I have a 1990 f150 with a 3:55 open rear axle and want to put a LS rear can I just changeout the diff inside the rear with a LS unit and put my old 3:55 ring gear on the new diff?here is what i`m looking at
Ford Racing 8.8 in 31 Spline Traction - LOK Limited Slip Differential w/ Carbon Discs at AmericanMuscle.com - Free Shipping!

 

Yes, but you will need to reset the backlash and pinion depth.

 

depending on your planned useage, might consider a powertrax lockright. You would just take out the spider gears,and replace them with the lockright. No re-setting the ring, and pinion back-up. That is what I did on my 88 4x4.

 

One thing with a limited slip is that it has a clutch pack. Commonly known as a trac-lock. Those setups aren't as good as a tru-lock posi. Also, trac-lock Limited slips have to have the rear end fluid that's designed for clutch packs.

 

Well the powertrax looks good but I don`t go off road much and it looks alittle bit harsh for me.How do you set the pinion depth and backlash on these ford rears?

 

Using somewhat expensive and special tools and a lot of patience. Because if you do it wrong, it will fail in a very catastrophic way in a short time.

Upgrading to a limited slip is not a plug and play thing.

However, a powertrax does not require you to set up the gears again. If you are intending to do this yourself, skip the Ford unit and get a powertrax.

Unless the tracloc is brand new, expect to have to rebuild it. The clutches wear out and by the time you hit 100k miles, it's an open differential as the clutches are shot.

 

I also vote Powertrax. It's in my F-150 and my dad's El Camino.

 

Installing the powertrax took about 1.5hrs, or so. Not really to bad. There is a difference in the handling of the truck, but nothing bad to me.

 

aslong as you are using your old gears pinion depth shouldnt need to be reset. back lash is easy to set up with a cheap dial indicator with magntic base from harbor frieght will work fine. before initial disassembly measure the backlash in the gears because it has to be set within one thousandth of an inch or it will start to wear in a different place and factory gears have a thin heat treatment and will whip out the gears fast.

 

yeah I work with dial indicators at work all the time so thats not a problem and I`m using my old gears.I believe you just have shims on the right and left of the diff unit and adjust it either way for backlash I think.

 

(see below)

 

NOT QUITE! 3 things must be achieved:
1. The tooth "contact pattern" must be correct. This is checked by using dye on the tooth faces and rotating the installed gears. The pattern depends on BOTH the side-to-side adjustment of the RING GEAR, and the PINION DEPTH, how far in or out the pinion is located.

2. So, moving EITHER the pinion in or out, or the ring gear side to side, affects BOTH the BACKLASH (clearance between gear teeth) and the CONTACT PATTERN.

3. See, this ain't so simple! Getting it right, without proper special tools, especially a PINION DEPTH GAUGE, CAN be done, but it's tedious, and requires quite a bit of mechanical savvy or experience with gearsets. The process might require taking the pinion gear out several times to change SHIMS on it's stem, between the bearings; those shims control 3 THINGS: backlash, contact pattern, AND pinion bearing PRELOAD, which is mighty important. Changing pinion shims involves pulling the bearings off, and pressing them back on. If you go ahead, please let us know how it goes. If you get stuck, come back and ASK!

Not trying to scare you; just be aware that getting the gears set up haywire will quickly ruin them, giving you lots of headaches and expense.