wat the diff

[paradox]

sure every one knows what a joy it is to just change the diff without a lot of room or jacks. finally pulled the 3.9 lsd out and put a 3.5 back. but i'd like to put it back with an LSD.

[JDSTAZ45]

You will need to change the crown wheel and pinion on the LSD from 3.9 to 3.5 (if same size diff). Probably a $400-500 job at a diff repair shop!

As the 3.9 is more desirable to most, you better off selling it and buying a 3.5 LSD.

You would need to figure out which 3.9 LSD you have Large (shorter torque tube) or Small (longer torque tube)?? Small in 3.9 are extremely rare!

[Lummy]

3.9 and 3.54 both share an 11 tooth pinion, so as long as you're going small-small or large-large, fitting an LSD centre isn't a big job, just slip the old open centre out and the new LSD centre in. You may want a diff shop to check tolerances unless you're good with backlash.

I have a spare 3.9 "large" LSD centre if you're interested, PM me. I was going to keep it as a spare, but these things tend not to break, so I doubt I'll need it. It has been checked out by a diff shop and is 100%.

[JDSTAZ45]

3.9 and 3.54 both share an 11 tooth pinion, so as long as you're going small-small or large-large

Can someone confirm this????
From memory the 3.5 crown wheel had 39 teeth and the 3.9 had 41 teeth. How does one pinion cover both ratios??

Also, Airbats801 was selling a group of 3.9 ratios recently, both the crown wheel + pinion were sold together?? I'm confused :?

[Lummy]

It's 39 and 43.

39/11 = 3.545

43/11 = 3.909

But please, if anyone else can back this up, then please do so - I'm just running from memory of when I pulled a few of these apart a year or so back.

[redzone]

You should keep the crown & pinion set together as they're matched, unless you get the gears lapped..

[Lummy]

Yep, they are matched. That's why I say you may want a diff shop to check it out for you once you put it together - a lot of the time they're fine, but sometimes they need shims to match them up perfectly.

You can educate yourself on the lap-pattern needed (heaps of info available on the net), then check it yourself with coloured grease - if it's out, that's when I get the diff guys to adjust it.

[panda]

Found this article on Starquest, maybe of help.

Setting up Starion diffs
Pinion depth is adjusted with the thick spacer behind the large pinion bearing, it is specific to the housing NOT the pinion used , it's been a long time since I was thru Chrysler’s diff school class but I was told that the machining of pinions is to a VERY close tolerance, and if there was any variation from "0" it was stamped on the pinion face, where you see your numbers, I see no + or - numbers so I'd assume the depth adjustment to be "0" that would mean reuse the same spacer that is in the housing already, again assuming that it IS THE ONE belonging to that case housing.

Pinion bearing preload is set by use of the small shim spacers behind the smaller front bearing, too tight you need to add a slightly thicker shim, some diffs use a crush collar, Mitsubishi did not, do so carefully, a little goes a long way.

Also with the Mitsubishi LSD diff you need to keep the back lash to the low side as there is no way to preload the diff side gear bearings, they allow for the torque thrust during diff operation, meaning if the allowed back lash is .004 - .008," stay closer to the .004 " if making changes, the actual back lash will gain slightly during heavy torquing.

Also always replacing side gear bearings is not always necessary, good bearings are good bearings, however with out changing any shims you can gain a little less back lash by replacing the old side gear bearing on the ring gear side this will ever so slightly move the ring gear closer to the pinion lessening the back lash a thousands or more, new bearings on both sides should keep every thing exactly where it is now.

I’ve set up 3 diffs with 3:90's. I reused all old shims and spacers and back lash on all 3 were .004-.005". The only new bearings were the large pinion bearing due to having had to press the old ones off the old pinion to get the housing spacers.

This post has been edited by Shelby: Oct 2 2008, 11:15 PM

[Lummy]

Good info, cheers.