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How To Build A GM 12-bolt Version 8.8 Ford For Any Leaf-Spring Car

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Say what? A 12-bolt version of the Ford 8.8? That's not a typo. It's actually a unique idea by Car Craft family member Tim Moore. We were looking for a stronger rearend for our S-10 V8 conversion when Tim said, "Why don't we use a Ford 8.8? They're dirt cheap in the junkyard, and we can convert it over to use GM axles and brakes."

Here's how this works. The stock S-10 7.5-inch rearend is far too weak to survive behind our small-block Chevy. It's also narrower than a typical Fox-body Mustang 8.8. The advantage of the 8.8 is that it is practically a direct copy of the GM 12-bolt. The ring-gear diameter is essentially the same (8.800 versus 8.875 inches), the pinion-shaft diameters are identical, and the Ford shares the same pinion and carrier bearings with the Chevy passenger-car 12-bolt. The Ford Traction-Lok limited slip is similar to the Eaton clutch-type posi and is rebuildable. The only 8.8 weak point is the early version's 28-spline axles compared to the 12-bolt's 30-spline.

But that was Tim's inspiration. We checked to make sure that a GM 28-spline axle would fit the Ford side gears and that the button on the end of the axle was the same so that stock Ford C-clips would work. Tim's idea was to narrow the Ford housing to the S-10 axle width, weld on GM 12-bolt housing ends, and then use a stock S-10 28-spline axle that would give us the 5x4.75 GM wheel-bolt pattern and GM axle offset so we could bolt-on any rear GM brakes. Tim's suggestion borders on pure Junkyard Builder brilliance. The only tricky part would be narrowing the housing, and that turned out to be an exercise in careful measurement. Here's how it all came together.

We found a Mustang 8.8 that appeared to have a posi already in place because both axles turned in the same direction. We yanked the rear out of the Mustang, paid $42.50, and took it back to Tim's shop. After we removed the rear cover, we discovered it was not just a posi but it had a 4.10:1 gear! That means we purchased the equivalent of a 12-bolt with 4.10:1 gears and a posi all for $42.50! We had decided that we would install new axles, but Tim said if we were really on a strict budget, we could have substituted S-10 axles when we bought the housing because the junkyard would neither know nor care. The price would have been the same, which would've reduced our overall cost. Tim is always thinking of ways to make used parts work.


Here's our Ford 8.8 ready to be installed in our V8 S-10. This could be the first hybrid 8/12 rearend! The rear cover and pinion flange says Ford, but it's nearly as strong as a Chevy 12-bolt.

The first step after we completely stripped the 8.8 was to carefully remove all the Fox Mustang brackets because we didn't need them for the S-10. We used a cut-off wheel and grinder. Do not use a torch. The heat from the torch can easily warp the housing because the 8.8's axletubes are very thin.

This is part of our Mittler Brothers narrowing tool. The multiple holes in the plate mount to different rear ends. The slot positions the tape measure at the pinion centerline to allow precise measurements of housing width. Keep in mind the pinion gear is offset relative to the centerline of the housing width. We simulated a GM offset of 1⁄2-inch.

After carefully measuring the stock S-10 axle width at 481⁄2 inches (housing flange-to-flange), Tim cut the Ford housing (which was more than 54 inches), taking into account the width of the replacement housing ends.

We used Moser housing ends that are designed as a stock replacement end for a Chevy 12-bolt. The small step is designed to fit inside the stock 12-bolt housing tubes, but it was too large in diameter to fit into the Ford tubes, so Tim removed it.

This was also a good time to install the Moser spring perches. Tim measured the pinion angle on a stock S-10 and duplicated this on the Ford rear. He MIG welded the perches to the housing before installing the housing ends. That way, the install would compensate if the tubes distorted.

The Mittler Brothers tool employs a 60-inch-long steel mandrel and aluminum and steel adapters that create a true centerline for the housing ends. Tim carefully measured the overall length, then tack-welded the housing ends. He also clocked the housing ends exactly like GM did because the 12-bolt pattern is asymmetrical.

After double-checking the fitment, Tim slowly MIG-welded the ends with 1-inch beads alternating between sides and taking time between welds to minimize heat concentration.

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4 comments
Montrose Patriot
Montrose Patriot

im considering this swap - the 7.5" in my 92 S10 Blazer has the oddball toner rings on the axleshafts where im retaining the stock drums (the article stated that the 28 spline axleshafts from the 7.5 (S10) can be used with the 8.8 although any 8.8 from a Fox chassis (Mustang, T bird, Cougar) or a Panther (Crown Vic, Grand Marquis, Town Car) with 28 spline axles will function...

Jamie111
Jamie111

I did the 8.8->'67 Mustang swap after reading the '07 article.  Thanks!    In my case, I tacked the leaf spring brackets then brought it to a rear end shop that welded them for a few bucks.

I used an LSC rearend with I think 12" disk brakes.

The housing is the right size, but the axle/brake bracketry makes it too wide.  You can swap the brake brackets left to right (pulling the calipers in) and snag shorter axles (I think fox mustang or 2 ranger driver side - google it :)       Maybe not worth it if you can find a cheap  rear end from a more common car, but definitely workable.

Paul Smalley
Paul Smalley

When narrowing an 8.8 rear for a leaf spring car, most people are using Explorer rear end's because they come with 31 spline axles, good rear ratios (3.73, etc)  and limited slip differentials.  Some years will have disc brakes too. 

Jamie111
Jamie111

Pro-tip:   When seriously upgrading rear brakes, always put in a proportioning valve and adjust it before you need to do a panic stop.     I accidentally did a super-smokey 180 (more like 90..wave sheepishly through the smoke, finish eating my burrito.. 90) in front of a line of CHP cars even though I couldn't have been going more than 50.   I tell myself was prop-valve related, not driving skill :)

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