Ask Away! with Jeff Smith: Examining the Viability of Chevrolet’s Oft-Overlooked “Corporate” 10-Bolt Rear-End
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.
Got a set of iron World Products Sporsman II angle plug heads on my 406 SBC. At about 2500 miles on the fresh engine,it started using oil real bad. I quart every 400 miles! Did a leakdown and ring seal is great. I heard horror stories of these heads having horribly loose valve guides. Am I better off installing new “bronze” guides or replacing the heads altogether. If so whats the best head for a mildly cammed 406 with a small 4bbl. and iron manifolds in a 40 ford tudor streetcar. Th350 auto,mild stall converter 3.00 ford 9 inch rear
Thanking you in advance…..Ron in Montville.
Though having new guides installed would be less expensive in the long run, with your mild motor build, heavy car and low numeric gear ratio; a new cylinder head with a smaller intake runner volume (180cc) would help you make power in a more useable RPM range. Feel free to call 1-300-630-0240–we’ll need a little more motor info but, we can help you pick out just the right set.
So the 10 bolt in the 72 is good?
The beefier Corporate-style is fine, yes.
Raced mine 5 years with a 500 hp small block, no problems.
I think the 12 bolt is lighter than a 9″ (not sure how much) but the 8.5″ 10 bolt should be even lighter, generally speaking.
lol i still run a 8.2 ( not stock ) in a 69 Camaro, runs mid 12’s