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  1. #1
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    Default BTR M78 Diff question

    Hi guys or is it g'day?
    Anyway, I'm looking for information on the diff used in the 1990-93 UK/European Vauxhall/Opel Lotus Carlton/Omega. It's not Australian (although there are known to be at least two down under) and it's not a V8, but a few of us here in the UK are attempting DIY rebuilds and are scared we may be biting off more than we can chew. The diff is colloquially know here as the BTR diff, and I think versions of it are also used in TVR's and Morgans. It's an IRS diff with a 3.45 ratio LSD, and 28 spline driveshafts. Externally it looks like this :

    Internally the crown wheel is marked : 0578-029019 YZ 3 45:1 D

    So far I've only managed to get one of the carrier support screw-in bearing things out - I need a longer breakers bar for the other! On the inside the carrier bearing is marked 0576-015900 and 6007

    Unfortunatley the dog tags gave been replaced by Lotus, so the part number on them is a Lotus part number and therefore it's difficult to cross reference it.
    Can anyone here confirm or deny my conclusions, which are : Its a BW/GM M78 diff with a cone LSD and 3.45 ratio drive? We have a supposed service manual, and the parts explosion is shown below :

    If I ever manage to get the other drive shaft screw retainer out then I'll be wanting new parts - the pinion stuff (1,3,4,5 & 8), and all the driveshaft stuff (10/12/13/14/16). Any ideas where to get them from and part numbers? I'd like Timkem or similar since we can probably get them here, r at last from eBay

    And if the crown/pinion need replaceing, are there any options for new 3.45 ratio sets?

    Cheers
    Malcolm

  2. #2
    Part of the furniture Deuce.'s Avatar
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    I have a 94 commodore with IRS and partiality rebuilt my diff while adding 3.45 gearing.
    That breakdown looks spot on to my diff (m78). I will be very surprised if parts were not interchangeable between our versions.

  3. #3
    Sure why not? 76lxhatch's Avatar
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    Although a common misconception, very few Commodores ever got M78 diffs, they were mostly M75 with an ever so slightly smaller crownwheel (where yours shows 0578 on the crownwheel, these show 0575). However the two do share exactly the same centre so many of the parts are useful.

    1 (pinion nut), 3 (pinion seal), 4 (front pinion bearing), 5 (collapsible spacer) will likely interchange with Commodore parts, 8 (rear pinion bearing) will not as its larger in M78.

    The remaining driveshaft parts should be the same as IRS Commodore assuming there are no oddball differences in the main housing or stub axles.

    All but the lowest spec Falcons got M78 diffs from EB series II to AU (lowest spec ones still M75), then low spec BA and BF are M78 but most are M86. Most of the pre-BA models are solid axle rather than IRS, but there are some.

  4. #4
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    VR Commodore with IRS was the M76, only some VY's got an M78

    M76 was the IRS version of the M75 and the time period is right for this to be an M76.

    Crownwheel diameter will be 7.5" or 190.5 mm for M75/76
    Crownwheel diameter will be 7.75" or 196.85mm for the M78

  5. #5
    Part of the furniture Deuce.'s Avatar
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    Well there you go. Good info 76lx and hq368.
    LC if you need anything easy measured, I have a spare 'M76' in my parts shed. Happy to help as required.

  6. #6
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    Thanks guys,
    We're fairly sure its a 7.75" crown wheel, but can't actually measure it till it's out.

    Two of us have our diffs in bits so far, and neither of us can get the passenger side (other side to the filler plug) carrier retainer out. I've made a reasonable extraction tool :



    And I've tried heating (perhaps 100 degC) but so far all I've done is bend two 1/2" breakers bars (ok extended to about 2m with a scaffold pole!), and rip the surface of the workbench that I'd secured/bolted the diff to.

    Any tips on how to get them out?

  7. #7
    Sure why not? 76lxhatch's Avatar
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    Quote Originally Posted by HQ 368 View Post
    M76 was the IRS version of the M75
    M76 is a slightly different version again, the IRS version of the M75 is an M75 (but the centre, as shared with other variations, has circlip grooves in the side gears).

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    Quote Originally Posted by 76lxhatch View Post
    M76 is a slightly different version again, the IRS version of the M75 is an M75 (but the centre, as shared with other variations, has circlip grooves in the side gears).
    You might be right but I have never found that in practice or mentioned in any of the material I have received. NZ may have some odd ball things going on.
    All the factory manuals and production information I have been fortunate enough to receive from Albury denotes that M76 is IRS only with the bulk of parts being identical to M75, side gears, bearing retainers, housing and a few other parts being specific for M76.

    Supposedly M75 is beam axle exclusively.

    That's just from the factory literature, you may be able to download some of the info from Motospecs / Drivetech or if you can view their online portal will have some info.

  9. #9
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    Thanks guys,

    I posted a reply yesterday but it appears to have disappeared into the ether-web never to be seen again.

    We're pretty sure it's a 7.75" crown wheel, but at the moment I can't get one of the screw retainers out so it's difficult to measure accurately. I've built a good socket tool, but can't exert enough torques on the thing to budge it. So far I've bent two breakers bars, and ripped the top of the workbench I fixed it to.

    Any tips for getting the screw in carrier retainers out ?

    Cheers
    Malcolm

  10. #10
    Sure why not? 76lxhatch's Avatar
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    Its not NZ specific info I'm referring to but M75/76/78/80/86 only specifies the gear set anyway (and specifies the crownwheel diameter in inches as you mentioned). Many other parts interchange but are also different for the same gear set (e.g. 2 vs 4 spider centres in the Commodores, all M75) so I'm not really sure how you're supposed to reliably identify a whole assembly especially with all the different housings. According to the BTR parts list I have there is no M76 in Falcon/Territory or Commodore/Ute/Crewman; you'll note that the crownwheels will be marked 0575 as shown in the above photos for IRS models as well.

    BTR started making these after Borg Warner stopped too, so there are multiple different manufacturers to add to the confusion. Valiants and older Falcons used the same family of diffs well before they made an appearance in Commodores, the listing even shows M80 used in later MG and Morgan...

    One interesting thing I noticed is that some gears are hobbed while others are milled, potentially that could account for the minor difference between 75 and 76?

  11. #11
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    Sorry to hijack the tread: So would you be able to use a M76 centre in a M75 late model VK using the std crown wheel & pinion with 28 spline axles????

    Just that I see on eBay a BW M75 series 28 spline "Torque Worm" centre for sale "M80 LSD & adapter to suit M75 axle"???? Or would there be pinion spacing issues???

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    M80 center has a different offset for the ring gear than the m75/76/78.

    If by "adaptor " they mean a shim to properly space smaller m75 ring gear on the m80 Centre than it may be possible.

    Somewhere in a thread I posted pics of a m80 eaton truTrac next to a m78 Harrop trutrac which should show the different offsets.
    Last edited by immortality; 22-09-2018 at 01:22 AM.

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    Quote Originally Posted by 76lxhatch View Post
    Its not NZ specific info I'm referring to but M75/76/78/80/86 only specifies the gear set anyway (and specifies the crownwheel diameter in inches as you mentioned). Many other parts interchange but are also different for the same gear set (e.g. 2 vs 4 spider centres in the Commodores, all M75) so I'm not really sure how you're supposed to reliably identify a whole assembly especially with all the different housings. ?
    I don't think you can. As someone recently showed me the centres also m numbers cast on them. The Centre from our Calais has m80 cast into it and the Centre out of my old VN has m78 cast into it. Unfortunately I no longer have an IRS Centre to confirm what is cast on that.

  14. #14
    Sure why not? 76lxhatch's Avatar
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    Quote Originally Posted by Lacksballs View Post
    Sorry to hijack the tread: So would you be able to use a M76 centre in a M75 late model VK using the std crown wheel & pinion with 28 spline axles????
    Assuming you are talking about a VP-VS (and some VT) M75 IRS diff, then yes. The difficulty is getting 28 spline axles in the appropriate length to suit - note that the VK 1/2 diff has different axle lengths to VL (the centre offset is different).

    Quote Originally Posted by Lacksballs View Post
    Just that I see on eBay a BW M75 series 28 spline "Torque Worm" centre for sale "M80 LSD & adapter to suit M75 axle"???? Or would there be pinion spacing issues???
    28 spline axles are generally interchangeable, but an M80 centre would require M80 gears and they definitely won't fit your M75 housing. As you can see there are issues with naming conventions not matching the gears and lots of rumours from the school holiday crowd so eBay is going to be even less reliable.

    Quote Originally Posted by immortality View Post
    As someone recently showed me the centres also m numbers cast on them. The Centre from our Calais has m80 cast into it and the Centre out of my old VN has m78 cast into it.
    Where are those castings? I'll have to have a look at the ones I have here, maybe that's the reason people think they have M78 gears in their Commodores? If you want to guarantee you have the right parts the crownwheel markings are the clincher, once you know what the gears are you know which centres will fit and you just have to choose IRS or not.

  15. #15
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    My replies/additions appear to be being moderated - Test to see if this 'puter works.

    Ahh, that worked - strange. Anyhow thanks for the answers guys. I finally got the other carrier screw in shaft out this arvo. Took a 3m scaffold pole on a 3/4" breakers bar, and me swinging my full weight on the end (> 80 kg). I thought I was applying enough force to stop the earth spinning on it's axis and then it finally went with a very (VERY) loud bang and I thought I'd broken something. Thankfully not so I've now got the carrier out, and can get some pictures of everything.

    One thing I can confirm now though - the crown wheel is 196mm diameter, which is 7.75".
    Last edited by LC0112G; 22-09-2018 at 08:36 AM.

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