Here’s the stock outer axle...
Here’s the stock outer axle roller bearing. Note that it lacks any ability to control side load or to retain the axle in the case of a broken internal C-clip.
For ’67-’72 Chevy pickup owners, there are a number of reasons not to get rid of that 12-bolt Chevy rearend under your 1/2-ton. For one, the darn thing fits. All the suspension components bolt right up, with the traction bar pickup point cast into the housing from the factory. And unless you’re going to add some serious tire under your truck or a wheel with a deep offset, the width is good to go as well. They’re also tough as nails, equally so to the much more touted Ford 9-inch, with a more efficient design thanks to a lower pinion position in relation to the ring gear centerline. This results in a 3½ percent horsepower increase through reduced friction and lower gear lubricant temperatures.
But as great as the 12-bolt truck rear sounds, there are some drawbacks. For one, they retain the axle C-clip design carried over from the lighter weight passenger car rearends. Snap the axle-retaining C-clip and the entire axle assembly, wheel, tire, and all, can make an abrupt run to the hills. Not such a good scenario. The second problem with these rearends comes in the form of the outer roller bearings. These non-tapered bearings ride directly on the axle itself, which means that if your rearend is suffering from sloppy axle bearings, you’re most likely going to have to replace both the bearing and the axle as there is no bearing race to replace. That can become both costly and irritating if you drive your truck hard. The other problem with this style bearing is that by its very nature it suffers from side load difficulties. Without a way to keep the axle tight against the housing, bent axles, broken C-clips, worn-out bearings, and an overall sloppy nature is the result.
The “race” on the stock Chevy...
The “race” on the stock Chevy axle is actually just a machined face on the axle itself. Once worn, the axle needs to be replaced to remove any slop.
But there are some solutions to these problems that make retaining that factory 12-bolt a very real option to even the most demanding C10 enthusiasts. And when compared to the cost of building an equally strong, custom Ford 9-inch from scratch, the 12-bolt makes even more sense.
I pulled the 12-bolt rear out of my latest project, a ’68 Chevy longbed, while we were getting rolling on the suspension upgrades and took it down to the local go-to guys when it comes to rearends, Currie Enterprises. They pointed out the aforementioned shortcomings the 12-bolts suffer from and made their recommendations to create a bulletproof GM rear. When it was all said and done, the upgraded rearend bolted right back into place, strong as an ox and with a posi third member to replace the stock peg legger. We’ll start the suspension upgrades next month, but first let’s get this rearend rebuilt and ready to go! CCT
The other end of the axle...
The other end of the axle is where the C-clip retains the axle to the rearend. The button is a well-known weak spot when it comes to Chevy rearends and is the only thing that keeps the axle from becoming a projectile.
A bar is placed through the...
A bar is placed through the axlehousing with the new Ford ends at either end. Here, the width is double-checkeded before the new ends are tacked to the housing.
A big chop saw makes quick...
A big chop saw makes quick work of removing the housing ends and yields a fairly square end.
A big chop saw makes quick...
A big chop saw makes quick work of removing the housing ends and yields a fairly square end.
A bar is placed through the...
A bar is placed through the axlehousing with the new Ford ends at either end. Here, the width is double-checkeded before the new ends are tacked to the housing.
To ensure that the ends are...
To ensure that the ends are welded to the housing in the proper relation, the center section flange is set level…
…followed by the axlehousing...
…followed by the axlehousing ends.
Before moving forward, Currie...
Before moving forward, Currie stamps the work order number on the axlehousing so that it can be tracked throughout the process.
The housing is then put into...
The housing is then put into a steelabrator to clean off any grease, oil, and grime.
While the housing is being...
While the housing is being cleaned, our attention is then turned to fabricating the new axles. Currie temporarily installs the posi unit into the housing to measure for axle length of the new Ford-based axles and cuts raw forgings to length.
The axle is then machined...
The axle is then machined to accept the various races and seals as well as the corresponding spline count (30 for the 12-bolt Chevy).
Once machined, you can see...
Once machined, you can see that the end of the Ford axle is much stouter looking than the button-ended Chevy item.
Since we’re replacing the...
Since we’re replacing the stock axles, we’re going to take the opportunity to change the bolt pattern over from the stock six-lug pattern to the much more common 5x4¾-inch. Currie has drill jigs for most popular bolt patterns.
Once drilled, new 7/16-inch...
Once drilled, new 7/16-inch wheel studs are pressed into place.
Next the retaining plate,...
Next the retaining plate, bearing and seal are pressed onto the axle end, completing the axle assembly.
Here’s the 12-bolt rearend...
Here’s the 12-bolt rearend housing after the steelabrate process.
Randy’s Ring & Pinion supplied...
Randy’s Ring & Pinion supplied a bearing seal kit since our rear will still use some Chevy components.
Since we have the third member...
Since we have the third member out, might as well upgrade to a posi unit right? Our gears looked good, so we’re simply going to swap the peg leg third member for a new Eaton positrac unit.
Currie carefully inspects...
Currie carefully inspects and deburrs both gears to reduce their likelihood of gear noise.
The 8.875-inch diameter Chevy...
The 8.875-inch diameter Chevy ring gear is attached to the Eaton third member with 50 lb-ft of torque.
Next the carrier bearings...
Next the carrier bearings are installed onto the posi unit.
To improve oil flow to the...
To improve oil flow to the outer axle bearings, Currie machines a small passage in the axlehousing.
Pressing the input shaft bearing...
Pressing the input shaft bearing races in is the next step before the ring-and-pinion gear’s load pattern can be checked.
Shims are placed in the housing...
Shims are placed in the housing on either side of the posi unit to set the backlash (0.011-0.015-inch is recommended by the gear manufacturer).
The posi unit is then dropped...
The posi unit is then dropped into place…
…before the carrier caps are...
…before the carrier caps are torquedto 75 lb-ft.
The pinion bearing is then...
The pinion bearing is then pressed onto the pinion.
The pinion seal is then pressed...
The pinion seal is then pressed into the housing…
…followed by the yoke. The...
…followed by the yoke. The pinion nut is torque to spec (20-25 in/lb), compressing the crush sleeve, thereby setting the bearing preload.
The backlash can now be checked...
The backlash can now be checked using a fixed position dial indicator. Three separate positions on the gear are checked with a minimum of 0.011-inch “slop” deemed acceptable.
Next, the gear tooth contact...
Next, the gear tooth contact pattern is checked. Gear marking compound is applied to both the ring and pinion gear and the resulting pattern is compared to the manufacturer’s specs. To correct the pattern, a general rule of thumb is that if the pattern is heavy at the toe (top), subtract shims. If it’s heavy at the heel (bottom), add shims. At this point, it may be necessary to remove or add shims or reset the pinion to achieve the proper backlash and gear contact pattern.
Currie’s guys are well versed...
Currie’s guys are well versed so it was only necessary to make one small adjustment before we were installing the Ford backing plates and new axles.
The axle retaining plate hardware...
The axle retaining plate hardware is torqued to 45 lb-ft, accessible via holes on the axle flange.
The rearend cover attaches...
The rearend cover attaches via twelve fasteners (hence the name). Before fastening the cover, each hole is given a quick cleanup via a hand tap.
Each fastener is then tightened...
Each fastener is then tightened to 25 lb-ft.
And there you have it! A 12-bolt...
And there you have it! A 12-bolt rearend that will give any 9-inch Ford a run for its money!