Mustang Rear Gear Swap Part 2: The Install
June 9, 2010 by Hechtspeed
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 AmericanMuscle.com.
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.
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.
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).
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!
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.
Install the diff ring gear, put the shims back in. It should have a tight fit preload.
Next step is to install the right shim.
Then you can put the caps back on and torque them to 75 ft-lbs.
Measure and adjust the side shims to achieve a backlash clearance of .008″ to .012″.
Install the axle and re-install the C-clip. Push the axle out to seat it.
Here the axle is seated with the C-clip and you’re ready to install spider gear drive pin.
There’s the drive pin. Simply line it up with the hole in the diff.
Fasten the bolt and tighten down. Now you’re ready to replace the diff cover. Dont forget the gear oil!
Not a great picture here with the flash, but this was to show you the gear pattern on the drive side.
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
Just wanted to thank American Muscle for letting Chris and I do this how-to story. If any body has questions, post them here and I’ll try and answer them. As you can see I have a lot of mods done to the rear end to make it stronger and easier to work on. I highly recommend the 31 spline axles if your drag racing and using slicks. Those 4,800 rpm clutch drops are axle twisters ..dont ask how i know. The gurdle, main cap studs and the pinion spacer/shim pack is a must have. I’m sure American Muscle can provide these parts. They sell a do it yourself rearend kit too that comes with bearings and seals and a bigger assortment of shims to make adjustments with.
Swaaeeet i just got back from a test drive of the American Muscle 3.73 gears in my 95 stang. im happy to report the gears are smooth and very quite . im stoked got to love Ford Racing Gears . the rpm at 70mph was @2300, great street gear and it still had the punch from a slow roll to smoke the tires! oh i just wanter to add that i s American Muscle stocks the Eaton posi unit .thats what i have in mine and it is a very strong unit and works well. also another mod i have done is welding the axle tubes to the cast 3rd member housing ,along with beefing up the control arm mounting areas of the unibody . good luck with your stangs guys ,and give American Muscle your support they have great parts to mod your stang with .
So I have a question regarding the pinion preload. I think I’m understanding this correctly, but just want to verify. In order to check the preload, you put a torque wrench on the pinion nut, and it should reach approximately 25 in-lbs for new bearings or 18 in-lbs for used bearings before the pinion starts to spin? Is that correct? Thanks, great write up with lots of helpful pictures.
Hello there ,to answer your ques. yes use a needle type torque wrench and as you are turing the pinion you should see the needel in the ranges in the text. it feels really tight but this is how it should feel. and using a pinon spacer/shim kit makes this part of the job a lot easier than using the “crush Sleeve” that comes with the gear kit . do your self a big favor and order one of these shim kits from American Muscle. you might have to take it apart once or twice to add or take out a shim but the effort required to crush the sleeve is a lot harder than takeing the nut back off popping the pinion back out and changing a shim. i mearsure the old crush sleeve’s thickness and build my shim pack to that measurement to start with .that will get you in the ballpark . if you have any more Ques. feel free to ask. im very happy with the Ford Racing gears i got from American Muscle.