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I know the gearbox case and diff cover are a matched pair but I built up a gearbox for a friend a couple of months ago with supplied bits . When it came to the diff I pointed out to him they should be matched as that's what I've always been led to believe . He told me the diff cover was off another box that had the case damaged and that he had swapped diff casings before and had no issues . There was a slight step in the casings on the side . Where the diff bearings run seemed ok . On his instructions and my warnings not heeded I built the box for him . Fast forward five months of driving and not one problem , no oil leeks , no diff locking up , in fact no problems at all .... So is he lucky ? Or is the matched pair thing not as important as we are led to believe .... Ken

I've been forced to do the same from time to time, though I don't like to.

When I've done it, First I check the tunnels and maybe try a few caps to get one that may line up, if not, then I remove the dowels and try again. The bearings will 'dowel' the two halves together much better than the dowels will in any case, just need to be sure that the holes for the studs (or bolts) have enough clearance.

After getting the tunnels sorted ^^ I 'lick' the sides to square it all up and get them on the same plane again;-



(That was a special diff cap, but we've done the same for stock types too)

It's surprising just how many we find that are out of square.

In regards to the side step, I'd suggest that you've been a bit on the lucky side, though how big was the step?

What your friend may find in time is that it may cause the output flange bushes to wear prematurely as the output flanges are no longer square.

Spider wrote:The bearings will 'dowel' the two halves together
much better than the dowels will in any case<SNIP>

I'd never thought of that myself, but have previously considered
how round the resultant hole would be, for example, is the
split-line exactly half way through the circle, less any allowance
for a gasket? ...

We could end up with a sort or "sideways-figure-of-eight" shape,
or otherwise a "vertical-rugby-ball" shape. (Much exaggerated,
of course...)

My thoughts here would be these:
1) Hole is narrower than it is high: Clamp up and line-bore, taking the
metal from the cover side, NOT the gearbox. Machining the gearbox
may mess up the meshing of the crown-wheel and pinion.

2) Hole is wider than it is high: Face off either the 'box or the cover,
or a bit off both dependent on what is needed to make the hole
round again.

3) Don't forget to allow for the thickness of the gasket.

4) As Spider suggests, sort out the flange-faces, or
"double-gasket" the low side.

Ian

As regards to the side step , I never measured it so can only say it was just enough to feel a difference in the two . As you say time may tell on this one if he develops premature wear of any sort . Thanks for answer . .... Has anyone else any experience of changing a diff cover from one box to another ? .... Ken

ianh1968 wrote: I'd never thought of that myself, but have previously considered
how round the resultant hole would be, for example, is the
split-line exactly half way through the circle, less any allowance
for a gasket? ...

I have always check this as a matter of course on just about every gearbox that I have done and they are surprisingly good actually, certainly well within gasket take up. If you are ever able to measure the centre to centre distance of a few cases I think this aspect would be the least of your concerns!

Most of the Gearbox I tend to do these days are Rod Shift types and one needs to be careful when doing any A+ Gearbox as some used gaskets between the diff cap faces and some did not. This seems to be something not widely known about. Those that did not, the factory recommends Silicon Sealant for. Further, the only way to pick whether it is Gasketed or Gasket-less is to dry assemble with and without gaskets and measure the tunnel. Nice of the factory to mark these cases in some way to let us know which were and which were not! This is a trap for unsuspecting players!! I have had a few through here that were gasket-less, however had been previously overhauled and fitted with gaskets. Usually there are no adverse problems, however I have had two in which the Bearing (on the Crownwheel side, which is the loaded side) had been 'walking' and so did do some damage to the cases. I was able to 'correct' these but it does leave a less than satisfactory gearbox case.

I'm in the process of doing this to a '190 box without a diff housing. As Spider has pointed out the two dowels hold the 'wrong' diff cover out of position. The width of the gearbox casing and the diff cover are the same so removing the dowels allows the diff cover to align better. I've made a jig which locates on the bearing diameters and the two side cover faces, I've then machined out the dowel pin holes oversize in the gearbox casing and fitted a stepped dowel, the jig is then transferred to the diff cover and the dowel pin hole is machined out and a sleeve is fitted to take the stepped dowel. This should locate the diff cover correctly and then its just a matter of checking if the bearing diameter is within tolerance.
Did it work? You'll have to ask me after Christmas when I might have finished it

Impressive!!!!

I've swapped covers a couple of times, but then I've had ten casings to choose from (1100/1300) cases. I've had satisfactory reseults with playing around and measuring, but I'd NEVER use one with a step.
Machining the sides is a good idea anyway, as this will require less shims

When the next 1128 blows up, I've got a load of ADO16 cases ready for transformation.

Another cracking engineering solution Bob!

I feel a little let down that 'bobs impressive tool' has 'top' written on it with a sharpie

something as impressive as that should be either engraved or have a little screen printed brass tag

Nice aproach 1071bob.

There's a bit of work in that jig. Is it settable to use on any gearbox case?

I actually like the sharpie touch, it makes it feel more human

When I made that 'special' diff cap, I plotted off the original with the DRO on the Mill, then did a dummy on some sheet metal. I could have likewise plotted from the gearcase too, but then it would have been mirrored and I'm sure i would have stuffed that up!

That's great BOB . Now were getting technical I feel I should have mentioned that when I used the diff cover from another box I did indeed take out the dowels and probably a bit of crude engineering , did file them down on one side as to move the cover over to were it felt it lined up . I then bolted the side casings on without the diff unit in place . Then I tightened up the diff casing , then removed the side casings to see if there was a step ...... Yes there was a step but you could only just feel it . Maybe this step is small enough to get taken up in the gasket/ hylomer thickness . When the diff unit is in place there's so much play in the output shafts through the bearings anyway , so would a casing with say a two thou step in the casings make any difference at all in the running of the diff unit . I'm kind of now thinking it's not so important and as long as it's not leaking oil there must be some tolerance in there as far as the diff unit and output shafts are concerned . Could that slight step in the casings magnify itself into the diff running not true . My personal opinion is that it won't because there's plenty of play in the bearings / diff pin / planet wheels etc anyway . ...... Maybe I've doubled the price of gearbox casings on eBay now that don't have a diff casing ..... Ken

My innocent approach here (feeler, snap and dti gauges), all aligned perfectly so I went for it: http://rsn-mk1.blogspot.pt/2013/04/dont ... -home.html

Not a problems so far (1.5 years).