March - April 2003 Lane Racing And Rodding Article By Jim Kaekel, Jr.
Although the 9" Ford rear end has become the rear axle of choice for drag and circle track racing the 12-bolt Chevrolet still enjoys a fair degree of popularity among drag racers and street stock racers from the GM contingent who still wish to run a corporate rear end. While it's not as easy as the 9-inch when it comes to servicing or changing gear ratios, the 12-bolt rear end requires less horsepower to drive because it is more efficient, with less frictional loss. The 12-bolt derives its name from the number of bolts that hold the ring gear to the carrier (although it also has 12 bolts securing the inspection cover). Offered from 1965 to 1972, it was standard on all full-size Chevrolets as well as many Buicks, Oldsmobiles, and Pontiacs depending upon applications and assembly plant locations. It was also used in high performance Chevelles, Novas and Camaros. Experts feel that a properly prepped 12-bolt is more than adequate for most moderately powerful, full-bodied cars.
Mark Williams Enterprises introduced an all-aluminum modular housing assembly several years ago, based on the original GM design, that uses conventional ring and pinion sets and is perfect for Competition Eliminator type drag race cars. Aftermarket steel replacement housings have become feasible, too, as the salvage yard supply of used rear ends has all but dried up. Available directly from Moser Engineering, and also supplied to Mark Williams through K.T.R.E., the new housings are not just reproductions, but significantly improved castings with sturdier carrier bearing caps and special housing ends for positive axle retention and elimination of "C-clips."
Because it had a relatively large, 8-7/8" ring gear, it didn't take long for the 12-bolt to become popular with GM fans, especially those contemplating a big block engine swap. The 12-bolt was so appealing to the owners of pre-'65 GM drag cars that NHRA tech officials uncovered racers who were disguising the 12-bolt rear by drilling and tapping the housing to accept an early 10-bolt cover and make it appear to be a stock 10-bolt.
Production 12-bolt rears used two-, three- or four-series carriers, depending upon the stock gear set in the carrier. The size of the carrier, identified by measuring from the carrier bearing shoulder to the flange of the ring gear, determined the range of ratios that would work. Two-series carriers (.590") were common to rears with high 2.56 or 2.73 ratios, three-series carriers (1.020") were used with 3.08 to 3.73 ratios, and four-series units (1.325") were used with gear sets which were 3.90 and lower (numerically higher). Care must be exercised when swapping gears to be sure that a replacement gear set is compatible with the carrier being used. If the final ratio is not compatible, either the proper series carrier must be substituted, or a thicker ring gear, available through Richmond Gear, can be installed in the existing carrier. These thicker gears allow mating a gear ratio of 3.90 and lower to a three-series carrier. A ring gear spacer which serves the same purpose is also available for the 12-bolt (Mr. Gasket #MRG920A), but it should only be used on light duty applications, according to Travis Podratz of Mark Williams. Never, under any circumstances, use both a thick ring gear and a ring gear spacer to stuff a 3.90 into a two-series carrier!
When preparing a 12-bolt rear end for drag racing, a spool and aftermarket axles are highly recommended. A spool eliminates both the axle and spider gears and reduces rotating mass for greater durability. Both rear tires will always turn at the same speed with a spool (don't use a spool on the street). Spools for the 12-bolt, available from Mark Williams, Moser, Richmond and Strange, replace stock carriers. Intended for racing only, they may only be paired with the same ratios as a four-series carrier. Mini-spools, popular in factory or hobby stock circle track classes, can be used to convert "open" type 12-bolts to provide power to both rear wheels, however, they should not be used in street or serious drag race applications. In a mini-spool installation, both the axle and spider gears are removed and the mini-spool is installed, retained by the spider pin, providing a fully "locked" rear end.
Heavy duty, replacement positive traction carriers, available from Auburn, are ideal for street/strip vehicles. Auburn's clutch-type 12-bolt differentials, available as "three" and "four" series units, can be used for a performance upgrade or to convert an "open" style carrier rear to a positive traction unit which will deliver power to both rear wheels under acceleration and still allow smooth cornering with little clutch "chatter".
A "C-clip" eliminator kit is a wise addition to a 12-bolt in a drag race, circle track or street performance application. The original axles are retained only by small "C-clips" and if either clip fails, the axle will pull free of the rear end creating a dangerous situation. For this reason, both NHRA and IHRA require a "C-clip" eliminator on any car quicker than 10.99 (1/4 mile elapsed time) or any car fitted with a spool.
All production units used 30 spline axles which were produced in about half a dozen different lengths. If a spool is to be used in place of a stock carrier, spools and axles are available with 30 , 33 and 35 splines. When upgrading to an aftermarket spool or axles, these components should be purchased from the same manufacturer.