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10 Bolt BOP - What's Your Problem?

The 10-Bolt Bop

Phil Blake, Ortonville, MN: I didn't see in your article ("Build a 10-bolt on a Budget," Mar. '06) anything about what size gears would fit in what size housings. I have been wondering what to do with a 10-bolt Posi out of a '71 LeMans and was told I can't put 3.42 gears in it because it has 2.70-somethings in it now. I've been told I would have to start with at least a 3.08 housing. The rearend I have looks like the one in your story with the cutouts in the cover.

I also have half a dozen or so BOP 12-bolts out of Cutlasses laying around and have yet to find one that actually has 12 bolts on the ring gear (haven't checked them all). Only one of these so-called 12-bolts is a 3.08. I would be more than happy to sell any of these fine rearends to you California boys for that $500 figure you mentioned. (I know, it's not out of a Camaro.)

Jeff Smith: You bring up a few interesting points worth illumination, Phil. Let's start with the basics. GM began building 10-bolt, Salisbury-style integral rearend housings in 1964 (as opposed to the drop-out style used from '55-'64) for Chevelles and later Camaros and Novas. These were 8.2-inch ring-gear-diameter housings. The Buick-Olds-Pontiac (BOP) stuff also used 8.2-inch-diameter 10-bolt ring gears, but the internal parts, including the gearset, are not interchangeable with the Chevy-style axle. The 12-bolt Chevy rears were 8.875 inches in diameter. In these rearends, the number of bolts on the rear cover is the same as the bolts used to attach the ring gear to the differential.

From '68 to '70, Oldsmobile used an axlehousing with a 12-bolt rear cover, but internally, it employed an 8.5-inch-diameter ring gear with 10 attaching bolts. You mentioned that you were looking through your 12-bolt Olds housings for a 12-bolt carrier; you can stop looking because you won't find one-they're all 10-bolts. This is still an 8.5-inch-diameter carrier, so it is stronger than the 8.2-inch 10-bolts, but it is somewhat rare, as its internal parts are not the same as other "corporate" 8.5-inch housings used in later GM models. Richmond Gear does make 3.42, 3.90, and 4.10 gearsets for this Olds-only (not BOP) axle assembly, and according to Richmond's catalog, those gears will fit a 2.93 through 3.23 ratio differential. The basic rule of thumb is that a 2-series differential will only work with gear ratios in the 2 series, as in 2.56 or 2.78. The other ratio series follow a similar path. This is not a hard-and-fast rule, however. For example, one Buick/Pontiac 8.2-inch rearend differential will mount 2.87 through 3.31 gears, while another will accommodate 3.36 and deeper ratios well into the 4s. So you need to be careful here when matching a differential with a given ring-and-pinion. Just to make it more confusing, many aftermarket gear companies offer, for example, a 4.10 gear for a 12-bolt Chevy housing that will mount to a 4-series carrier as you might expect. But some companies also make a 4.10 gear that will mount on a 3.08 through 3.73 (3-series) differential by making the ring gear thicker.

To answer your question about swapping from your 2.79 gear in your '71 Pontiac to a 3.42 gear, the information you received is correct. Unfortunately, you will need to come up with a 3.36 or deeper (higher numerically) style differential to use that 3.42 gear. Keep in mind that the additional ratio of a deeper gear will make it much easier to spin the tires. This means that a limited slip (also known generically as a "posi" which comes from the GM term Posi-traction) is a great idea to add in at the time you add the gears. Auburn, Precision Gear, and others offer limited-slip differentials that will bolt right into your 8.2 housing. The price hovers around $400 to $450.

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