At the end of the previous blog post, I mentioned I’d ordered a Torsen unit for the rear. That arrived, and I built up the old front diff with the Detroit Truetrac inside it. I went to swap it over with the rear differential, and unfortunately had two problems - firstly, I noticed the old threads were in terrible shape. Secondly, it wouldn’t go in.

Parts diagram

The bolts are part 12 in the diagram. The bevel pinion housing is 11, and the locker replaces parts 2, 5, 6, 7, 8, 9 and 10.

The threads situation had to come first, so I ordered some new differential bolts. Once those arrived, I tried to wind the old bolts out (into the axle case) but in the process I also noticed the securing wire around the perimeter was broken on both sides (there are two wires, one on each side of the cutout for the ring gear).

This is a picture off the internet showing the axle housing - you can see the two cutouts for the ring gear, and the bolts in situ around the perimeter.

Axle housing

Not only that, but I also noticed while breaking the bolts loose that two of them were bent, and badly enough that they would’ve damaged the threads in the axle case, which would’ve been very difficult to repair. I decided to just cut the old bolts short, so I’d only have to wind out the 10mm of thread already in the axle casing. This worked fine (about the only thing in the rear ATB installation that did), and I installed the new bolts.

I decided not to replace the locking wire, and use Loctite instead. They can’t back out once installed, since the nuts on the outside of the bevel pinion housing (part 37 on the diagram) hold them. The only time that’s risky is during installation, where it would be possible to wind the bolts further into the axle casing.

Once the new bolts were installed, I had another shot at installing the diff, and it still wouldn’t go in. It wasn’t even going as far as the studs, so it seemed like the bevel pinion housing was incompatible with the axle case. With this in mind, I rebuilt the Torsen unit into the old rear diff, figuring that the diff that came out would fit back in.

It didn’t.

In fact, it was exactly the same problem - something was getting caught up, and it felt like the ring gear was hitting the axle case. I did some research into the bevel pinion housing part numbers to check if the locker should’ve fit my housing. It turns out all three of the differentials I have (the two originals, and the one I bought that’s now in the front axle) have part number 219636, which is actually from a Rover P4. This is considered an upgrade, since it’s off a more powerful car and is stronger. The part that the manual says I should have is 528257, which is available but only second-hand from the UK. There’s also part number 219635, which is a late Series I diff. I believe either of these would likely be compatible with the Truetrac, but they’re not what I had.

I emailed Ashcroft to see if this was a known issue, and while they weren’t sure about my part number, they mentioned it was common that they had to grind down the bevel pinion housing to make things fit. That got me thinking, so I wrapped a layer of masking tape around the likely contact points on the differential to see if it was getting hung up on the housing itself.

Doing a test install and looking at where the tape got damaged, the problem was the corners of the head of the bolt protruding into the opening of the axle case! You can see a little bit of this in the axle casing picture above. So I ground the bolts down a little, and the diff could now get a lot further in.

The next problem was the bolts again. Whatever bent the bolts originally had also tweaked the axle case, so now two of the bolts stuck out from the casing at an angle at wouldn’t line up with the bevel pinion housing. I ran a bunch of nuts down the bolt and abused a socket and breaker bar to bend the casing back in line, and the diff slotted right in.

After that, it was the usual process of sealing things up, tightening it down, reinstalling the halfshafts, filling it with oil, and test-driving it. I’m happy with it, and now I have front and rear Torsens.

Apologies for the lack of photographs in this blog post, I simply forgot to take them as I was going.