Was it possible for the 1970 big block 396 (402) Nova to come with the 10-bolt rear-end installed from the factory? I know the 12-bolt was optional for around $150 from the dealer at the time but that was a lot of money to the car buyer in 1970. Did the factory turn out some 10-bolt rear-ends in the big block Novas from 1970?

 

Jeff Smith: I’ve learned over the years never to say never because someone will come forward with a build sheet that says otherwise, but as far as I know, Chevrolet always put the 12-bolt behind any big-block Nova in 1970. Remember that in 1970, the 10-bolt was still the spindly 8.2-inch diameter ring gear rear-end used behind a 283 and Powerglide combo. This is a weak rear-axle assembly compared to the 12-bolt’s much larger 8.875-inch ring gear diameter, so it is unlikely that Chevrolet would put the 10-bolt behind even the base 350-horsepower 396 (402). This makes sense because all 1970 big block automatic Novas came with the much stronger TH-400. It simply wouldn’t make much sense to put a 10-bolt behind the TH-400.

Just to broaden your outlook, starting in 1972, Chevrolet converted to the “corporate” 10-bolt in the Nova. This is a 10-bolt but with a compromise 8.50-inch ring gear diameter that is almost exactly in between the 8.2 and the 8.875 sizes. While ignored by enthusiasts, this is a very strong rear-axle assembly that uses the 12-bolt’s pinion shaft diameter. The weakest link is the stock 28-spline axles but there are aftermarket limited-slip differentials that upgrade to 30-spline axles. For example, Eaton offers a Truetrac limited-slip differential that employs 30-spline side gear. The best thing about these rear-ends is that you can still find them used for a cheap price. The same cannot be said for a 12-bolt.

How strong is this 10-bolt? Strength is a relative term, but I have a close friend who still runs a second-generation Camaro in Fastest Street Car racing from time to time that has run as quick as 8-teens at 179 miles-per-hour using the stock, GM 8.5-inch rear axle assembly. He uses much larger spline count axles but the rear-end has lasted for years. That should give you some idea of the 10-bolt’s strength.

Keywords
Ask Away, GM 10-Bolt, GM 12-Bolt, Jeff Smith, rear-ends
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.