Probably the most common of the mini gearboxes is the 4 speed rod change. I don't profess to know much about these gearboxes, but I know there were 5 speed versions available when I had my last Minis. 

I looked for one of these as soon as I started this project, but the cost of them is silly.

So in the spirit of "Because I can", I decided that I would make my own adapted gearbox. And given that there is so much space in the end of the gearbox, why not make it a 6 speed!

Most front wheel drive cars use a two shaft gearbox, but the mini box is the same as most rear wheel drive gearboxes being that it is a three shaft box.

That means that the research needs to start with rear wheel drive cars.

Cleaning the box

Before I had the crazy idea of modifying the gearbox, it was simply going to be rebuilt with all new bearings etc and cleaned. All the cases were stripped bare and it was sent away for vapour blasting. This uses a blast media supported in water to clean the aluminium back to near perfect. The cylinder head had also had the same treatment.

Ready to go

The gearbox is ready to go back together, all new bearings and shimmed correctly.

But its still standard, so will it cope with the power?

The car is not being built to be abused, I never intend to use it on a track, so it's unlikely to ever reach it's full potential. Based on this, I decided to stick with the original diff and not upgrade it. I also don't want to add straight cut gears to it, I want to be able to use my ears when I arrive at my destination.

So this is a back up, it is ready to use and wrapped up safe for future. But there are four other gearboxes here in pieces, and a plastic version too!

The BMW influence

Not everyone is a fan of the BMW mini, and some people also proudly sport he sticker "100% Mini, 0% BMW" on their classics. Well this one is different. Given that the head is BMW, and my background in working on BMW's, then it makes sense to make the gearbox a BMW.

I have obtained two 6 speed rear wheel drive gearboxes, they both coincidentally come from Z4s. The plan is to modify the BMW clusters to fit in the Mini casings.

The arrow shows where the Mini pinion is needed for the differential.

Will it fit.

Because I can, I will make it fit somehow. The problems seem to be:

  • Input shaft too long
  • Synchro hub too large in diameter to fit for 4th gear
  • Synchro hub for reverse being very close to end of gearbox, possibly even in the speedo housing.
  • Need to make room in the middle for the Mini pinion.
  • Layshaft is huge and is too far forward.

Back to the 3D design and printing.

The key elements must be

  • Input shaft must be in same location to pick up from drop gears
  • Mini pinion must line up with mini crown wheel
  • End case for flywheel must be able to bolt on as original as possible.

Anything else can be changed with a welder, grinder and machine shop.

Input shaft

The BMW input shaft has to be considerably shortened to fit the Mini box.

The splines had to be machined and then a 13/16" thread cut for the nut. 

Both bearing faces had to be machined to hold the bearings in place, albeit the smaller bearing is held in place by a circlip.

This took some work as all gearbox shafts are case hardened to ensure longevity and prevent wear. Now that this has been made, it will be case hardened again before assembly.