Do you own a 1981 or newer Chevy or GMC truck? Or maybe a Blazer, Tahoe, or Suburban? Does it have a 14 bolt rear axle? If not, do you want one?
If your answer is yes, then a 14 bolt rear axle swap may be in your future.
In this article we’ll share some info about swapping out your 10 bolt rear axle or 12 bolt rear axle (aka GM 8.5″ or 8.875″ axle) for a 14 bolt, talk about the pros and cons, and share some helpful resources.
First, What’s A 10 Bolt?
Most 1/2 ton trucks offered by Chevrolet and GMC between 1980 and 1994 were equipped with a 8.5″ or 8.875″ ring gear axle, which many people call a 10 bolt axle. This axle is also found on some 4WD trucks from the late 70s and is common on Chevy and GMC full-size SUVs (Suburban, Tahoe, and full-size Blazer). According to Hemmings, you can find a 10 bolt axle on at least some of GM’s 4WD trucks and SUVs produced from 1977 to 1997.
GM benchmarked the old Dana 44 front axle when they designed the 8.5″ front axle, so a lot of the dimensions are similar. The ring gear, axle spline count, axle shaft diameter, are all fairly close (not identical, but close). And while there is always a debate about the relative strengths of any axle, a Corporate 10 bolt, front axle is considered a solid, yet perhaps unexceptional axle. Most people agree that the GM 8.5″ axle (aka Corporate 10 bolt) is just fine for vehicles with 33″ tires (or smaller). After all, it is basically the same as the Dana 44 front axle.
Second, What’s A 12 Bolt?
Many 1/2 ton GM trucks got the upgraded 12 bolt axle. Unlike the 10 bolt that was used in front and rear applications, the 12 bolt was only used in rear applications. The 12 bolt was used in trucks from 1965-81. It is stronger than a 10 bolt, but not even close the the legendary 14 bolt.
Why Swap A Corporate 10 Bolt or 12 Bolt For A 14 Bolt?
The main reasons to swap out your Corporate 10 bolt for a 14 bolt are:
- Strength. A 14 bolt is a monster of an axle compared to a 10 bolt or 12 bolt – larger housing, larger axle shafts, larger ring gear, etc. If you’re doing any serious off-roading, for example, you’ll appreciate the upgrade.
- Lots of upgrade options. It’s typically easier to find upgrades for the corporate 14 bolt full-float axle (like disc brake conversion kits!) than it is for the 10 bolt or 12 bolt. There are more options for axle shafts, ring and pinion sets, trusses, and so on.
- The swap is fairly easy. Because the 10 bolt, 12 bolt and 14 bolt rear axles were often offered side by side on the same vehicles (GM offered 14 bolt axles on a lot of trucks), it’s fairly easy to do the swap (depends on your vehicle, depends on your situation).
- The swap is relatively affordable. While it’s becoming more and more difficult to find a good 14 bolt at your local junkyard, they’re definitely around. The same can’t be said for the Dana 60, which is similar in terms of size and strength, but more expensive to source.
While the 10 bolt is adequate for a lot of day-to-day activities, a stock 14 bolt axle is a monster. If you’re concerned at all about durability, why not?
Remember, Not All 14 Bolts Are The Same!
If you’re new to the 14 bolt axle, it’s important to note that there are full-float and semi-float versions of the 14 bolt. The 14 bolt full-float is the more desirable axle as far as strength is concerned, and they’re also fairly inexpensive to build up. However, the semi-float 14 bolt is also an upgrade over a 10 bolt or 12 bolt, and some people have found that the semi-float is an easier upgrade (the spring perch width may be the same on the 14 bolt semi-float and the 10 bolt, depending on your vehicle).
If you’re looking for a 14 bolt axle, here’s an identification chart to help you determine which type of axle you’re looking at (full-float or semi-float).
Know Your Widths
When you’re swapping, you’re going to want to know:
- Wheel mounting surface to wheel mounting surface width for both axles
- Spring perch to center point width for both axles
- Shock mount to center point width for both axles
These widths will determine how easy or hard your swap will be.
You want to measure wheel mount surface to wheel mount surface because 14 bolt widths are different for single rear wheel, dual rear wheel, and cab-and-chassis applications. You can address the width differences easily enough, but it’s more work.
The spring perches and shock mount distances will help you determine if you’re going to have to “move” the spring perch (or pay someone that knows how to weld to do it for you), but there are many stories of 14 bolt swaps that took place with stock spring perch mounts on both axles. Some say it’s only semi-floats from certain year ranges that swap over, some say there are full-floats that will work..but instead of trying to find the perfect donor axle, just measure it.
Gear Ratios, Conversion U-Joints, and Misc. Hardware
When you swap out your 10 bolt for a 14 bolt, you want to think about a few more things:
- Gear ratio. If you’ve got a 4×4, front and rear axle gear ratios need to be the same. Figuring out the gear ratio in a 14 bolt is simple enough if you can spin the wheels on the donor vehicle and count the pinion rotations. Otherwise, you’ll need to count the teeth on the ring gear and the teeth on the pinion and do some math.
- A “conversion” U-joint. To connect your existing driveshaft to a 14 bolt axle, it’s probably easiest to find a so-called “conversion” u-joint that fits both (the stock U-joint won’t work).
- U-bolts and U-bolt plates. Because the housing is bigger on a 14 bolt than a 10 bolt, you’ll want to get the U-bolts and U-bolt plates off the donor vehicle (well, at least the bolt plates – new U-bolts are probably a good idea).
- Wheel lug count. Your front and rear wheels might not have the same lug count when you’re done with your swap. You can fix this with a spacer/adapter that converts an 8-lug mount to a 6-lug (or a 6-lug to a 5-lug).
- Driveshaft. Since the 14 bolt axle is bigger, you will most likely need to have your driveshaft shortened.
- Matching lug pattern. Most 14 bolt axles are 8 lug. This means if you want to match your front axle you may need to upgrade it to 8 lug also. You may be able to find wheel adapters to convert your front 6 lug axle to 8 lug.
- Wheels. Now that you have a heavy duty 8 lug rear axle you will need to upgrade to 8 lug wheels also. Note: There is a rare 6 lug semi float 14 bolt axle that can be used to keep the same lug pattern.
What About Keeping The 10 Bolt?
Swapping out your 10 bolt for a 14 bolt isn’t always easy. If you can’t find a 14 bolt that’s close enough in size to your 10 bolt, you might think about skipping the swap and building up your 10 bolt instead. While you can certainly do this – upgraded components are available for 10 bolt axles – it’s fairly expensive.
Many people spend just a few hundred dollars doing a 14 bolt swap. Building out a 10 bolt with stronger gears and axle shafts can cost twice as much as a swap, and when you’re all done your 10 bolt may not be any stronger than a stock 14 bolt. Unless a swap is out of the question, it’s definitely the way to go.
Tips For Finding A Good Used 14 Bolt
If you’re going to acquire a used 14 bolt, it’s a good idea to inspect the following:
- Is it straight? If you lay a 6 foot long 2×4 alongside the housing, does it look bent? How about if you look at it sideways?
- How about the axle shafts – are they bent? A steel straightedge is a good tool. A bent axle shaft is fixable, but you want a discount.
- Visually inspect the ring gear and interior gears. Look at the teeth and check for chips, scoring, and/or cracks. If you see wear, you want a discount.
- Inspect the housing for excess rust. Surface rust is common, but any penetrating rust is a problem.
Because 14 bolts are still relatively easy to find, you don’t have to settle for an axle that’s damaged. It’s okay to be a bit picky.
Don’t Forget To Convert Those Drums To Discs!
Last but not least, converting from drum to disc brakes is beneficial in many ways. We offer disc brake conversion kits for all types of 14 bolts, you just need to know which type you have (full-float or semi-float, 6 lug or 8 lug).
The only thing to keep in mind here is that, when you replace drums with discs, you lose the parking brake. We have a solution for that – an emergency brake caliper kit – but it’s important to note. If you can’t get your vehicle registered without a parking brake, for example, you’ll want to plan ahead.