If you’re not sure how a differential works, get out the shop manual and familiarise yourself with your car’s type. Even though the principles are the same for most of them, they can differ in important aspects, such as how you adjust the pinion gear in relation to the ring gear. When you feel sure about what you are up against, here is how to proceed.

Pull the pumpkin

Place a piece of four-by-two between the brake pedal and the floorboard to prevent someone from inadvertently depressing the pedal while you have the rear brake backing plates off the car. Remove the rear wheels. Next, pull off the rear drums, and to accomplish that you may need a drum puller. Which type of drum puller you require depends on the type of car you have.

If your car uses the kind of puller that grips the edges of the drums, you can pound on the centre of the drum puller with a hard rubber mallet to pop the drum loose, but if your car requires a puller that attaches to the studs, don’t pound on the centre of the drum puller. You will damage the thrust block in the differential spider gears if you do.

Disconnect the brake lines from the wheel cylinders and remove the brake backing plates. Save any shims you find between the backing plates and the axle flanges, and don’t mix them up. Next, pull the axles out of the axle housing. On some cars, you will need an axle puller to do this. Finally, split the rear U-joint and wire or tape its bearing cups together so you won’t lose the needle bearings inside them. Pull the prop shaft off the transmission and set it aside.

Wash down the pumpkin with solvent and a brush. You don’t want any grit falling into the bearings or gears when you pull the gear case out. Now drain the gear oil from the differential. Roll the transmission jack under the differential carrier and run it up to just below the axle housing. Don’t be tempted to lift the differential assembly out with your bare hands, because they are heavy and slippery. Cut the heads off of a couple of three inch-long bolts the same size as the ones that hold in the differential assembly to make a pair of studs.

Take the bolts out at the three and nine o’clock positions and install the studs in their place. Remove the rest of the bolts and slide the differential carrier straight forward.

Inspection and repair

Bolt your diff to an engine stand, or mount it in a large, smooth-jawed vise. Inspect the ring and pinion gears. Are they chipped, broken or pitted? Are they worn to a knife-edge? Are they discoloured? If any of these problems exist, you will need to replace both gears. Ring and pinion gears are machined as matched sets, so you can’t replace one and not the other. If they look good, try to move them in relation to one another. If there is a lot of play between them, your problem may be that the bearings are worn.

Grab a punch and hammer and mark the bearing caps and bearing pedestals in relation to one another so you won’t get them mixed up when you reassemble the differential. Also mark the big, disk-shaped bearing adjusting nuts in relation to the bearing pedestals.

Now loosen the bolts holding the bearing caps in place enough to allow you to remove the bearing adjusting nuts. Carefully count the number of turns required to remove each bearing nut, and write it down. This will help you get the bearings back into approximate adjustment when you put things back together. Lift the differential case and ring gear out of the differential carrier.

To get the pinion gear and bearings out, remove the U-joint yoke, then the pinion nut. Save any shims you find. Now slide the pinion shaft out the rear of the diff carrier. Pull the pinion seal out of the front of the casting.

The bearing and adjusting cone (if there is one) should come out next. If, after inspecting the bearings, you decide they must be replaced, you can drive the bearing races out of the carrier casting with a hammer and drift.

Wash out and inspect all the bearings. Shoot them with light oil and turn them slowly. They should turn smoothly and be firmly held in their cages. There should be no pitting, discoloration or galling on the rollers or their races. If you see signs of wear, replace the bearings.

Remove them with a bearing puller. Bad bearings are the most common problem with differentials. They often fail because of neglect and/or lack of lubricant. Take your old bearings to a bearing supply house and have them matched up.

You will also want to inspect the differential spider gears inside the gear carrier, although they almost never need replacing. They don’t turn at all when the car is going in a straight line, and unless you have been doing doughnuts they aren’t likely to be worn.

Reassembly

New bearings will need to be pressed on using a hydraulic press. Many auto spares stores and machine shops can help you with this task if you don’t have a press. If you need to replace your ring and pinion gears, when you install the new ring gear, tighten its bolts evenly in three stages to about 45lbs/ft (this is a rule of thumb for most old car differentials, but check your shop manual to be sure. Yours may differ). If the bolts were wired into place originally, rewire them during reassembly.

Tap in new races for the pinion shaft. Slip it back into place and install the front bearing. Set the gear case back into its bearing saddles and, using new keepers for the bolts, tighten the bearing caps loosely into place. Install the bearing adjusting nuts and turn them back into place the same number of turns as when you removed them.

Adjustment

This is the most critical part of the job. If you don’t get the relationship between the ring and pinion gears just right, they will self-destruct. Also, if the carrier bearings aren’t preloaded properly they won’t last long either. Some differential designs use shims to adjust the pinion gear and shaft in relation to the ring gear; others use a cone of soft metal inside the differential carrier. Be sure to read up on how your car’s diff is to be adjusted before undertaking the job. All we can offer here are general tips based on common designs.

The big, disk-shaped adjusting nuts beside the differential carrier bearings adjust the ring gear in relation to the pinion gear, and they also set the preload on the differential bearings. Check the lash between the ring gear and the pinion gear using a machinist’s dial indicator mounted on a magnetic base (usually, it should be between o.003-inch and 0.005, but check your manual for the figure for your car). Also, use a little methylene blue liquid on the gear teeth to check for proper contact. The chart at lower left shows what to look for, and how to tell when you have it right.

To set the preload on the differential bearings, use a large bull calliper, which looks like an iceman’s tongs, for those of you old enough to remember the iceman. With the bearing caps still slightly loose, check the distance between the bearing pedestals with the bull calliper. Now tighten the adjusting nuts equally so as not to disturb the ring gear adjustment, until a feeler gauge of the correct thickness (0.010-inch is common) will just slip in under one tip of the bull calliper.

Finally, it is well worth the effort to recheck everything one more time before popping your pumpkin back into its housing. As we said at the beginning, if the gear lash and bearing preloads aren’t right, your differential will be noisy and won’t last long.

Guide to rebuilding a rear-end

Things you’ll need

• Jack stands

• Transmission jack

• Brake tubing wrench

• Ball peen hammer, punch, drift

• Bearing puller

• Drum puller

• Magnetic base and dial indicator

• Sharp putty knife

• Methylene blue liquid

• Silicone sealer

• Bearing press

• Seals and gaskets

• Gears and bearings as required

• Hypoid gear lube

Differentials get dirty in service. Be sure to clean yours thoroughly before taking it out. You don’t want grit in the gears and bearings
Mark the bearing pedestals using a hammer and punch before taking the end caps off so you won’t get them mixed up
Also mark the big, disk-shaped adjusting nuts in relation to the pedestals the same way
Bend the keeper tabs enough to allow your wrench to clear, then remove the bearing caps
Remove the adjusting nuts, counting the number of turns it takes to remove them as you do, so you can put them back approximately in adjustment
The rear pinion shaft bearing will have to be driven off with a hammer and drift. A new one will need to be pressed in place using a hydraulic press
Components of a typical differential. The one in your car may differ slightly
Adjusting differential bearing preload Bearing preload is also critical to the life of your differential. One way of setting it is with a bull calliper and feeler gauges
Adjusting clearance and backlash Use methylene blue dye liquid for checking gear teeth contact between ring and pinion gears. Modern hypoid differentials are not forgiving of poor adjustment
Typical Morris open drive differential Morris differential is fairly typical, but check your shop manual before you begin work on your car

Words & Photos: Jim Richardson