The Ute Diff Thread

[Gus The Snail]

Hey all, thought i'd start a Q and A thread for diffs. Im after some info on centre replacements, mainly info on the trutrack centre and IRS gearsets. I have an IRS from a 1996 VS ststo i wrecked, now i havent pulled it down yet, but its a 3 bolt, which apparently is M78. The diff i plan to rebuild, has an LSD centre, still in ok condition, but the gears are very noisy.

 

[Immortality]

Diffs, FAQ

diffs, all you need to know (updated) Seeing as many of these questions get asked multiple times I thought I’d put this together for future reference. history The Borg-Warner diff was first fitted to later model VK’s, it had a 2spider center with 25 spline axles. On the VL’s they fitted 4...

forums.justcommodores.com.au

 

[Gus The Snail]

I did do a search but i couldnt find the specific information i was chasing. After reading what you linked, it is saying the pinion will fit, but someone else is saying it wont. I was hoping someone had actually done what im hoping to do, to confirm it CAN actually be done.

 

[Immortality]

There is a huge misnomer with the naming of the diffs used in the 2nd gen Commodores.

All car manuals refer to the diffs as M78, this is both right and wrong. These diffs have an M78 centre ( 4 spider 28 spline) but use M75 gear sets. If you pull the cover off both diffs and check the gear sets they will both show M75 3.08 stamped on the outside edge of the ring gear. If you pull out the centre M78 will be cast into the housing.

The IRS centres can also be called M76 but again it's the same as the M78, the only difference I can see is that the side gears in the centre have an extra groove internally to retain the output hub shaft. I have put/used an IRS centre in a standard semi-trailing diff so there is no problem there.

The Gear sets in both diffs are identical (been M75) and are interchangeable. But note that you must keep the pinion and ring gear as a set.

The only 2nd gen Commodore to have an actual M78 diff was the HSV's fitted with the hydratrac diffs (that I'm aware off). These have proper M78 gear sets. The M78 gear set uses a different pinion and are not interchangeable. Now I'm told it is possible to swap these over grinding clearance in the housing but I've not seen it done so can't confirm.

Only Harrop supplies a genuine Truetrac centre (built under license from Eaton) but is very expensive. There are other brands available from suppliers like MACE which operate on the same principle.




Maybe I'll update my Diff FAQ with this info clarified.

 

[Gus The Snail]

So here is the centres, in exploded view, IRS on left and Solid on right. I'll take a photo of the stampings on them after lunch.

Apart from the centres ( IRS is open centre, from a 304 statesman, Solid axle is LSD centre from a previous IRS diff i dismantled- reusing my gearset in the ute solid axle housing ) there doesnt seem to visually be much difference.

 

[Deuce]

I used 3.45 gearset from a solid axle and put it in an irs diff reusing the irs centre.
I hope that answers some of the puzzle with 1st hand experience for you.
Immortality is always pretty bang on with diff info (esp 1st and 2nd gen commo) so you can trust what he said.