Friday, March 20, 2015

Differential Rebuild

<This post is part of the 'Winter Project 2014/15 - (Gearbox Refurb)' project>

Having already stripped the differential unit down in this post, the final part of the project was to put it back together and reunite it with the gearbox. Obviously before it gets reassembled, it's very important to lay all the parts out for no reason other than they look nice.

Don't they look nice!

Video Guide
To help me through the rebuild, I found it very helpful to use some step by step videos by Mini Mania posted on YouTube. The first part covers problem areas to look out for, assembling the differential unit and then how to shim it up.

The second part covers the assembly of the speedo unit (which I've already sorted in a previous post), and then moves on to bolting the differential unit to the gearbox.


New Parts
The first thing to establish is what new parts are needed. After watching the video and doing a little more research on the internet, the main area to look out for is a worn differential pin and planet wheels.

In the post where I stripped the differential unit down, I established that both the differential pin and both planet wheels were too worn and would need replacing. While I was buying new ones, I also ordered two copper thrust washers and two new fibre washers. All the parts came from Mini Spares, except for the planet wheels which came from Guessworks.

New bits.

Differential Unit Rebuild
With a new fibre washer, the output shaft was loaded into the carrier unit.

Output shaft with new fibre washer

Carrier unit.

As the differential pin was slid through the carrier unit, the copper thrust washers, planet wheels and central thrust block were loaded into place. It was all quite a tight fit, but I suppose that's to be expected with new parts.

The thrust block itself had slight wear on it's original thrust faces, so when I put it back in, I rotated it round to make sure it was using different thrust faces against the output shafts. The old thrust faces are easy to spot from the circular wear marks. This is important as the thrust block not only keeps the planet wheels in place, but also keeps the two output shafts pressed tight to stop them moving in and out when in use.

Thrust Block.

As the pin reached the other side of the carrier, its notch was lined up with the hole in the carrier body to enable the locking pin to be hammered through.

Hole for the locking pin.

Pin in...

...and hammered flush.

With the other output shaft and fibre washer mounted in the crown wheel, it was bolted to the carrier unit.



Once the six bolts were torqued up on the crown wheel to the required 60lb ft, which is just over 81Nm, the lock tabs were bent into place.



Next the bearings were pressed onto the output shafts. They're a fairly tight interference fit, but were knocked into place with an appropriately sized deep socket and mallet.




Shimming up
In order to determine the correct shim size, the differential unit needs to be set in place on the gearbox. To do this, it's easier to tip the gearbox onto its front face and drop the differential unit in from above. After fitting with new oil seals, the two output shaft side covers were bolted in place with their gaskets using only the two bottom bolts.

New oil seals...

...and in place with the diff

Wedging a screwdriver between the crown wheel and the gearbox case, the whole differential unit was levered as far to the right as it would go.

Crank it over.

Then the gap on the left hand side was measured with a feeler gauge. I measured it to be 12 thou.

Measure the gap

The work was now put on hold while I ordered the correct shims from Mini Spares and waited for them to arrive. Unfortunately they were ordered on a Friday so had to wait over the weekend before they were delivered. When the shims did arrive though, I test fitted them to find they were a perfect fit.

No gaps!

Attaching to the gearbox
Now finally it was time to bolt everything together properly. This particular gearbox doesn't require a gasket for the differential housing, so after cleaning all the surfaces, RTV was used to create the seal. I used my fingers to dab a nice even spread over the gasket face.

Just RTV for the gasket

Dabbed around the gasket face.

After the output shaft side covers were removed again, the main differential housing (with a new oilseal) was lowered down into place over the differential unit and the bolts put in, but not tightened yet.



Before the right hand output shaft side cover could be bolted on, the 'shifter detent ball', spring, sleeve and oil seal were pushed in their hole.

 'shifter detent ball', spring, sleeve and oil seal.

Go in here...

... like this.

Next, the right hand output shaft cover was bolted on with it's gasket and a thin smear of RTV. It's important to leave the top three bolts loose at this stage.

RTV'd gasket

In place with the top three bolts left loose.

Using the shims that were sorted out earlier, the left hand output shaft cover was bolted on also, again leaving the top three bolts loose.

Shims in...

...cover on.

With the side covers on, all the bolts on the main differential cover were tightened down. I didn't use a specific torque setting, but just made sure they were tight. Once the differential housing was tight, the remaining side cover bolts were tightened up also. All the bolts for the differential housing and side covers in this post were bolted in using threadlock rather than spring washers.

Bending the lock tabs over on the main bolts of the differential housing completed the job and all that remained to do was to put the gearbox the right way up, stand back, and admire the glory.


Conclusions
This post marks the end of the gearbox refurb project and I have to say that it's been one of the best projects on the Mini so far. I have learnt so much from the project and gained a valuable insight into the inner workings of the gearbox.

I have to admit that I when started there was a shadow of doubt in my mind as to whether I was going to be able to pull this off. As it turned out it wasn't as difficult as I first thought it was going to be.

As much as I've loved this project though, I'm glad it's finished now so I can press on with the rest of the engine rebuild. That said, I do have another two spare gearboxes to refurbish and I have to admit I'd love to get stuck into them right now, but I'll have to resist and save them for later in the year as I also have an MOT to worry about!

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