Gearbox - Close-Ratio Conversion - Pre-A+ gearbox
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 in the relevant article. 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.
The following can be applied to either the remote or rod type gearboxes.
When utilising the pre-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 A+ variants are available/sourced, then you will also need to use an A+ mainshaft, pinion retaining nut, layshaft and layshaft centre-web thrust washer, and relevant bearings. The mainshaft is required to provide the larger 18mm first motion to main shaft support bearing, the pinion nut because the thread is different to the earlier types, the layshaft for the increased 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. The layshaft is the bugbear here.
Although the two ends are the same diameters as the pre-A+ item, it has an extra step on the first gear end to accommodate the bigger, single needle roller bearing used. The A+ casings have a recess around the bore in the centre web that carries the first gear end of the layshaft, the pre-A+ ones don't. This prevents the A+ layshaft passing through the web far enough to get the locating plate in its relevant slot on the other side. It is not caused by the smaller layshaft thrust washer as believed by some. There are two solutions to this problem.
Either grind the locating plate slot wider by 0.375" towards the end of the layshaft, or grind the first step on the layshaft back 0.375". I would recommend the latter as it's generally easier to do, and doesn't compromise shaft strength at all. The other way brings the slot perilously close to the end of the shaft! And the shaft will have to be ground - it's pretty hard. If grinding the step back, make sure a slight radius is left in the new 'corner' to avoid a stress riser. I finish polish the rad with aluminium-oxide abrasive tape. Wet and Dry paper will do, something in the order of 400 to 600 grit. Doing either of these fixes will not cause bearing lubrication problems, but I would advise properly de-burring the oil feed holes on the layshaft/bearing surface area.
Re-assembly and shimming is as per the standard method/tolerances.