considerations when shortening a diff

[Shifty]

Hey guys

Just interested in any things that should be considered when shortening a diff.

Obviously straightness is a major issue - any particular pointers or tips here would be appreciated - eg where & what method is best to cut & join, if you can do it without sleeving, etc.

With the placement of leaf mounts, applying some logic then the mounts need to be the same distabce from the centrepoint of the axles (rather than the same height off the diff).

Brakes all look like they'll clear fine with the width I'm going to.

Anything else shortening-wise?

[HoonBoy]

You won't be able to do it straight, even the pros can't do it straight. It's all about getting it as close as possible and then using an oxy torch or other heat source to bend it straight.

[Shifty]

Interesting.

Obviously the further along the housing is cut, the better - the closer to the centre the cut is, the more any inaccuracy will be amplified.

Any other comments?

[9triton]

check gassed250 posts -think he did a shortening himself for his V8 sigma (...may be a link to another forum in his post ..)

[9triton]

check gassed250 posts -think he did a shortening himself for his V8 sigma (...may be a link to another forum in his post ..)

edit - here you go:
http://www.xfalcon.com/forum/showthread.php?31701-Gassed-s-Clevo-sigma

[PLAYA]

its easy, just cut it and sleeve it, but if you want it to be actually straight you will need something to put through the diff, a guy I know uses an old hydraulic ram with some spun up mounts where the bearings go. Bolts it in and then oxys it up till its straight. He said even sheet metal comp engineering housings are way off.

[Fondles]

Just interested in any things that should be considered when shortening a diff.



Remove the axles before you cut otherwise they might be a tad short to be of any further use.

[9triton]

its easy, just cut it and sleeve it, but if you want it to be actually straight you will need something to put through the diff, a guy I know uses an old hydraulic ram with some spun up mounts where the bearings go. Bolts it in and then oxys it up till its straight. He said even sheet metal comp engineering housings are way off.

and thats how gassed250man did it:

http://i796.photobucket.com/albums/yy248/Gassed250_album/SAM_0201.jpg

http://i796.photobucket.com/albums/yy248/Gassed250_album/SAM_0210.jpg?t=1271996032

[no1200]

ive shortened a couple of hilux diffs. Not really hard to shorten the housings and keep them straight, just need to weld them in order correctly. Hardest part I found was finding someone to accuratley recut the splines for me, and induction harden them.

[Gassed250]

and thats how gassed250man did it:

http://i796.photobucket.com/albums/yy248/Gassed250_album/SAM_0201.jpg

http://i796.photobucket.com/albums/yy248/Gassed250_album/SAM_0210.jpg?t=1271996032


yeah that was pretty easy to do all up cost including the diff was $320

the "housing bar" is a 2m long piece of 40x10mm hollow bar with some collars machined to the same dia as the bearing cups, i didnt need to oxy the housing or anything, I just made sure i could turn the bar as the housing was being welded

[Shifty]

Got any pics of how the tubes come apart from the housing? Apparently they're plug welded - grind back, drill out, what's the hotness? And then the tubes just slide off a sleeve? cut tubes down to required side, then slide back in & weld up?

[Gassed250]

Got any pics of how the tubes come apart from the housing? Apparently they're plug welded - grind back, drill out, what's the hotness? And then the tubes just slide off a sleeve? cut tubes down to required side, then slide back in & weld up?

didnt do any of that shit it was just a cut and shut job (no it wont break)

next time i'll cut the ends off and weld new axle flanges on

[dave1600]

http://spriso.com/content/products/mazda-rx-7-gsl-se-rear-end-swap/

[bigmuz]

Shifty- pay the man.

[no more wife]

If you have to post on the net to find out how to do it, I'd be giving it to someone like dan who does them for a living.

[jmac]

You could probably make a decent argument for cutting it close/at to the outer axle flange/tube intersect, shortening it and welding it there.

For one thing, *(if the flanges are 'right' then having it reattached there and getting it (for arguments sake) 1 degree crooked, it would't alter the axle centreline much. Alternately, if you remove and shorten and re-attach where the axle tube locates in the centre, well a few degrees out here would result in the tyre/axle tube skewed forward or rear by a lot more.

some diff shops can and do have to heat and 'bend' just a little after the welding is done, not necessarily any sign of a lack of skill (though ther are imbeciles out there who stuff it up and it requires a larger heat and rebend to straighten it.

You may have seen some 9" diffs where a profile cut length of RHS is welded across the rear of the housing and along each axle tube. This obviously stiffens the housing, and some level of warping would be practically inevitable. I have never actually tried doing that, but I would imagine that 'the go' in this case would be to heat and push the axle tubes a few mm forward first, and then cut and fit/weld the strengthening beam on there - in the hope that when it cools and shrinks it'll pull the axle tubes back to straight, and no more. I suppose it might take a few trial efforts to find out just how much to bend it before welding, so it will return to dead straight.

In all seriousness, it'd be hard to imagine (given startup costs) it'd be cost effective to even dream about resplining the axles yourself.

A quick note on diff specialists. Some of them are absolutely gold (and sometimes aren't even the dearest) but there are also some small percent of them that couldn't be trusted to pour piss out of a gumboot, if you told them the instructions were on the heel. One such example is those that shorten the diff so the new axles will be identical lengths. This is easier to do (they can often just shorten one side) but it will put the tailshaft and diff yoke well off centre). SO the ones that shorten it evenly, to maintain tailshaft angle etc, are a step up.

When it comes specifically to 9" diffs in mid 70s holdens, there's some more specific issues and since this is a common conversion, although not specifically relevant to this topic, are worth a quick discussion.

Added on top of that, some of the 'short cut' guys shorten to the same axle lengths, and use axles out of a commodore (I think around Vn era) - both from the same side of the car (i.e. two shorter ones) . This is problematic in many ways. First, they (i.e. the shonky ones) use a commodore stud pattern, for HQs. There's 0.6mm difference in the stud pattern. you can't usually 'spot' it by eye, and teh wheels will fit on ok, but this creates dangerous loads on teh studs, and can lead to them cracking and failing. And they'll likely fail when under the most load - ie some hard spirited driving, or even drag racing - about the last time you'd want it to happen. Now if the studs didn't break because of that, using axles from the same side of two donor cars - well that means one is now being fitted and after a 'lifetime' of only being heavily stressed in one direction of rotaton, it's now being stress the opposite way. Again potential for real danger there.

Point of all that being - if you aren't 100% sure, get someone reputable to do it, better a few dollars out of pocket rather than a few dollars saved but the part failing and doing 10 times as much damage/costs to repair.

[sv_i]

^ adding to what JM has said, I found when looking into getting a diff shortened many years ago, many diff shops could shorten the housings without a problem, but most (in melbourne anyway) were sending their axles to get shortened and resplined by the one guy.

Having said that, I've also seen some shocking conversions done, one that springs to mind was for an RX-3. Complete lack of ability to do simple maths = car crab walked down the road

I like that jig gassed.

ps. Don't forget to think about brake hard lines.

[Ben.]

i'm with jmac, cut the flange off and shorten it by cutting off the end off rather than cutting a chunk out of the middle, that way the axel tubes will stay straight as you basically dont touch em