Getting the right 73-87 c10 rear end specs is one of those things that saves you a massive headache when you're under the truck trying to figure out which parts to order for a weekend project. If you've spent any time working on a "Square Body," you know that while these trucks look pretty similar across that fourteen-year production run, GM changed things up just enough under the skin to keep us on our toes.
When it comes to the rear axle, these trucks are legends for a reason. They're simple, tough, and mostly easy to work on. But whether you're hunting for a replacement at a swap meet or trying to decide if your current setup can handle a new crate motor, you need to know exactly what you're looking at.
The Tale of Two Axles: 12-Bolt vs. 10-Bolt
The biggest point of confusion for most guys is usually the 12-bolt versus 10-bolt situation. From 1973 until about 1981, the 12-bolt rear end was the king of the C10 hill. It's widely considered the "stronger" of the two factory options because of its larger 8.875-inch ring gear. If you look at the diff cover, it's got a bit of an oval shape to it, and—shocker—it has 12 bolts holding the cover on.
Somewhere around 1981 or 1982, GM started phasing out the 12-bolt in favor of the 8.5-inch 10-bolt. Now, don't let the name fool you. While the 10-bolt gets a bad rap sometimes, the 8.5-inch version used in these trucks is actually pretty stout. It's got a rounder cover with two distinct "ears" or protrusions at the bottom of the housing. If you're just cruising or doing light towing, a healthy 10-bolt is more than enough. However, if you're planning on doing burnout contests or dropping in a high-horsepower LS, that's where the 12-bolt (or an aftermarket upgrade) starts to look a lot more attractive.
Understanding the Width and Wheel Fitment
If you're moving from an older 1967-1972 truck to a 73-87 model, the first thing you'll notice is that the rear end got a little wider. The 73-87 c10 rear end specs for width are generally around 62 inches from wheel mounting surface to wheel mounting surface.
This is a crucial measurement if you're buying custom wheels. Because the axle is wider than the previous generation, the offset and backspacing requirements change. You can't just swap wheels from a '68 C10 and expect them to sit perfectly in the wheel wells of an '85. Most of these trucks came with a 5x5 bolt pattern (5 lugs on a 5-inch circle), which is the standard for half-ton GM trucks of this era. If you've got a heavy-duty chassis or a 4WD model (K10), you might run into 6-lug setups, but for the standard 2WD C10, it's almost always 5-lug.
Identifying Your Gear Ratio
Nothing kills the fun of a truck like having the wrong gears. If you've got 2.73 gears and you're trying to pull a trailer, your transmission is going to hate you. On the flip side, if you've got 4.10s and you're trying to do 80 mph on the interstate, your engine is going to be screaming.
The easiest way to find your factory gear ratio is to look at the Service Parts Identification (SPID) sticker. On most Square Bodies, this is located inside the glove box. You're looking for "GU" codes. For example: * GU2: 2.73 ratio * GU4: 3.08 ratio (very common for highway cruising) * GU6: 3.42 ratio (a great middle-ground) * GT4: 3.73 ratio (better for towing and off-the-line pep)
If your sticker is long gone or painted over, you can do it the old-fashioned way. Jack up the rear end (use jack stands, please!), spin the tire one full rotation, and count how many times the driveshaft turns. If it turns a little over three times, you're likely looking at a 3.08 or 3.42. It's not pinpoint accurate, but it gets you in the ballpark.
The Infamous "Gov-Lok" Differential
When digging into 73-87 c10 rear end specs, you'll likely come across the RPO code G80. In the truck world, this is the factory locking differential. GM called it the "Gov-Lok," but many enthusiasts affectionately (or not so affectionately) call it the "Gov-Bomb."
Here's the deal: the Gov-Lok is designed to stay open during normal driving but lock up when it senses a difference in wheel speed (like when you're stuck in the mud). It works great for a farm truck or a daily driver in the snow. However, it's not designed for high-performance use. If you try to do a massive burnout and the locker engages suddenly under heavy load, it has a tendency to shatter into a million pieces. If you see G80 on your glove box sticker, just know what you have and treat it with a bit of respect—or plan on replacing it with a Truetrac or a traditional Posi unit if you want to play hard.
Axle Shafts and Spline Counts
Inside the housing, the axle shafts themselves have their own specs. Most of the 10-bolt rear ends in the early 80s used 28-spline axles. Toward the very end of the Square Body run (around 1987-1988), some of these trucks started seeing 30-spline axles, which are slightly stronger.
If you're ordering replacement shafts or an aftermarket locker, you have to pull a shaft and count the splines first. There's nothing more frustrating than having your truck apart on a Sunday afternoon only to realize you bought 30-spline parts for a 28-spline housing. The shafts are C-clip retained, meaning you have to pull the diff cover, remove the cross pin, and push the shafts in to pop the clips out. It's a messy job, but it's the only way to be 100% sure of what you've got.
Common Upgrades and Swaps
Even though the factory 73-87 c10 rear end specs are decent, a lot of guys choose to swap things out. If you're building a dedicated drag truck or a high-end pro-touring build, the Ford 9-inch is the most popular swap. It's almost infinitely adjustable and can handle massive amounts of power.
However, if you want to keep it "all GM," finding an old 12-bolt from a 70s C10 is a bolt-in upgrade for the later 10-bolt trucks. Just remember that you might need to shorten or lengthen your driveshaft depending on the specific yoke and housing dimensions. Another popular "budget" upgrade is finding a 14-bolt semi-floater from a heavy-duty half-ton or light-duty 3/4-ton. These are significantly stronger than the 10-bolt but still maintain a reasonable weight and size.
Maintenance and Keeping it Alive
Regardless of which axle you have, keeping the fluid fresh is the best thing you can do. Most of these old trucks have been sitting for years, and the gear oil inside probably looks more like chocolate milk than lubricant. A standard 80W-90 gear oil is usually fine for these units. If you have a limited-slip or Posi-traction unit, don't forget to add a friction modifier, or you'll hear all kinds of nasty chattering and popping when you go around corners.
Also, take a look at your leaf spring bushings and U-bolts while you're under there. A lot of the "clunking" people blame on the rear end is actually just worn-out rubber bushings or loose hardware. If you're checking your 73-87 c10 rear end specs because of a noise, start with the easy stuff first before you go tearing into the differential.
Final Thoughts on Square Body Rears
The rear end in a 73-87 C10 is a workhorse. Whether you have the early 12-bolt or the later 10-bolt, you're dealing with a design that helped build the reputation of these trucks. Knowing your specs—from the 5x5 bolt pattern and the 62-inch width to your specific gear ratio—is the secret to making sure your truck performs exactly how you want it to.
It doesn't matter if you're restoring it to factory fresh or building a restomod; these axles are the foundation of the truck's driveability. Take the time to identify what you have, give it some fresh oil, and it'll likely keep spinning those tires for another forty years. Just maybe take it easy on those Gov-Loks if you value your gear teeth!