Build A Budget Ford 8.8 Axle For Any Leaf Spring Muscle Car

Words And Photos: Jeff Smith

Car guys are a creative lot. We’re constantly looking for ways to make those parts live longer without breaking while also adding more power that will tax even the strongest part. Plus, we don’t want to spend any money. The easy way is to just drop the cash on the good parts and sometimes that’s the only real way to solve the problem. But then there are those creative guys who just seem to pull these amazing ideas right out of the dirt and grease of the junkyard and make them work.

At a recent bench racing session, we were discussing how to add a stronger rear axle assembly to early leaf spring muscle cars. The classic answer has always been to stuff in a big Ford 9-inch and be done with it. One reason that the Ford 9–inch is so durable is because the pinion gear is located lower on the ring gear than other typical rear axle assemblies. This lower position increases tooth contact, increasing the gear’s capacity to accept high torque levels. This also increases the amount of power it requires to spin those rear gears. We tested a 12-bolt against a Ford 9-inch and a Strange S-60 (Dana 60) several years ago and found on a small-block Chevelle that it the 9-inch eats about 8 to 9 rwhp compared to either a GM 12-bolt or the Dana. Plus, the 9-inch is heavy and not everybody needs a big 9-inch under a small car.

Here is where my buddy Tim Moore came up with a really great idea. The challenge was to find a rear axle assembly that was nearly as strong as the GM 12-bolt but was less expensive to build and do this for a ’66 Chevy II. At first, we came up with using a ’71 and later Camaro 10-bolt. Beginning in 1971, GM merged the 10-and 12-bolt housings into a single, stronger 10-bolt. The 8.5-inch ring gear carrier diameter was in between the 10-and 12-bolt and the pinion gear shaft diameter is the same as the 12-bolt. But the hitch in this plan was that we couldn’t find many ’71-’81 Camaros in the junkyard. They’re just too old.

That’s when Tim suggested the Ford 8.8. If you look at the 8.8 specs, that rearend housing is essentially Ford’s version of the GM 12-bolt. The ring gear diameter is 8.8 while the 12-bolt is 8.875 inches. The pinion gear shaft diameter is exactly the same as the 12-bolt. In fact, they use the same pinion bearings – the PN’s don’t lie. The only place where the 8.8 is weaker is with axle spline count. The GM 12-bolt uses 30-spline axles while the early 8.8 Fords use a 28-spline count. But this turned out to be an advantage. But best of all, the Ford 8.8 rearend is used in literally thousands of V8 Fords, including Mustangs, full-size cars, Broncos, and ’90-’01 Explorer and Mountaineers.

But Tim’s idea goes further than just trying to find an 8.8-inch Ford rear axle assembly that’s close in width. His plan was to find a Fox body Mustang 8.8, purchase the housing assembly with the gears but leave the axles and Ford rear brakes in the dirt. Then we’d remove the coil spring brackets and weld on leaf spring perches. But the best idea was to cut off the stock Ford housing ends, narrow the housing to the Chevy II width and weld on stock Chevrolet housing ends to the Ford tubes. With the housing now at the width of the stock Chevy II, we could use the 10-bolt Chevy II axles and c-clips that would fit right into the stock Ford differential. Because the housing ends are now GM, we also converted the rear brakes to rear discs. We’ll save that for a follow-up story, but the key is to use third generation Camaro rear disc brake caliper brackets and everything bolts on.

In our opinion, this is near genius. But at the junkyard, the plan got even better. We ran across a Fox body Mustang that already had a posi. We removed the housing, stripped the axles and brakes and paid barely $50 for the assembly. Once back to the shop, we counted the gear teeth and discovered this rear was fitted with 4.11:1 gears and a functional clutch type limited slip.

After removing all the coil spring brackets and carefully measuring for the stock early Nova housing width, Tim used a chop saw to narrow the Ford housing and then used a Mittler Brothers mandrel tool to ensure that the ends were on straight and true to the axle centerline before welding. He also established the proper pinion angle for the leaf spring perches, and welded them on to fit the stock spring width of the Chevy II. Finally, we added stock Chevy II 10-bolt 28-spline axles. The 4.11:1 gear was actually too deep for our needs, so we rebuilt the limited slip and added a used set of 3.45:1 gears from a different Ford 8.8.

The parts included new axles, new bearings and shims to set up the limited slip all from Ratech, used gears, new Moser GM housing ends, and a pair of new axles. This came to roughly $800 (not including labor) and we now have a brand new rear end that will last, if not abused, forever. If you were really on a tight budget and found an 8.8 with acceptable gears and a posi that did not need to be rebuilt, it’s possible to pay a shop to narrow the housing, weld on the housing ends and reassemble the rearend for around $500. Compare that to the cost of a brand new 12-bolt assembly from one of the axle companies and depending upon options – this could run in excess of $2,000. Even good used GM 12-bolts are selling for $1,800.

There are also many more options for this 8.8 plan. If the 28-spline axles are too spindly, it’s possible to step up to a better 8.8 posi carrier with the matching side gears that would accommodate the much stronger 31-spline axles. In that case, you would probably opt for the Ford 3.150-inch bearing housing end to use bolt-in, 9-inch style axles instead of the c-clip versions. Think creatively – you could spin this idea off into running an 8.8-inch axle housing into nearly any muscle car or hot rod with leaf springs and keep a little cash in your pocket at the same time.

Parts List

Description PN Source Price
Ford 8.8 housing  w/gears, posi N.A. Junkyard $50.00
Mittler Bros Rend End Narrow tool 1000-RENK-1 Summit Racing $574.97
Ratech Pinion Depth tool 10006 Summit Racing $28.97
Ratech Rearend Rebuild kit 305K Summit Racing $73.97
Moser 28 spline axles (need 2) A102809 Summit Racing $259.97
Centric 10-bolt axle bearings (need 2) 41364000 RockAuto $10.40
Moser GM housing end, c-clip style 7905 Summit Racing $99.97
Moser 2 ¼ spring perch, weld on 7200 Summit Racing $19.97
Moser Chevy II mono spring perch 7210 Summit Racing $49.97
Lucas 85w-140 gear lube 1qt. (3) 10042-1 Summit Racing $7.97

Sources

Mittler Bros. Machine & Tool
mittlerbros.com

Moser Engineering
moserengineering.com

Ratech
ratechmfg.com

Tom’s Differentials
tomsdifferentials.com

RockAuto
rockauto.com

Summit Racing
summitracing.com

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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