Gearbox - Close-Ratio Conversion - A+
As outlined in the 'Gearbox - standard production gearbox types' article, it is entirely possible to change the common 4-syncro gearboxes to the helical close-ratio set-up as used in the Cooper S, 1275GT, 1300GT, and some early MG Metros providing the right parts can be sourced. And you don't already un-knowingly have the close-ratio kit fitted! Part numbers are detailed at the end.
The other pre-requisite for this conversion is that it can only be applied to the early, pre-A+ type mainshaft gears. That's those that have the square-topped teeth as supposed to the distinctively pointed A+ profile. This is because despite being endowed with A+-type fittings for the mainshaft end and layshaft, the conversion gears have the pre-A+ tooth profile. This is not to say that the conversion cannot be put into an A+ gearbox casing - it can and is detailed below. What is essential to understand is that the pre-A+ second and third mainshaft gears are going to be needed. First gear is the same in all 4-syncro gearboxes irrespective of whether they're pre-A+ or A+.
It should also go without saying that doing this to an otherwise clapped out gearbox is a waste of time. Take the opportunity to give the unit a good over-haul and fitment of a centre-oil pick-up pipe. Utilise a good manual for general gearbox disassembly/reassembly method. Haynes is possibly the best as it's quite comprehensively detailed. Of course there is also my 'Mini Expert – Issue 1' magazine available from this site where I cover the stripping/re-building and parts wear assessment in far greater detail than you get in the Haynes manual.
When utilising the A+ first motion shaft and laygear, this is very straightforward. Simply exchanging the existing ones for the close-ratio versions is all that's required. Everything else remains the same.
If only the pre-A+ variants are available/sourced, then you will also need to use a pre-A+ mainshaft, pinion retaining nut, layshaft and centre-web layshaft thrust washer and relevant bearings. The mainshaft is required to provide the smaller 14mm first motion to main shaft support bearing, the pinion nut because the thread is different to the later types, the layshaft for the reduced carrier bearing sizes used and it's accompanying centre-web thrust washer for correct layshaft clearance/bearing retainer hole. Fitting the mainshaft is a simple swap for the pre-A+ one with no other fitting problems.
Fitting the laygear and layshaft is straightforward without any fettling or fixing required. It is imperative that the pre-A+ layshaft centre-web thrust washer be used. The A+ one does not locate the bearing properly, and would allow a certain amount of 'float' in the shaft to occur.
Re-assembly and shimming is as per the standard method/tolerances.