1973 Mini that has the old remote-type gear-change mechanism, but not insurmountable. It's dealt with in the 'Ancillaries' appendage to this series.

Practically all Metros had a 3.44 FD including the MG, the only oddballs were the Turbo that had a 3.2, and rare ‘economy’ models that had a 3.1. The 3.44 FD gives the best all- round performance - that’s why John Cooper fitted it as part of his performance package on the late 998 Coopers. The 3.2 is a bit neither here nor there. The 3.1 being good for long distance work and on BBUs tuned for better mid-range, and the same as that fitted in all A+ engined Minis from 1980 onwards. In fact the whole gearbox from the Metro is exactly the same as that fitted to the A+ powered Mini - only crown-wheel and pinions were changed to achieve the different FDs.

Rod-change Minis were fitted with a 3.44 FD up until 1980, when they went to a 3.1. The only exception here was the City E and Mini 25, endowed with a 2.95 - absolutely killed the poor 998 engine’s meager performance stone dead!! Mind you, the French Minis were fitted with a 2.76 FD (as fitted to the very latest Mini)!!

The only way to be 100% certain of the FD ratio if the exact origin isn’t known is to count the teeth on the crown-wheel (big one in diff housing) then divide this by the number of teeth on the pinion (small gear on end of mainshaft by the big retaining nut where the speedo drive spindle is connected).

Ratio-wise, the A+ gearbox used in the Metro isn’t quite as good as the old pre-A+ rod-change set-up. First gear is lowered to cope with the taller FD, and there’s a bigger gap (rpm drop) between second and third. Much of this was due to the utilitarian application to the whole A-series front-wheel-drive range. Identification is easy, the A+ gears have a markedly pointed profile whereas the earlier ones are square topped (blunt). In fact the early type is the same as those first used at the inception of the four-syncro gearbox!! It's also the exact same set-up used in all the A+ Minis and the A+-powered Allegro. See what I mean?

Austin 1300s/AAs and 1300GTs used a remote-type gearbox. The main difference is the horrendous rubber/metal sandwich plate between the remote housing and the back of the diff housing. Made for quite a gooey gear-change once a few miles old. These will fit in place of the Mini remote type, although the rear mount is different. My advice here is to change it to the Mini mounting by drilling/tapping the two holes necessary to carry the Mini remote mounting. Trying to alter the bracket to take the Austin 1300 one is more of a job! Getting a remote-housing type gearbox into a rod-change shell a worse one, taking a fair bit of fabricating and fettling. Internally they use the same gears as the old Mini - square-tipped teeth with the same ratios - except the 1300GT. This used the S/1275GT close-ratio set-up employing the 18-tooth first motion shaft and 22G1040 laygear. FDs-wise, you could get anything from a 3.76 to a 4.1. Best to double-check by the tooth-counting method!

Crown wheel and pinion ratios data;
Tooth count -

Note:
A+ Pinions are flat on both sides - Pre A+ have ridges.
Although some ratios have the same tooth count on either
gear they are different angles and part numbers and therefore
not interchangeable.

Useful part numbers:
Remote housing rear rubber mount 21A956
Remote housing rear bracket 21A745