The Diff

Over the last few weeks I’ve not had a great deal of time to get stuck in on the Spitfire restoration. I’ve been bolting some of the suspension parts together but there’s not really anything I can do on the rear end without getting the diff done because the transverse leaf spring bolts to the diff. The vertical links bolt to the spring.

I’ve never rebuilt a diff before. Before I took the car off the road there was excessive backlash and it was beginning to whine quite loudly. I’ve done the gearbox a couple of times but had limited success due to lack of parts. Before I took the car off the road, synchromesh was struggling on all gears but 4th. I’ll be taking the gearbox to a specialist for reconditioning when the time comes as the cost of reconditioning is roughly equal to the cost of the parts, some of which I can’t source myself.

I want to have a go at the diff myself though as I can do it much cheaper than the cost of a reconditioned unit. It will be a challenge but it’s also one of the most interesting parts of the restoration. The Haynes manual is useless in this area so I bought a copy of the original British Leyland Workshop Manual which covers everything in excellent detail. There’s also quite a bit on YouTube, I found this video particularly helpful for understanding the principles. I’ve read the workshop manual several times as well so I think I have a pretty firm understanding of the process now.

The workshop manual calls for the use of a “dummy pinion” which is used to measure the difference between the pinion head and the carrier bearing bores in the diff casing. The average of these two measurements plus the thickness of the standard spacer theoretically gives you the size of the pinion spacer you’d need for correct pinion depth. The problem is the tool is no longer available but having asked around on the Triumph Experience forum this only really gives you a relatively rough starting point anyway. I will follow the manual as best as I can but it ultimately comes down to running a contact pattern check and making adjustments from that.

So far I’ve stripped the diff down and pulled some of the bearings. Next I’ll need to order some bearings, six in total. Some cheap varieties can be found on the internet by searching for the part numbers but I’m going for Timken bearings where I can as I think this is definitely an application that calls for quality ones. I’ve read some stories of people having issues with run out on the crown wheel/diff carrier with cheap bearings. There’s not a lot of room in the tolerance specifications. For example, crown wheel run out has to be within 0.08mm and crown wheel/pinion backlash is 0.127mm. The spec for the sun and planet gear backlash is zero. The thrust washers only come in one thickness having checked several suppliers so there’s not much choice there.

Bearing Puller

I’ve seen people fashion steel bars to hold the pinion in place so they can undo the nut. I didn’t have any steel lying around to do this so I used a couple of old bolts through the flange against the mounting bracket bolt heads. Seemed to do the trick.

While I’m at it I drilled and tapped a hole for a drain plug as there isn’t one on the Spitfire diff, only a filling plug on the side. The oil that was in the diff when I emptied it was black. OK for an engine where combustion is involved but not so good for something like a diff so occasional oil changes might be a good idea and a drain plug will make it a fairly quick job.

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