Back axle blues

Good old Lee - he'd brought some overalls just in case

Well, we had great plans the other day to fit a luvverly pair of specially shortened drive shafts into the rear axle of the Candidi Provocatores Allard, refit the axle and do the Testing Trial the following weekend but it didn't work out.

This was the view along the good shaft and - as you can see - it's twisted along the length of the keyway and probably wouldn't transmit power for much longer

We did a trial fit of the driveshafts and turning them in the diff it sounded "funny" - funny as in odd.

This is the end of the broken shaft

Bolting it up locked the diff and not in a good way.

A lot of head scratching followed but we were heartened by the arrival of one of our buds, Lee Peck, motorsport nutcase, engineer and all round good egg.

After more scrutiny, it would appear that the newly machined driveshafts are not a matched pair and that one of them has the gears machined into it at a slightly place. Consequently, the gears aren't engaging at the right point and lock up.

Lee suggested we spray the gears with paint, fit it all together and turn it (or try to) before examining where any bright spots developed. This proved the idea that the distance from the rear face of the bevel gear on one of the drive shafts was slightly too great.

This sort of variance is understandable when you think about the length of the production run involved with Ford V8 axles. they were also made in the US, the UK, France and Germany on a variety of machine tools and over decades.

The diff pinion wheels have these domed thrust washers to locate them properly

With the benefit of hindsight, we should have checked the replacement half shafts on the diff before we got them machined to make sure they engaged correctly.

But people with 20/20 hindsight only have their head up their arse anyway, so we don't listen to that sort of nonsense.

We might take it on as a learning point, though.

It probably wouldn't take a lot of machining to get it to fit but the process would involve assembling the diff and taking a tiny bit off at a time until the bright spots denoting tooth contact moved into the right place and we had a quiet diff with the correct engagement. This process of machining, assembling, checking, disassembly and more machining would take bloody ages.

The red gears on the driveshaft on the right are just a little bit too proud once fitted into the diff to engage with the crown wheel properly

However, the resulting shaft would be slightly weakened and would not stand up to the sort of treatment we have in mind for it. It would be alright for the more genteel motoring associated with a cosseted classic car but no good for competition

The preferred solution is to use a matched pair of shafts but that will involve narrowing them again.

So that's why we weren't at the Testing Trial.