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Budget 10-Bolt Upgrade

How To Build A Bulletproof, Posi 10-Bolt For $400 Or Less!

Jeff Smith Feb 1, 2000

Step By Step

All it takes to swap an 8.5-inch rearend into a ’67-’69 Camaro is a ’72-’75 Nova housing, a clutch-type posi unit from any 8.5-inch rearend, and a set of used factory 3.42 gears. All of these parts can be found in the recycling yards for $200 or less, depending upon your bargaining skills.

Factory-clutch-type posi units are relatively easy to find for the 8.5-inch 10-bolt, especially in mid-’70s Pontiac Firebirds and Trans Ams.

Tim Moore found this ratchet-style limited slip in a mid-’80s truck. It looks weird but uses clutches to apply the limited slip.

The 8.5-inch 10-bolt is very similar in appearance to the weaker 8.2-inch rear. According to Moore, the best way to identify an 8.5 is to remove the cover and inspect the ring gear bolts. If the head of the bolt requires an 11/16-inch wrench (7/16-inch bolt diameter), then it’s an 8.5. If the ring gear bolt requires only a 9/16-inch wrench (3/8-inch bolt diameter), then it’s an 8.2-inch rear. Be aware that the ring gear bolts on the 8.5 are lefthand threads.

There are two gear thicknesses used in the 8.5-inch 10-bolt. All gears with ratios 2.73 or numerically higher (3-series) require the thinner-style carrier (right). The 2.56 gear requires the thicker flange carrier that also offset slightly more (left).

Ring gear spacers like this steel unit from Summit can be used to mount a 3-series gear on a 2.56-style carrier. The carrier has a thicker inside-diameter gear register that offers more surface area than other carriers, so Moore feels this is not a bad compromise. Summit offers this steel spacer with ARP bolts for under $30.

While the 8.5-inch Nova housing will bolt into any ’67-’69 Camaro, the ’67 cars use a monoleaf spring. You can make a spacer plate to accommodate the difference, or buy the leaf springs that came with the Nova and bolt them in place of the stock ’67 monoleaf spring. This is the best solution since it offers the later-model springs.

New axle bearings are a good idea even with used axles. The GM 10- and 12-bolt rearend housings use the axle as the inner bearing race. If the axle bearing surface is worn, the best solution is new axles. However, you can purchase axle-saver bearings (left) that place the bearing surface in a different place on the axle. Keep in mind that you can reuse your 8.2-inch rearend axles and brakes in the new 8.5-inch housing.

Several companies offer complete overhaul kits for the GM 8.5-inch rearend. This Federal-Mogul kit includes carrier and pinion bearings and races, a new pinion nut, a crush sleeve, gear marking compound, and a tube of RTV in place of a gasket. PAW, Summit, and many others offer these kits for under $100. Axle bearings are not included.

Older rearends often will need replacement axles. Mark Williams has just introduced a new line of high-performance, induction-hardened Master Line street axles. The axles feature multiple wheel bolt patterns and a thicker axle flange. The Master Line axles are affordably priced compared to the gun-drilled race axles also offered by Mark Williams.

How strong is the 8.5-inch 10-bolt? Well, let’s put it this way: Both Dave Henninger and Kurt Urban run these 10-bolts (with spools) in their respective Camaros. Henninger has run as quick as 8.68/151 mph, while Urban’s Camaro runs 9.20s/145. Henninger has never broken a gear and has logged hundreds of runs. That should settle the question of durability.

The ’67-’69 Camaros are probably the most sought-after and popular performance cars Chevy ever built. If you have one, then you know that finding a 12-bolt rearend for it is even harder than finding a president with high moral standards. The 12-bolt is coveted mainly because it’s as strong as the standard 8.2-inch ring-gear 10-bolt is weak. Abuse an 8.2 10-bolt and it’ll break. Sure, you could bolt in a 9-inch Ford, but there is a more elegant solution that’s also downright inexpensive.

GM enthusiasts assume all 10-bolts are weak. While for the most part this is true, there is one exception—the GM 8.5-inch ring-gear 10-bolt. The truth is the 8.5-inch 10-bolt is almost as strong as a 12-bolt since its ring-gear diameter is only 0.375 inch smaller than a 12-bolt (8.50 versus 8.375 inches) and the pinion gear shaft is an equal diameter. Even better, the 8.5 rear axle assembly was the universal rearend for millions of GM cars and trucks from 1971 through 1996. This means there are a bazillion to choose from.

Our pal Tim Moore, of Moore Automotive, turned us on to this low-buck rear-axle swap for the early Camaros. The corporate 8.5-inch 10-bolt was first used in ’71 Camaros (the ’70 Camaro retained the 8.2) along with several other body styles, including the second-generation Novas. While the Camaro might seem the likely swap, the ’71-’81 Camaro rear axle is 1 inch wider and employs a wider leaf-spring mount. The ’72-’75 Novas also used the 8.5-inch housing and its dimensions make it a bolt-in for first-generation Camaros. But this is just the beginning.

While ’72-’75 Novas offered gear ratios from 2.73 to 3.42 both with and without posi units, most came with open differential 2.73 or 3.08 gears. After dozens of recycling-yard jaunts, Moore has discovered that many ’71-’81 Firebirds and Trans Ams came with posi units that can be purchased relatively cheap. As for gear ratios, Moore likes to look first in ’77-’94 ½-ton Chevy trucks and vans equipped with the 8.5-inch ring gear to find the 3.42 gear sets.

Moore recommends purchasing a bare 8.5-inch housing unless it’s already equipped with a posi and the right gears. By purchasing the gears and posi separately, he can usually find all three major components for less than $200. Chevy’s famous interchangeability comes into play here as well since the stock ’67-’69 Camaro 8.2 brakes and axles will bolt right on the Nova 8.5-inch rearend. What could be easier? Before assembling the rearend, Moore recommends buying a Federal-Mogul rearend assembly kit (PAW offers a great price on these) that includes all the bearings, seals, and small parts you’ll need to assemble an 8.5-inch ring and pinion. The only other parts you’ll need are a pair of new axle bearings.

The only other major expense involved with this swap would be paying someone to set up the ring and pinion. This can cost around $200, which could drive the entire cost of this swap up to as much as $400, but this is still cheap. Even at this price, that’s a third the cost of building a similarly equipped 12-bolt or a Ford 9-inch. This is a classic example of how to use inexpensive factory parts to create a very strong drivetrain that will last for decades in even the most brutal street/strip applications.


Moore Automotive
Sun Valley, CA 91352

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