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Swapping Rear-End Gears

Some jobs are better left to the pros

In early September 1964, a southern Californian who loved the styling of the new 1965 GTO checked all the right boxes on the dealer order form except one: the rear gearing. This musclecar was delivered with a 3.23:1 non-positraction rear end, and it stayed that way until a recent horsepower upgrade and transmission rebuild demanded more low-end performance. The decision was made to yank the one-legger and install 3.55:1 gears and a posi.

Many suppliers carry the parts to accomplish a rear end gear change. Complete installation instructions are usually included with these kits, but a word of caution is required: This is not a job for a novice auto enthusiast. If you are not experienced in working on rear ends, then this gear change is best left to a professional. The most difficult part is setting the pinion depth with the correct shims and installing the correct amount of shims on the carrier. If it is not done properly, the gear meshing will not be correct and the rear will howl while going down the road. It's time-consuming and sometimes frustrating, but it must be done correctly. Also, we manufactured our own tools to press on the carrier bearings and the pinion bearing from a piece of steel pipe (1 7/8-inch OD, 1 1/2-inch ID).

While the average weekend hobbyist should probably have a pro do the changeover, this overview shows what it takes and how much work it really involves. The numerical difference between 3.23 and 3.55 is not that great, but the low-end performance and torque are very noticeable and make all the effort worthwhile. We are working on a 1964 GM 10-bolt, an axle that basically remained the same through 1972. We used normal shop tools for the R&R, but a few specialty tools like a dial indicator and bearing puller make certain parts of the procedure go a lot smoother.

Axles

Obviously, we have removed the rear wheels and brake drums. Before removing the four nuts securing the axle to the rear-end housing, we cleaned the entire brake area to remove debris and accumulated brake dust. The nuts securing the axles are accessed through holes in the hub by simply turning the hub until you can get a ratchet on each nut and remove it. Then pull the axles out of the tubes. The axles might require a little coaxing, but will come free fairly easily. While the axles are out, it's a good idea to have new axle bearings installed so you don't have to worry about them in the future.

Carrier

Two caps and four bolts secure the carrier. Mark each cap with top and left or right—these must be replaced in the same position as you removed them. With the caps removed, the carrier is now pulled forward and the shims removed from one side at a time. These shims may be reused, so mark which side you removed each shim from and set them safely aside. The carrier can now be rolled forward and removed. This item is heavier than it looks, so having a spare set of hands will be helpful. Make sure that the install kit you use has new bearings, bearing races and a selection of shims for the carrier and pinion gear.

Pinion Gear

In order to remove the pinion gear, the driveshaft yoke must be removed. A large nut secures it to the pinion shaft, and this nut must be removed with either an air tool or breaker bar. It is seriously tight on the pinion. After removing the driveshaft yoke and front pinion bearing, we tapped the pinion out and removed it from the case. This gear, along with the 3.23 carrier, will be put on the shelf and kept with the car. Have an extra set of hands at the rear of the case to grab the pinion gear as it comes free.

New Races

The new pinion-bearing races must fit flush with the surface of the case. We tapped the race in slowly until it was in the exact position as the original. This is a project that is not accomplished quickly, so have patience. New races should be installed in both the front and rear of the case. A short piece of 2x4-inch wood and a hammer is the at-home installation method; a professional shop has a special tool called a "race driver" to accomplish this task. Some specialty tools are required to save time and make for a better install.

Ring Gear

Before installing the ring gear, carefully clean the mating surfaces of both the ring gear and carrier. This must be a perfect mating surface, so it must be squeaky clean. The ring gear is now secured to the positraction carrier. Once the bolts are finger-tight, a torque wrench is used to tighten them to spec (50 lb.-ft. in this case), using a crisscross tightening pattern. After all bolts are tight, double-check their torque to be sure. The axle splines slide into each end of the carrier. The driveshaft turns the yoke and pinion; the pinion meshes with and turns the ring gear, which in turn spins the axles through the carrier.

Backlash

A dial indicator is now used to check the backlash. The pinion must be fixed so it can't turn, and the dial needle measures the free play (backlash) between the ring gear teeth and the pinion teeth. Ours was ideal at .008-inch. If you want to increase the backlash, you must move the ring gear away from the pinion and visa versa. The instructions will give the ideal setting for your rear end. Spacing is accomplished using shims of varying thickness. It took us several attempts to get it right. We used a micrometer to measure and mark the shims to make selection easier.

Tooth Contact

Here we see an ideal contact pattern. Moving the pinion closer to the ring gear will move the pattern deeper on the tooth (flank contact) and slightly toward the toe. Shimming the pinion away from the ring gear will move the pattern toward the top of the tooth (face) and toward the heel. Gear-marking compound should be included with your installation kit. Brush the compound on several of the ring-gear teeth, then rotate those teeth back and forth through the pinion gear teeth to establish a contact pattern. This procedure must be repeated after each shim installation to guarantee the correct tooth contact.

Precision Check

Here's the completed assembly with everything turning smoothly and looking good. Since this is such a critical procedure, make sure you use only the best parts available. Using a lift or removing the rear end from the vehicle is practically a necessity, taking into account all of the checks and rechecks required to get the contact pattern precise. This exercise took three of us almost eight hours to accomplish; most of that time is fitting and shimming to perfection. If we had all the correct professional tools and considerably more experience, we probably could have cut the time considerably. That's another reason to have a pro do the install.

Final Steps

The case cover was bead-blasted and painted, and the rest of the rear end cosmetic resto will follow. A new posi tag added the final touch. We used GM 90-weight gear oil and positraction additive, which is available at most GM parts departments. Fill until the fluid leaks out the filler hole in the side of the case (about 2.5 quarts here). Brush a light skin of gasket sealant on both sides of the gasket, then torque the bolts about the same as you would a valve cover bolt and you're done. The gear manufacturer recommended three 15-minute cruises, with a cool-down period in between, to set the gears and break them in. After that, it's driving as usual.