BW78 diff gear backlash

[VK_3800]

I always find it interesting that people have problems with the collapsible spacers, I've never once had an issue with one that was properly installed. Remember if you go too far with the preload you cannot back the nut off - you have to start again with a new spacer.

Never adjust the pinion installed height for gear mesh or any other reason - this is set when the gears are lapped together and if you change it they will not mesh correctly. Pinions are generally marked for installed height, the figure is the offset from standard depth, e.g. if it was marked "+1" it would need to be 0.001" deeper. The simplest way is to compare the old and the new; compare the pinion that came out with the one going in and add/remove shims to the existing to make up for any difference. If its not marked or you don't know what was in the housing originally it gets a lot trickier.

Backlash is _only_ adjusted by sideways location of the carrier, but as above its really only a sanity check - tooth pattern is of primary importance. Don't forget to preload the carrier when turning it to check the tooth pattern.

Of all the Borg Warner (and later BTR-built) M75s etc that I've done, many have been quite accurately machined and don't require much if any shim changes.

Careful with the Max Ellery manual, it will give the general idea but don't rely on any specific details as they are quite often incorrect.

[TdracerTd]

Thanks guys, I feel reasonable well equipped now to have a bash at it. Lots of good information came out. I'm glad I asked I'll let you know how it goes.

[immortality]

My diff guy always fits solid spacers when rebuilding diffs.

I machined up one huge spacer at work and now only have to part it off to the correct length to suit. I also keep the carrier shims from any diffs we strip so we have a good collection. It's a lot easier with IRS equipped cars though. Just need to make a adaptor to suit the threaded bearing hubs.

[vlad01]

Yeah my future Vp project. plan is to get a trutrac center and maybe stronger axles. Also heard about a solid spacer, but definitely will get that done after my experience of pinon bearing failure related to the crush tube.

Any recommendation for aftermarket axles? I noticed the ones I saw were unbalanced unlike factory. (1 hole in flange instead of 1 big and 2 smaller counter weighting holes)

[Charlescrown]

Gotta throw my bit in here about the collapsible tube. I think we all agree what it's role is but think about it. Under acceleration/load the pinion is being pushed hard against the bearing which has nowhere to go except against the machined housing. Only on overrun is it trying to pull the pinion head into the crown-wheel and I would be very surprised if it will collapse at all under these conditions. I have found many diffs do get end float in the pinion but I would like to think it's caused by bearing wear not the spacer collapsing.

[delcowizzid]

when we were manual turbo with 4.11's we could kill the crush tube instantly on a test drive LOL .3.45's been in the diff since it was built never had the pinion out but have changed the centre to our spooled one been good for years.we called nz's best diff guy after a few faliures by our local diff builder and the diff guy said hes a good diff builder and sends all the local jobs there LOL its all in getting that pinion in at the correct height

[Charlescrown]

It doesn't make sense. How can it put load on the tube when the bearing takes all the load on acceleration. If that were the case even a std HSV should be able to destroy it fairly quickly. Pinion height is vital and the best diff people have their own jigs to set pinion height where others just go on the + or - on the pinion head. Many unwary don't know that a +2 means the pinion is oversize and you need to reduce the shim by 2thou.

[jenko]

a few on off throttles in low gear in race traffic is enough to do it

[vlad01]

Gotta throw my bit in here about the collapsible tube. I think we all agree what it's role is but think about it. Under acceleration/load the pinion is being pushed hard against the bearing which has nowhere to go except against the machined housing. Only on overrun is it trying to pull the pinion head into the crown-wheel and I would be very surprised if it will collapse at all under these conditions. I have found many diffs do get end float in the pinion but I would like to think it's caused by bearing wear not the spacer collapsing.

Im my experience overrun can be the most aggressive abuse the drive train can see.

In a manual with large TB and lots of compression, overrun in low gears is like hitting a brick wall. I've seen even rods getting bent in overrun. Not sure how but it happen in overrun lol.

One of my old boss's 4.1L Ford van with BW diffs, blew 4 diffs in a row. All blew on overrun down hill.

He has a 9" in it now and been good for 15 years.

[HQ355]

Yep 9" 's can take a heap of abuse, or so I've heard