Speedy Auto > Car Tech > 4x4 > Transmission Drivetrain

14-Bolt Basics - Inside The Ultimate Junkyard Axle - Four Wheeler Magazine

Ali Mansour Brand Manager, 4WD & Sport Utility

When it comes to strength, value, and vast availability, the full-float 14-bolt rear axle from General Motors is about as good as it gets. Fitted with an impressive 10½-inch ring gear, three pinion bearings, and backed by a strong array of aftermarket support, the GM 14-bolt can easily handle up-to 44-inch tires in most applications. In case you're not familiar with the 14-bolt, the axle gets its name from the 14 bolts that secure the differential cover. First appearing in 1973, the 14-bolt is one of the most used rear axles in the General Motors stable.

14 Bolt Basics Inside The Ultimate Junkyard Axle trimmed 14 Bolt Axle Photo 44113306

Found under ¾- and 1-ton GM trucks, vans and SUVs, the 14-bolt received a max torque rating of 6,242 pounds. Up until 1994, GM manufactured the 14-bolt in-house. Post 1994, the 14-bolt was built by American Axle & Manufacturing. While the 14-bolt can still be found under many late-model GM 2500HD gas trucks and Suburbans, most '01-and-newer ¾- and 1-ton diesel trucks use the AAM 1150 rear axle. The AAM 1150 uses an 11½-inch ring gear, but retains the same axleshaft diameter as the 14-bolt.

Although there are a few versions, the full-float, 8-lug 14-bolts are the most desirable for those looking to build an off-road rig or upgrade their pickups light-duty rear axle. The 3¼-inch axletubes provide plenty of room for custom brackets and some of the pickup versions can be easily swapped into other makes with minimal fabrication. Maybe the best part about the 14-bolt is that they are plentiful in junkyards and can typically be picked up for less than $200 in operable condition. Aside from a few brake variations and a 18-inch diameter increase in the outside pinion bearing in the late '80s, the 14-bolt went relatively unchanged throughout its long run.

In this article we've taken hold of a late and early model 14-bolt axle to show you the ins and outs. We've even dug-in (literally) to our late model 14-bolt housing to install a shave kit from Ballistic Fabrication. What makes the Ballistic Fabrication Shave Kit a key upgrade for the 14-bolt is that it slices 2 inches off of the bottom of the massive 14-bolt housing. This is no small feat, but a tremendous upgrade for those looking to get the most out of their junkyard jewel. Ultimately, the 14-bolt is one of the most DIY-friendly axles you will find and can be made as high-end or basic as you choose.

PhotosView Slideshow

The majority of the GM 14-bolt axles have a full-float design. The advantage of a full-float axle is that it removes the load from the axleshafts and transfers it to the hub. This full-float design not only allows for a stronger axleshaft, but an increase in the load carrying capacity over a semi-float axle. The full-float 14-bolt weighs around 500 pounds and is fitted with an 8-on-6 1/2 -inch wheel bolt pattern. The axles Gross Weight Rating can range from 5,500 to 8,600 pounds.

On the left is a stock Dana 35 rear axleshaft and to the right a 1 1/2-inch 14-bolt ’shaft. The 30-spline 14-bolt axleshafts are comprised of 1040 carbon steel and are very stout. If you do manage to break one, they are easy to come by in wrecking yards and can also be replaced with stronger aftermarket chromoly units.

Prior to 1999, all 14-bolts were fitted with drum brakes. Like the axles, there are brake variances throughout the years. For off-road use, we suggest ditching the heavy drums for a lighter, and more dirt friendly, disc brake conversion. Companies such as Offroad Design offer bolt-on disc brake conversions which use front rotors and calipers from ’73-’87 3/4-ton GM trucks. It’s important to note that not all disc conversion kits are parking-brake compatible.

Early 14-bolt housings had less ribbing than later housings. To get a little more ground clearance from the early models you can cut off the lower fin from the bottom of the housing. Then, using a grinder, you can smooth out the bottom of the diff, which helps prevent it from being hung on obstacles as easily.

Unlike a Dana 60, the 14-bolt’s pinion-support assembly actually bolts to the housing. Once bolted in place, the pinion head is supported by a third internal bearing.

This support bearing prevents the massive 1 3/4-inch 30-spline pinion from deflecting, which is especially important when behind vehicles producing a large amount of torque such as a modern diesel truck.

Inside of the differential sits a massive two-piece carrier, which uses the ring gear bolts to merge the two halves. On this particular unit, a Grizzly Locker sits in place of the factory spider gears. If you plan to re-gear your 14-bolt with a numerically higher ratio such as 5.38:1, most aftermarket gearsets will require a 4.10 and down differential carrier. The case break on the 14-bolt is 4.11 and down and 4.56 and up.

Without question, two of the biggest drawbacks of the 14-bolt are the size of the differential housing and its massive low-hanging pinion. No matter what year, every 14-bolt housing simply hangs down extremely low, which absorbs valuable ground clearance. To combat this problem companies like Ballistic Fabrication have developed shave kits, which flatten the bottom of the housing and net nearly 2 inches of clearance.

Trimming the bottom of the housing for the Ballistic kit is no small job. We used a Sawzall, Porta-band, cut-off wheel, and grinder to carefully trim the correct amount off. Expect to spend a fair amount of time with a grinder to get the bottom plate sitting correctly and at the suggested 6-degree angle.

To prep the cast housing for welding, Ballistic suggests pre-heating the emptied diff to approximately 400 degrees. The use of ER70 or ER80 welding wire is recommended as well when using a MIG welder. If you have a heat blanket available, use it to allow the housing to cool slowly.

For the Ballistic shave kit, you’ll need to machine the massive 10 1/2-inch ring gear down to 10.063 inches. We ordered our 5.38:1 gear set already machined from Ballistic, which saved us some machine work and downtime. Since this particular 14-bolt is being placed under a relatively lightweight Jeep Wrangler, we are not too concerned about any strength we may have lost from the machined gear.

Setting the gear pattern is very straight-forward on the 14-bolt. The external collars allow you to easily set backlash without using any internal or external shims. While Ballistic does machine a valley under the ring gear for an additional fluid pool, it is a very tight fit.

To finish out the shave kit, Ballistic provides a 3⁄8-inch-thick diff cover, which is a huge improvement over the laughably thin stock 14-bolt cover. Since the shave kit uses 15-bolts to secure the cover we’re not sure if we should still call it a 14-bolt!

Two of the most common 14-bolt axle widths (measured from wheel mounting surface to wheel mounting surface) are 63 and 67 inches. The 14-bolts found under vans are near 70 inches and are more highly sought after. Later model 14-bolts can be found with disc brakes, but may fetch a higher premium over older models.

Differential Decisions
Gaining 2 inches of differential clearance is nearly the equivalent of moving from a 31- to a 35-inch-tall tire. While we think the Ballistic Shave Kit is an excellent option for those wheelers looking to get the most off-road potential from their 14-bolt, it definitely isn't for everybody. For a ¾- or 1-ton truck that still hauls heavy loads and/or pulls trailers, the differential reduction may have more cooling and strength related cons than pros. The sheer amount of work involved and equipment required for the conversion are also serious items to consider.

14 Bolt Basics Inside The Ultimate Junkyard Axle trimmed 14 Bolt Housing Photo 44113315

Body Chassis Transmission Drivetrain Electrical Engine Suspension Brakes Wheels Tires Tech QA

14-Bolt Basics - Inside The Ultimate Junkyard Axle - Four Wheeler Magazine

You Might Also Like

DON'T MISS