Mustang Rear Gear Swap Part 2: The Install

So, if you missed Part 1, The Tear Down, read that first, then come back for this, Part 2: The Install.  So, at the end of Part 1, we had removed the old ring and pinion.   Now we’re ready to install the Ford Racing 3.73 ring and pinion gear set from

Ford Racing, 3.73 gear set, gear swap, Mustang, SN95

First, I wanted to show you what the Ford Racing 3.73 Gear Set comes with.  Ring gear, check.  Pinion gear check.  Crush sleeve, nut and shims, check, check and check.  But remember, its SUPER DUPER HIGHLY recommended that you ditch this crush sleeve and get yourself the Shim Spacer Pack for easier preload setup.  You’ll thank yourself later, trust me.

Ford Racing, Mustang, pinion gear, calipers,

OK with Ford OEM/Racing gears i have found that they are pretty much ground on the same centerlines. So if you measure the pinion and shim after you’ve removed the pinion bearing and match the length to your new gears by adding the correct shim to equal the first measurement, you will be very close.  Now that being said, I always have to check the gear pattern and add or subtract shims to get the gear pattern correct. it is recomended to change the shim in .003″ increments. I ended up with a .027 ” shim.  I started with a .024″ shim.

If the gear pattern is “heavy” on the toe side of the gear (contact on the inside of ring gear) you need to add shim. If it is “heavy” toward the outside edge (heel of gear) you need to remove some thickness of the shim.  Now this is where having the pinion preload shim and spacer kit really makes life easy.  Crushing the the crush sleeve is a major gig and really takes a torque multiplier to do the job right and a serious bar tool with a flange to hold the pinion drive flange.  Check out my homemade tool shown in the picture below.  As you change the pinion shim you need to add or subtract from the preload shim pack to compensate for the pinion shim, as it will change the preload on the pinion bearing. 

These Ford Racing 3.73 gears have a lightened ring gear that should decrease 60ft times due to less rotating mass.

Ford Racing, 3.73 gear swap, Mustang how-to, SN95

So, now you will install the pinion gear with the pinion preload shim pack and spacer.   Tighten to 125 ft lbs.  Again, check out the homemade cheater bar that comes in handy (this is the 5th gear set for this Mustang, its had 3.08’s, 3.55’s, 3.90’s, 4.10’s and now the 3.73’s.  Not much else to try.  haha).

Ford Racing, 3.73 gear swap, Mustang, SN95

Once you have the shim pack dialed in, set the preload to 25 in-lbs for new bearings or 18 in-lbs for used bearings.  Dig the big ‘n’ little wheel/tire setups in the background.  Also peep the stock mufflers.  Connected to an off-road H pipe there aren’t many automotive sounds that are more music to my ears!

Ford Racing's, 3.73 gear set, torque spec, Mustang, SN95

Next you’ll bolt the gear onto the diff and torque ’em down.  Torque value depends on bolts you’re using.  These are ARPs.  They’re around 50-60 ft-lbs.

Ford Racing, 3.73 gear ratio, Mustang, SN95, hot rods, muscle car

Install the diff ring gear, put the shims back in.  It should have a tight fit preload.

ARP studs, Ford Racing, 3.73's, gear swap, Mustang, SN95, muscle car

Next step is to install the right shim. 

ARP bolts, Ford Racing, SN95 Mustang, gear swap how-to, muscle car

Then you can put the caps back on and torque them to 75 ft-lbs.

Ford Racing gear set, Mustang, SN95, muscle car, rear diff

Measure and adjust the side shims to achieve a backlash clearance of .008″ to .012″.

Mustang, SN95, muscle car, 3.73 gear swap, hot rod, 31-spline axles

Install the axle and re-install the C-clip.  Push the axle out to seat it.

 3.73's, Carbon clutches, limited slip diff, Mustang, SN95, muscle car

Here the axle is seated with the C-clip and you’re ready to install spider gear drive pin.

3.73s, Ford Racing, Mustang, SN95, Muscle Car, LSD, posi

There’s the drive pin.  Simply line it up with the hole in the diff.

3.73's, gear oil, LSD, posi-traction, Trac Lok, 8.8" rearend, Mustang, Ford Racing

Fasten the bolt and tighten down.  Now you’re ready to replace the diff cover.  Dont forget the gear oil!

gear pattern, LSD, posi-traction, Mustang, muscle car, Ford Racing, 3.73s

Not a great picture here with the flash, but this was to show you the gear pattern on the drive side.

gear pattern, LSD, posi-traction, Trak Lok, Mustang DIY, Ford Racing

And this is the gear pattern on the drive side again.  Use gear marking compound.  Yellow works better!

Well, there you have it.  Please ask questions or make comments below.  Thanks to my pops for supplying the Mustang, tools, pictures and commentary, oh ya, and the wrenching.  Nice work!  Pops might not know how to work a camera too good, but he sure knows his way around the garage.  Even if people don’t wanna venture into doing a gear swap themselves, this gives you a good idea of what goes on when doing a rear gear swap. 

Thanks to AmericanMuscle as well for sponsoring this How-To feature.

Hechtspeed, Mustangs, SN95, S197, hot rod parts

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