Installation of a 7.5" Trak-lok Differential
 
 
This article describes a do-it-yourself installation of a trak-lok differential in a 2000 base Mustang.   The stock 3.27 gears were retained.   I'm no expert, and this article should not be taken as a "how-to".   There are several good articles on the net (linked below) that will walk you through the process better than I could.   I just want to pass on some info that may be of interest to other shade-tree mechanics who may be considering a similar installation.  Here are links to some of the articles I found helpful:

Ford 8.8" Ring and Pinion Installation Guide
Gear install in a Mustang 8.8" rear
Generic gear install with tooth pattern illustrations - 1
Generic gear install with tooth pattern illustrations - 2

By far my most useful resource was the 2000 Ford Workshop Information CD.   It contains step-by-step instructions with line drawings, tables indicating which shims to use to change backlash or preload by a given amount, and all the torque and other specifications you need to do the job.   You can find these CDs selling on ebay.com for about $10.

Shown above on the left is a 7.5" Trak-Lok differential ($194 shipped from www.gefracing.com) and on the right the same differential after installation of new bearings and races and the ring gear from my stock differential.   Installing bearings requires a big press.   I went to a local differential service shop to buy the bearings and races ($47) and they installed them along with the ring gear for $25.

  

I bought the machinists's dial indicator and magnetic base shown on the left for about $30 (Harbor Freight Tools.)   Here's a good link to info about dial indicators.   I also bought the basic ring & pinion installation kit shown above for $43.99 (Summit Racing P/N RAT-104K).   Since I did not touch the pinion gear, all I really needed out of this kit were the carrier shims (lower right) and the new ring gear bolts (upper right).

  

I already had a micrometer to measure the thickness of shims.   Ford uses a special tool to drive the shims into the carrier.   I made my own out of 1/2" birch plywood (above right).   I also picked up some gear marking compound and a brush ($3.50 Summit Racing)

Terms you need to know:

pinion depth
The distance between the pinion gear and the center of the ring gear.   It is adjusted by crushing a sleeve under the pinion bearing.  
backlash
The amount of play in the ring gear, determined by how close the ring gear is to the pinion gear   It is adjusted by putting different size shims on each side of the differential case to move the ring gear toward or away from the pinion. 
bearing preload
How tightly the bearings are pressed into their races when under no load.   It is increased by adding shims under the pinon bearing or on the sides of the carrier bearings.   Carrier and pinion bearing preload is needed to keep the pinion depth and backlash settings from changing when the differential is under load.  
ring gear runout
A measurement of how much the ring gear "wobbles" as it turns.   Runout should be close to zero.

The first thing I did after opening up the differential case was to practice measuring ring gear runout and backlash on the stock carrier.   The runout figure was good, but the backlash was at the low end of Ford's spec. (0.008" - 0.015", with 0.12" - 0.015" preferred).

After unhooking the parking brake cables, removing the rear brake calipers, pulling the pinion shaft and C-clips, and carefully removing the axles, I tried to pry the differential out of the case.   Ford uses a lot of bearing preload, and it was in there tight.     I found the the only way I could get it out was with my slide hammer (pictured above).   I put it through the pinion shaft hole, attached a nut on the end, and "whacked" the thing out.   You never know when old autobody tools will come in handy   :

I found that on my stock differential, Ford used only one fat shim on each side of the carrier (the two shims are shown above with the micrometer).   The one on the drivers side was 0.295" thick and the one on the passenger side was 0.265" thick.   I did not try to use these shims with the new carrier because they include so much bearing preload that they are almost impossible to get into the case.   It's easier to build up a number of shims and gradually add preload.

The results:

After playing with several combinations of shims, I managed to get the backlash figures into Ford's "preferred" zone of 0.12" - 0.015, while still keeping enough preload on the carrier bearings so that the differential is tight in the case.   The photo below shows the final gear pattern.   Reading gear contact patterns is an art, but this one looks pretty good to me.   In particular, the pinion depth (which I was concerned about) looks spot on.

Cost of this installation?

If you don't count the cost of tools or the supplies I had on hand (gear oil, silicone sealer, threadlocker), it came to $313.49.  Here's the breakdown for that figure:

ItemVendorCost
Trak-lok differential and friction modifierGene Evans Ford $194.00
Basic Ring & Pinion Installation Kit Summit Racing $  43.99
Gear marking compound Summit Racing $    3.50
Carrier bearings and races Western Differential $  47.00
Installation of bearings and ring gear Western Differential $  25.00

Driving impressions?

It's too early to say much other than the gears seem reasonably quiet.   There is a very faint whine.   I've only got about 10 miles on the new differential so far, and I'm going to take it easy for a while to let the gears adjust to the new backlash settings.

Update - August 2003:  I've now got 2,200 miles on the new rear.   The car feels no different when driving in a straight line, but it is very different in turns or when accelerating from a dead stop.   Lots more grip.   No more sitting with one wheel spinning on sand at an intersection.   The new rear is dead quiet up to about 50 mph, after which some gear whine can be heard.   That's probably due to my setting the backlash near the top of the range recommended by Ford.   That's the price I pay for doing it myself...  But I can live with it as is.


Last modified: 24 December 2003