Swapping diff internals and shimming

[Brian28]

Info from anybody who has done it/works in a place that rebuilds diffs please, I've already done a search but not found anything definitive. I have a 3.91 LSD and a 4.1 open diff both medium case, I need a 4.1 LSD. If I fit the 3.91 clutch pack into the 4.1 diff housing with the 4.1 crown wheel/pinion etc, will everything bolt back together with the correct clearances/tolerances if I use the original shims from the 4.1 housing? Thanks.

[GeoffBob]

First off, the big heavy thingy (with the planet and sun gears inside it) in the middle of the casing is called the differential (not the whole thing). Fit that into the casing and the whole lot is called a final drive.

My suggestion is you fit the crown-wheel and pinion from the 4.1:1 ratio final drive in place of the crown-wheel and pinion from the 3.91:1 ratio final drive. Retain the bearings fitted on the left and right side of the LSD with the original housing and you shouldn't need to re-shim (swap the LSD over into the casing that had the open diff in it and you will need to re-shim). Fitting the pinion gear from the 4.1:1 final drive is a little tricky (a new crush sleeve should be fitted) and will require that you clock the crown-wheel with a magnetic clock-guage for the backlash.

Short of me going into a detailed desciption of how to swap over the pinion gear, I suggest you lay your hands on the book High-Performance Differentials, Axles & Drivelines by Joe Palazzolo, available here



If the bearings are tired you might as well replace them (a bearing puller and hydraulic press will be required) in which case you will likely have to re-shim (unless you get lucky). You might also consider looking at replacing the clutch-packs in the LSD if they are poked (which they likely are). In other words, give the whole thing a thorough going over so that you don't have to take it apart again after only just putting it all back together.

Good luck

[N00b]

Any ideas of cost for overhauling an LSD?
I know this might be a "how high is up" type question, but just a ballpark figure would be helpful.

[Brian28]

Thanks Geoff, tho I was really hoping for an answer along the lines of "Yeah just stick it all back together and it'll be just fine" Will see if I can find anybody local and cheap to throw everything to so they can sort it, don't have the know how or patience to do it myself. Read an old post on it, involves building and stripping a few times trial and error to get everything right and using engineers blue, whatever that might be

As above, anybody got an idea of costs just to strip and reassemble the right bits into one case with new bearings?

[Rav335uk]

Give Gareth a shout, he might be able to sort it out fo you

[GeoffBob]

Thanks Geoff, tho I was really hoping for an answer along the lines of "Yeah just stick it all back together and it'll be just fine"

Sorry to disappoint Brian, but I would be lying if I told you that. Truth is the alignment of the pinion gear with the crown-wheel (just one example) has to be accurate to less than the thickness of a human hair. getting it wrong won't stop it from working, but will ensure that your final drive fails prematurely. Best take it to a pro if you don't know how to do it yourself.