Beefing the 7.5-inch 10-Bolt
Al Zak, Dawsonville, GA: The majority of articles published in your magazine are about engines or transmissions. What about at the other end of the driveshaft? I have an '85 El Camino. I've already put a revised engine in it and have a 200-4R ready to go in. The rearend and rear brakes are a challenge. The original 7.5 axle is weak. The 8.5 from a Buick GN or an Olds 4-4-2 are like hens' teeth and extremely expensive. Has anyone successfully transplanted an 8.5 with rear discs from a 9C1 Caprice or Impala SS into a G-body? The rear disc brake and park brake designs are much better than anything the aftermarket has to offer and are standard parts, so if you break down in Thyroid, North Dakota, you have a hope of getting it fixed. Would it make sense to narrow the housing and use G-body 8.5 shafts to keep it standard? A lot of Malibu, Monte Carlo, and El Camino owners could benefit from a project like this. Plus, in a time when money is scarce for many reasons, wouldn't this be a good magazine project for a low budget? After all, isn't that what real hot rodding has always been about?
Jeff Smith: We agree with you, Al, that the low-buck way offers more challenges, and we'll always look for the least expensive way that will produce good results. So to dive into this, your idea of swapping the 9C1 police car or Impala SS 8.5 has potential merit, but the upper control arm angle determined by the cast-in mounts in the rearend will not line up, so this is not a realistic conversion. We've heard the upper control arm angles on the early Chevelle 12-bolt are close, so we spoke to Strange Engineering. The company says the distance between the cast-in mounts and their included angle is different-so it's not a good swap.
If we're approaching this from a budget standpoint, then purchasing either a used or new 12-bolt isn't really an inexpensive deal. Frankly, there is no cheap (meaning less than $500 to $1,000) conversion to improve the strength of the G-body rear axle. Unfortunately, that's the reality. The price for a bolt-in Moser 12-bolt is around $2,800 with gears, 30-spline axles, and a limited slip through Summit Racing. A less expensive alternative is a Strange S-60 rear axle assembly for the G-body that's roughly $1,995. Either avenue is a significant financial hit, so let's examine other options.
First of all, your statement, "the 7.5 axle is weak" has credibility in comparison with the 12-bolt, the 9-inch, or a Dana/S60, but how weak? We've heard of many 12-second G-bodies that employ the existing 7.5 rearend assembly. Let's examine what it would take to beef up this rearend, since that appears to be the best solution within a limited budget. But keep in mind that the parts required to upgrade the 7.5 are roughly the same cost as similar parts for a 12-bolt, so if we invest $1,000 in a 7.5 to make it stronger, wouldn't that same money be better spent on a 12-bolt that you know will live? It's something to ponder. The other alternative is to try and find one of those 8.5s. Yes, they're rare, but if you look hard enough, I think you could find one. The 8.5 is virtually as strong as a 12-bolt, and again, the parts cost the same.
From a theory standpoint, the smaller ring-gear diameter does limit durability. What we're really talking about here in terms of strength is the amount of tooth contact between the pinion and the ring gear. With a given ring-gear diameter, one way to improve strength is to make our pinion gear tooth contact as large as possible. That means limiting the gear ratio. As the gear ratio becomes numerically larger from 3.08:1 to 4.10:1, for example, the pinion gear diameter becomes smaller because the tooth count is reduced. A 3.08:1 pinion uses 13 teeth while a 4.10:1 gear has only 10. A 3.42:1 gear, however, has 12 pinion teeth, so the contact area should be almost as good as the taller gear. This will improve strength. So this means you probably would not want to go much deeper than perhaps a 3.73:1 rear gear (its pinion has 11 teeth). Next, regardless of the ratio, it's imperative that you employ a solid pinion spacer instead of the stock crush sleeve. The advantage of the solid spacer is it offers additional pinion shaft support. This also means it must be set up properly by using shims to establish a specific overall length between the two pinion bearings. This creates the proper preload on the bearings. The solid pinion spacer can be purchased through Ratech (PN 4111, $16.95 from Summit Racing). If you don't do anything else to the rearend, be sure to do this.
You're also going to need a good limited slip. The Eaton clutch-type units (PN 19663-010, $519.95, Summit Racing) work well but are pricey. We've had excellent experience with the Detroit Locker TruTrac (PN 912A317, 28-spline axles, $375.95, Summit Racing), and the price is more affordable. It requires upgrading from 26- to 28-spline axles-order these as if they were for the Grand National rearend, since they are the same width. We found a set of 28-spline Mosers (PN A102808, $244.95, Summit Racing) for a reasonable price. Add a set of Richmond 3.42:1 gears (PN 49-0007-1, $205.99, Jegs) and a Ratech deluxe installation kit (PN 3001K, $109.95, Summit Racing), along with an aluminum rear support cover (PN 8510400, $149.95) and you have a complete, upgraded 10-bolt.
Adding up all these parts plus roughly $200 for a professional ring-and-pinion installation, we're looking at just under $1,000. If you are talented, you could do the installation yourself, but it will require some custom tools and significant expertise. Even doing the work yourself, the bottom line is around $800, and you've now invested that much money in a weak 7.5-inch rear axle assembly. But you must spend more to step up to an 8.5 or even a 12-bolt. If your car is only running in the high 12s-we'd suggest upgrading the 10-bolt and trying not to abuse it. If the car has the potential to run low 12s or quicker, then we'd go with the Strange S60-it's roughly 20 pounds heavier than a 12-bolt but has immense durability and torque capacity. It's your choice.
Eaton (Detroit Locker)
Morton Grove, IL;