R50 Midlands Transmission Rebuild Masterpost (Updated 1/9/18)

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01-09-2018, 09:51 PM

Tanner Stinson

2nd Gear

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Join Date: May 2017

Location: Vashon Island, Washington/Mesa, Arizona

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R50 Midlands Transmission Rebuild Masterpost (Updated 1/9/18)

The purpose of this article is to combine everything found,
throughout the entire internet about our transmission. Please feel free to comment anything else you would like to see added to the post! I will update this every so often, so please please please let me know what to add.

I will also be adding pictures here within the next few days. I currently have it all torn apart and am going to be putting it together within the next few days.
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What happens:
The "Midlands"/"R56" transmission has issues that cause it to blow up on itself.
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Symptoms:

The sound is like wheel bearing sound, but is noticeable only when in gear. There is a “balance” point where the noise sounds as if it is bouncing. This is because the busted bearing allows movement in the input shaft. This causes improper meshing of the gears and odd torque angles.
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Cause:
As stated before, this is from a bad bearing. On my transmission it was the input shaft, clutch side. The original was a ball bearing that was unsealed. I believe this is one of the main issues that caused my failure. However, there are multiple different possibilities of failure:

Improper head treating

According to one source, the bearing was "Hardened, but not treated" I.E. It went through a process that made it very hard, but in turn made it very brittle. Imagine a knife; You want it to be hard, but if you drop it and it fractures, what is the point?

Not possible to fill transmission to max

I can't find the page right now but I remember finding an article from someone saying that filling the transmission through the fill plug will not actually fill it to max. There are also areas that do not get lubricated enough because of blocked/tight passages.

Lack of thrust bearings/improper force vectoring

So here comes a bit of engineering. Transmissions gears are not cut "straight toothed," like your bicycle, to reduce noise. They also, unfortunatly, direct forces and torque in directions that aren't tangential to the gear (I.E, along the shaft). This forces the bearings to rub against themselves. Higher output engines have either tapered gears, like those in the differential, or have thrust bearings, which reduce the friction.

I personally think that a combination of all of these reasons causes the failure.

Teardown/Rebuild instructions

TO BE ADDED AT A FUTURE DATE

Expendable Part numbers and Sources:

This is the link to the MiniMania source for bearings

And this is the kit that I bought. If the link breaks, look up "Gearbox Parts online" on ebay and find the kit. I will update with a review once I get it installed and at mileage marks.

This is also a great source for part numbers and references. I haven't ordered anything from there, but I want to try it, and I probably will eventually.

Output shaft:
-Roller Bearing (Differential end):
---Part number: N.12680.S04.H100

-Ball Bearing W/ ring:
---Part number: AB.40087.S08
-----You can find these ranging from the $21-$100 price range, with shipping. It completely depends on where you are looking.

Input shaft:
-Input end:
---Part number: AB.40361.S01

-Support end:
---Part number: AB.12390.S01

-Output Shaft Lock Nut:
https://www.ecstuning.com/b-genuine-...t/23201037013/
Part number: 23201037013

-Input Shaft Lock Nut:
https://www.ecstuning.com/b-genuine-...t/23201037014/
Part number: 23201037014

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Preventatives:

I plan on swapping the fluid every 5k miles, roughly, to start. Once I am sure that there is nothing in the fluid, I will start swapping every 10k or 15k miles. I would rather spend $15 every couple months, than spend the money on Royal Purple or Redline and trust in it. But, again, this is personal preference.
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Sources:

Found on one of our sister forums. OP mentioned that he talked to a BMW tech who said that the Bearing was improperly heat treated:

Quote:

After 2 weeks I got it back and had a LONG LONG Talk with the mechanics at the BMW Dealership (Mini USA offered "Good Faith" discount of $500 after a $900 towing charge- IE: More expensive). The one mechanic worked for a Mini dealer in MD. He said his JOB was replacing transmissions, that's all he did (sound like there's a problem?). He said BMW knows the problem, knew of it when they put the Midlands boxes in the car but figured they would last past the warranty long enough to sell the cars and it worked we all got suckered into them. This is also why the 2006+ have 6spd Getrags (same ones used by GM on the SS Cobalt BTW).

He said the ONLY way to keep these from breaking is to change the oil at 7500 miles. Use synthetic and hope for the best. The problem is in the bearings on the shafts. He showed me on mine where it failed. The bearing shreds then the bits get jammed into the gears which causes them to break apart. There not double heat treated gears so they are brittle but strong. Double heat treating would give them some flex which may actually cause them to bend and break up faster.

I cannot confirm the SS Cobalt Getrag transmission theory. Something tells me that this is wrong, and it makes me not-so-sure about the article.

Found on our forum, posted by AutoCoarsen on page 15. Very good writeup including all of the part numbers, tools required, flaws, etc. Thank you very much if you see this, AutoCoarsen! Plenty of good pictures too, I followed his methods such as the needle nose vice grips for the clips.

Here is the MiniMania rebuild kit. It was too overpriced for my, being a college student, but if you want to do it right This is probably the best way to go. I have not cross referenced the part numbers to see if mine are the exact same model, but I would love to see. It also includes the bushings for the shift lever. Mine were fine and I did not need them, anyways.

Another great post on our forum about the process. Also includes great pictures and tips.

Last edited by Tanner Stinson; 01-10-2018 at 10:23 PM.

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