Pontiac 10-bolt rearend - How to "bulletproof"

How to "bulletproof" your Pontiac’s 10-bolt rearend

Story and photography by Jefferson Bryant

Swapping rear-end gears is just for drag racers. Stock gears are suitable for just about everything other than full-power blasts down the dragstrip … or for rock crawlers. If you believe that, don’t ever accept one of those “free” vacation offers – you’ll end up owning a timeshare.

The gearing is part of the drivetrain system that works together to put the power to the ground. The intended purpose of the vehicle – along with the transmission gearing, the tire size and the engine – factors into determining what ratio is best for your vehicle.

Differentials 101

Before we actually get to swapping gears, a brief primer on differentials and how they and the gears work is in order, for those less accustomed with how they operate.

As the engine operates, the crankshaft’s rotation is transferred through the transmission and driveshaft to a pinion gear fitted through the rear axle housing’s center section. The spinning pinion meshes at a 90-degree angle with a vertically-oriented ring gear that’s fitted around the differential housing (also known as a “planetary carrier”), causing it – and the axles connect to it – to rotate, ultimately turning the wheels and tires.

A “differential” is simply a device that splits power between two drive wheels, such as the rear two on classic Pontiacs. (Differentials are also used to split power between the front and rear wheels in all-wheel-drive vehicles.)

Because the pinion gear is smaller than the ring gear, it must rotate several times in order to drive the ring gear through one full rotation. This relationship is what multiplies torque at the differential: if it takes 4.11 turns of the pinion to turn the ring gear once, the torque multiplication is a 4.11:1 ratio – and, coincidentally, that’s the gear ratio.

If you are unsure of your rearend’s gear ratio, you can count it by jacking up the car and supporting it on sturdy jackstands, then mark the tire and the driveshaft. Next, count how many times you rotate the driveshaft to cause the tire to make one full revolution. Having an assistant can be helpful here. In most cases, you will have an odd number – like “three and three-quarter” turns of the driveshaft or “just over three rotations,” so you’ll need to figure what ratios were available for your rearend – such as 3.73:1 and 3.08:1, for these examples. There is some guesstimation in this process, and that’s OK.

Gearing makes a bigger difference than you might think in terms of acceleration and highway cruising – and fuel economy.

It’s worth noting, too, that changing the diameter of your tires is equivalent to changing – or fine-tuning – your gearing. A two-inch change in tire diameter can yield a 10 percent difference in torque. The taller the tires, the deeper the gears need to be. Gears are often referred to as “deep” or “low” and “high,” which is really confusing, considering gears are listed numerically. A “low” gear will be numerically higher than a “high” gear: i.e., a 4.11:1 gear is lower than a 2.90:1 gear. High gears are also called “highway gears” because they’re great for high-speeds. But a vehicle with low-numerical (highway) gears will take longer to get to top speed than an identical car with higher-numerical gears, but the highway-geared car’s top speed will be faster. This is great for fuel economy. In reality, when you are talking about performance cars, low-numerical gears are rarely considered, they simply suck the life out of the stop-light performance. High-numerical gears have the opposite effect. You get off the line much quicker, but the top speed is reduced. Just go for a ride in a ’60s truck and you will know. Cruising at 65 while the engine is revving to 3,500 rpm is not only annoying, but the mileage sucks.

Modern five- and six-speed transmissions can compensate, however, for highway gearing by featuring low first and second gears. For example, a classic Muncie M22 four-speed with its 2.20:1 first gear needs a 4.11:1 rear gear to have a 9.042:1 final drive ratio for good acceleration, but highway cruising will be horrendous. But with a 3.06:1 first gear in a 700R4 automatic, you would only need a 2.96:1 rear gear for the same final drive ratio. So, you’d get the same acceleration (thanks to the short first gear) but much friendlier highway cruising and economy, thanks to the tall rear gear … and the 700R4’s 0.70:1 overdrive fourth gear and lock-up converter only sweeten the economy and top-speed possibilities.

In a performance car, the trick is to match everything together and get the premium performance in all areas. The basis of this article is to cover the installation and set-up, not the details on picking the gears. For that, it is suggested you call the manufacturer for assistance. You will need to know the original gear ratio, as there are limits to how much larger or smaller you may go depending on the carrier you have.

We had everything picked out for the GM BOP 8½-inch 10-bolt under our musclecar-era Pontiac. While some scoff at the 10-bolt, those in the know understand that the 8½-inch 10-bolt is one of the best differentials available. Only three-eighths (0.375) of an inch smaller than the GM 12-bolt, the 8½-inch 10-bolt can be just as strong, yet weigh less. It should be noted that GM also offered an 8.2-inch rearend later on, which is not a very strong unit and is not suitable for high-performance usage without significant upgrades … and even then, they can be questionable.

The basic installation is straightforward; the dial-in is the tricky part. You need a few basic tools to set up rear gears:

• Dial indicator. Preferably with a magnetic base. This is needed to measure the backlash. There are ways around this, but these don’t cost much and have a lot of uses.

• Calipers. Either digital or analog, you need calipers to measure the shims. A digital set is much easier to read.

• Grease paint. Once the backlash is set, you have to check the wear pattern, that is done with grease paint. Gears typically come with a tube of the stuff.

• Hydraulic Press. Don’t have one? No problem, just take the old pinion gear to the local machine shop or even parts store to have the bearing pressed off. You also have to have the new bearing pressed on.

In all, the entire process can be done in a day, but more than likely it will take you a couple days. We learned a few tricks that we’re sharing to make it easier. The best advice is to follow the specs in the guide that comes with the gears and double check your work. You should drive carefully for the next 500 miles to break-in the new gears, so keep your foot out of it.