Installing New Gears in Chrysler’s 8.25-Inch Rear End
Mike Aguilar
Click Here to Begin
Unless you’ve got a car like a Challenger or Charger SRT, your car came with rear gears that are a compromise between performance and economy. This is great if you’re not planning on doing any form of racing with your car. However, if you’re building a strip burner, autocross car, or any other type of race car, you’re going to need to install better gears in the rear end. If the rear end is making lots of noise as you drive, you’re also going to need new gears. Let’s see how to do this in Chrysler’s 8.25-inch rear end, found in a large number of Chrysler/Mopar vehicles dating back to the late Sixties.
You’re going to need a mix of ordinary hand tools and some specialty tools for this project. You should be able to rent or borrow the specialty tools from places like O’Reilly Auto Parts and Autozone.
• Jack and two jack stands
• Lug wrench or air impact gun
• Socket set
• Ratchet and extensions
• Wrench set
• Breaker bar
• Slide hammer
• Axle puller attachment
• Torque wrench that goes up to 175 lb-ft
• Beam-type torque wrench calibrate in inch-pounds
• Gasket scraper cleaner
• Barrel sander or grinder with drill or die grinder
• New gear set
• 80/90W gear oil
• Gear oil additive if required
• Differential cover gasket
• Screwdriver or small prybar
• Catch pan
• Shop rags or towels
• Pinion yoke tool or large pipe wrench
• Small cold chisel
• Pinion bearing puller tool
• Small (3/8-inch diameter) punch-preferably brass
• Bearing/seal installation tool
• Safety glasses
Use this exploded diagram as you read through to get an idea of what is where in the differential.
We’ve got an article up on how to remove and replace the axles. You can find it here. To get started with this project, read the article and follow the instructions given in it to remove the axles. However, once you get the car up in the air, put the transmission in gear/park and set the parking brake. Put your safety glasses on.
Slide under the car with a 3/8-inch wrench and remove the four bolts securing the driveshaft to the pinion yoke on the differential. If you have an automatic transmission, secure the driveshaft out of your way. If you’ve got a manual, you can remove the driveshaft. Slide out from under the car, release the brake and put the transmission in neutral. Finish following the instructions to remove the axles. Once you’ve got the axles out, jump to the next step.
The pinion yoke is torqued on with about 150 lb-ft of torque, so it’s not going to be easy to remove. You’ll need to hold the yoke with the yoke tool or a large pipe/monkey wrench. The nut on the yoke is normally a 1.25-inch monster. It might be useful to have a friend help you with this step.
Slap the socket on your breaker bar and climb under the car with it, your pipe wrench, and your hammer. Place all your tools so you can reach them from under the car without having to slide out. Grab the yoke with the pipe wrench and stick the socket on the pinion nut. While you pull on the breaker bar to loosen the nut, have your friend pull the pipe wrench in the opposite direction. If you’ve got an impact gun, use it, although unless it’s really strong you may still need to use the breaker bar. Hit the back side of the yoke with the hammer until it’s loose enough to pull off by hand.
Using the cold chisel and hammer, make a mark on both carrier bearing caps (see exploded image above) indicating what side the bearing caps are on and what direction they’re installed in. Swapping sides or installation direction will cause problems down the road. Make sure you know which cap came off which side and what side was up on both.
The bearing cap bolts are normally 5/8-inch. Attach that socket to your ratchet and remove the bolts. Use the hammer to loosen and remove the bearing caps. If you’re lucky, the carrier will come right out by hand. You will most likely need to hit the end of the pinion shaft with your hammer to coax it out though. Remember, the ring gear bolts are reverse threads, which means they come off by turning clockwise and the new ones go in by turning them counterclockwise.
You can use a “special” seal puller to remove the seal, but I like to use the end of the pinion shaft in the same way I use the end of the axle to remove axle seals. Just slip the threaded end inside the lip of the seal at an angle and pry it out. The outer pinion bearing will come out by hand.
Look inside the pinion “snout” and you’ll see two or three small half circles cast into the metal that give you access to the inner bearing race. Use the hammer and punch to drive the race out. Move around to the other side and repeat the process with this inner race.
Find the bearing installer cup that is the correct size to install the smaller/outer bearing race. Place the race on the installer cup and hold the tool in place on the differential housing. Paying attention to the sound made when the hammer strikes the tool, sharply strike the tool with the hammer until the sound changes, indicating the race is properly installed. Place the smaller bearing into the race and lightly tap the pinion seal into the pinion snout with the hammer to keep it from falling out. Use the installer tool to finish properly seating it. After selecting the correct cup, repeat the race installation process to install the larger inner bearing race.
Slide out from under the car with the pinion and carrier assembly. Place the pinion with the gear down on your workbench. Open the pinion bearing remover tool and slide it over the pinion. You want the tool to go between the pinion and the bearing.
Install a couple of extensions into both sides of the tool until they’re both about two to three inches above the end of the pinion shaft. Tighten the nuts evenly on both sides until the tool is completely under the bearing and closed on the pinion shaft.
Find the crossbar and long threaded rod with a cup at the end of it. Thread this rod into the cross bar. Loosen the nuts at the top of the extensions enough to slide the crossbar under them. Tighten these nuts completely and evenly. Now, using a wrench or ratchet, tighten the long rod in the center of the tool while holding the crossbar until the pinion bearing is loose enough to remove. Loosen and remove the tool but don’t disassemble it; you’ll need it later most likely. Hold onto the shim if you’re installing the same gear ratio. If you’re changing the gear ratio, you’re going to need to adjust the shim size as you set pinion depth.
Here’s where the job gets technically interesting. In fact, this part of the job is so involved that I need to cover it in an article of its own. Read through this article and familiarize yourself completely with the rest of the gear swap process outlined here as I put together the article on setting pinion depth and backlash.
Once pinion depth is set, you can install the new pinion bearing on the new pinion gear. Installation of the new bearing is accomplished by flipping the pinion over and installing the tool from the opposite side with the long threaded bolt pushing against the pinion gear instead of the shaft.
For the purposes of this step in this article, we’re going to presume that you’ve properly set both pinion depth and backlash. Slide the crush sleeve onto the pinion shaft and install the pinion into the differential. Tap the yoke onto the end of the pinion shaft and run the pinion nut down as much as you can by hand. Use a ratchet to tighten it down until there’s no “slop” when you push and pull on it.
Now you’re ready to install the carrier for the last time. Push the bearings onto the ends of the carrier housing and slide the races over the bearings if they’re separate. Hold the carrier with the side that doesn’t need to be shimmed in place in the diff housing and tilt the carrier so you can slide the shim(s) you determined were needed when you set backlash. Slide the bearing caps in place (make sure side to side and up and down are correct) and insert the cap bolts. Torque these down to 60 lb-ft.
Head back over to the axle swap article now. Pick up from installing the new axles and move through refilling the differential. Before lowering the car, set the pinion preload (see next slide). Once the preload is set, you can lower the car.
Slide under the car with the breaker bar and large socket, plus both torque wrenches. Put the pinion nut socket on the torque wrench and torque the nut to about 160 lb-ft. Put the socket on the beam-type torque wrench and rotate the pinion through a full revolution as you check the amount of force that is required to rotate the pinion.
You’re looking for a force/torque of between 15 and 30 inch-pounds required to rotate the differential assembly. Increase the torque on the pinion nut by two lb-ft until you reach that. It should be around 170 lb-ft. However, instantly jumping up to 170 instead of doing it incrementally will invariably cause you to over-torque the crush sleeve, which will require you to disassemble the differential again and install a new crush sleeve. A crush sleeve that has been crushed too much will allow the outer/smaller pinion bearing to float, which will ruin it. So, start with a lower torque on the torque wrench and slowly work up to the final torque of 170.
Once you’ve got everything wrapped up and the car on the ground, it’s time to do the break-in for all the new parts. How many miles you need to drive carefully for will vary from manufacturer to manufacturer, but it’s usually around 500 miles. After that, drive it like you stole it! I’ll see you on the track.
