If you've spent any time putting together a steering system on a C10 — whether you're running a stock column rebuild, swapping to an aftermarket tilt column, or building a full custom shaft run after a rack-and-pinion conversion — you've run into the term Double D shaft (also written "DD shaft"). Maybe it was on a parts page. Maybe someone at a car show mentioned it. Maybe you're staring at your lower column output right now trying to figure out what bore your new u-joint needs to be.
Either way, the DD profile is one of the most important and most misunderstood connections in the entire steering system. Getting it wrong means your steering shaft doesn't connect properly, and that's not a problem you want to discover after the truck is moving.
At Don Dotta Solutions, we're lifelong Chevy truck enthusiasts who carry an extensive lineup of steering universal joints in DD configurations. We built this guide to give you the background knowledge that makes buying the right joint simple — and the installation confidence that keeps your build moving forward.
What Is a Double D Shaft?
A Double D (or DD) shaft is a cylindrical shaft that has two opposing flat sides machined into it, one on each side, 180 degrees apart. If you sliced the shaft and looked at the cross-section, it would look like the letter "D" facing both directions. That profile is what gives the DD its name.
The point of the DD profile is mechanical interlock. A smooth round bore relies entirely on a pinch bolt or set screw to prevent rotation. If that bolt backs out even slightly, the shaft can spin inside the bore — which means your steering input isn't fully connected to your steering box. A DD profile adds two flat faces that physically prevent the shaft from rotating inside the bore, even if a bolt loosens slightly. That's an important safety margin in a steering application.
Here's how the common shaft profiles compare:
- Smooth Bore — Round, no flats. Relies entirely on a pinch bolt or set screw. Common on stock factory shafts and some budget joints.
- Single D — One flat machined in. Gives some indexing but less rotational resistance than DD.
- Double D (DD) — Two opposing flats. Best mechanical lock short of full splines. The standard for custom and aftermarket steering builds.
- Splined — Multiple teeth around the circumference. Heavy-duty and OEM applications. Requires matched spline count.
A Brief History: How DD Became the Builder's Standard
The Double D profile didn't start in the hot rod world. It's been used in industrial machinery and agricultural equipment for decades — anywhere a removable connection needed to resist torque without the complexity of full splines. The profile is simple to machine, easy to inspect, and straightforward to assemble in a field environment.
In the automotive aftermarket, DD shafts became the dominant steering connection standard alongside the custom column industry in the late 1980s and 1990s. As builders started replacing factory columns with shorter tilt-adjustable or collapsible aftermarket units, they needed a universal way to interface those columns with factory or aftermarket steering boxes — which often used completely different output shaft profiles than the stock column expected.
The steering u-joint became the solution, and the DD bore became the standard because it was strong, affordable to produce, and compatible with a huge range of shaft diameters. Today, if you're building any kind of custom or resto-mod steering shaft, there's a near-certain chance a DD u-joint is somewhere in the stack.
For C10 trucks specifically, this matters a lot. The squarebody generation (1973–1987) came from the factory with a variety of shaft sizes and box configurations depending on year, trim level, and column type. Once builders started upgrading to rack-and-pinion setups, compact columns, or tilt columns from later trucks, the DD u-joint became the go-to bridge between mismatched components — and it still is today.
Where Does a DD Shaft Fit in the Steering System?
Your steering system runs from your hands at the wheel all the way down to the front wheels. The shaft section — where DD connections live — spans from the base of the steering column down to the input of your steering box or rack. Here's how a typical C10 steering shaft run stacks up:
- Steering Column Output — The lower end of the column shaft. On most aftermarket tilt columns, this is a 3/4" DD output.
- Upper U-Joint — Connects the column output to the top of the intermediate shaft. One or both bores may be DD.
- Intermediate Shaft — Bridges the distance between the column and the steering box. Both ends may be DD or splined depending on the configuration.
- Lower U-Joint — Connects the intermediate shaft to the steering box or rack input shaft. Often a hybrid: DD on one end, splined on the other.
- Steering Box / Rack Input — The connection point at the box. The input shaft spec here determines what bore your lower u-joint needs on the box side.
Every joint in that chain has to match the shaft it's receiving on both ends. That's exactly why knowing your DD size matters before you order.
DD Shaft Sizes You'll Actually Encounter
Not all DD shafts are the same diameter. The DD designation tells you the shape of the profile — not the size. Here are the most common DD shaft diameters in the custom steering world and where you're likely to see them:
- 3/4" DD — The most common size in the aftermarket. Used on the output of most custom and aftermarket tilt steering columns. If you're building a C10 with an aftermarket column, there's a strong chance this is what you have.
- 1" DD — Heavier-duty applications and some performance builds. You'll see 1" DD bores on the box side of the shaft run in some configurations.
- 13/16" DD — Found in select applications, particularly where a mid-size between 3/4" and 1" is needed.
- 11/16" DD — Less common but appears in some factory applications and specialized column builds.
- 9/16" DD — Smaller diameter, used in tight spaces and on certain column inputs.
- 17mm DD — Metric equivalent. Shows up when mixing metric-spec components into an otherwise imperial build.
Pro tip: Always measure your existing shaft across the flats to confirm the DD size — not across the round portion. A 3/4" DD shaft measures 3/4" from flat to flat. Measuring the outer round diameter will give you a larger number and throw off your selection.
C10 Corner: What Square-Body Builders Need to Know
The 1973–1987 Chevy C10 is one of the most-built trucks in the country right now, and the steering system on these trucks can get complicated fast once you start mixing generations, swapping columns, or converting to rack-and-pinion. Here are the key things to understand before you start buying joints.
Factory Column Output
Stock squarebody columns used a splined lower shaft — typically 3/4"-36 spline or a metric equivalent depending on year and market. When you swap to an aftermarket tilt column (which the vast majority of C10 builds do), the output is almost always a 3/4" DD. That means your upper u-joint needs to bridge two different profiles: 3/4" DD on the column side, and 3/4"-36 spline on the shaft or box side. This is a hybrid joint, and it's one of the most popular items in our steering catalog.
Steering Box Input Shaft
If you're running the factory Saginaw recirculating-ball box, the input shaft is typically 3/4"-36 spline. If you've swapped to an aftermarket manual or power box, verify the input spec — many use 3/4"-36 as well, but some heavy-duty boxes use 1"-36 or a DD profile instead.
Rack and Pinion Swaps
This is where builds get complex in a hurry. If you've converted to a rack-and-pinion — whether it's from a C5 Corvette, an S10, or a dedicated aftermarket rack — your pinion input shaft spec may be completely different from your column output. Plan the full shaft run before buying any u-joints. Most builders end up with at least one hybrid joint and often a custom-length intermediate shaft to bridge everything cleanly.
The Two-Joint Rule
Your steering shaft run should have no more than two u-joints where possible, and each joint pair should be phased correctly — meaning the yokes of the two joints should be in the same plane — to cancel out velocity variation through the joint angles. If you get vibration or a pulsing feel in the steering wheel after installation, incorrect phasing is the first thing to check. Keep your operating angle at each joint under 35 degrees for smooth, vibration-free operation.
If you want the full breakdown on building a clean shaft run as part of a larger C10 upgrade, the Ultimate C10 LS Swap Builder's Guide covers how steering, suspension, and the entire driveline relate to each other as a system. And if you're still in the early planning stages on your truck, the Complete Guide to Building a Chevy C10 Truck (1960–1987) is a great place to map out your full build sequence before committing to parts.
Unique Pieces Worth Knowing About
3/4"-30 × 3/4" DD
Most builders default to the 36-tooth spline, but 30-tooth splined shafts show up on certain factory applications and older steering boxes. If you're sourcing a used box from a junkyard and wonder why your standard joint won't seat properly, count the splines. You may need the 30-count version.
5/8"-36 Chrysler × 3/4" DD
This one surprises people: Chrysler used a 5/8"-36 spline on many of their factory steering shafts. If you're doing a Mopar-adjacent swap or sourcing a Chrysler-spec box for a custom build, this hybrid joint is the bridge. It's a niche piece, but when you need it, nothing else works.
17mm DD × 3/4" DD
Metric-to-imperial bridge joint. This shows up when mixing metric-spec components — often from an import-spec donor vehicle or a European-market part — into an otherwise imperial build. Don't guess on this one. Verify the metric measurement before ordering.
1" DD × 1" DD
Full 1" DD both ends. Used in serious performance applications where the torque load on the steering shaft is significant — think high-horsepower builds with heavy front ends or trucks that see road course or track use. Not a typical street build joint, but it exists and it's what some builds specifically require.
9/16"-26 × 3/4" DD
This compact joint appears in tight applications where the upper column section has a smaller-diameter output. It shows up in certain tilt column configurations and in builds where geometry puts the column output in a confined space.
Double D U-Joints Available at Don Dotta Solutions
We carry a full lineup of steering universal joints in DD configurations. Every joint below is available in Black, Raw Steel, Nickel, and Stainless Steel finishes. Here are the most common combinations for C10 and custom truck builds:
- 3/4" DD × 3/4" DD — Full DD both ends. Connects two DD outputs directly. Common where an aftermarket column meets a DD-output intermediate shaft.
- 3/4"-36 × 3/4" DD — The most popular hybrid for C10 builds. Bridges an aftermarket DD column output to a factory 36-spline shaft or Saginaw box input.
- 1" DD × 3/4" DD — Steps between a 3/4" DD column output and a 1" DD intermediate shaft or box input. Common in heavier builds.
- 1" DD × 3/4"-36 — Connects a 1" DD shaft to a standard 3/4"-36 splined box input. Heavier-duty connection for performance applications.
- 13/16"-36 × 3/4" DD — Hybrid joint for specific column swaps and specialty applications where the spline count doesn't match the standard 3/4"-36.
- 11/16"-36 × 3/4" DD — Appears on earlier column builds where the factory spline count differs from the common 36-tooth standard.
- 17mm DD × 3/4" DD — Metric-to-imperial bridge. Use when a metric-spec component is one end of your shaft run.
- 7/8" Smooth Bore × 3/4" DD — Connects a smooth-bore shaft (set-screw type) to a 3/4" DD output. Useful in OEM-replacement scenarios and budget builds that retain a factory-style shaft on one end.
- 3/4"-30 × 3/4" DD — For the 30-tooth spline applications. Verify your spline count before ordering if you're running an older or sourced box.
- 5/8"-36 Chrysler × 3/4" DD — For Chrysler-spec shaft applications mixed with a DD column output.
Each joint is sold individually. Order all your steering hardware in the same finish at the same time — mixing black and chrome or raw steel in the same shaft run is one of those details that shows up and is hard to un-see once the truck is done.
Installation Tips for DD U-Joints
- Torque your pinch bolts properly. Most DD u-joint pinch bolts spec between 18–25 ft-lbs. Under-torquing leaves the joint loose. Over-torquing can crack the yoke. Check the spec for your specific joint and use a torque wrench — don't guess.
- Phase your joints correctly. Both u-joints in a shaft run should have their yokes in the same rotational plane. If you get vibration or a pulsing feel in the wheel, incorrect phasing is the first thing to check — not a defective part.
- Use anti-seize on the shaft before assembly. DD profiles can gall and seize, especially raw steel shafts in humid environments. A thin coat of anti-seize on the shaft before inserting it into the bore makes future disassembly much easier.
- Check for binding through full lock-to-lock. After installation, turn lock-to-lock slowly by hand before driving the truck. Any binding or tight spot indicates either a steep operating angle or a phasing issue. Fix it before the truck moves under power.
- Don't overlook the finish. A chrome u-joint against a black column or raw steel hardware next to a polished shaft looks unfinished. It's a small thing, but finished trucks have consistent hardware throughout the steering shaft run.
Questions? We're Here.
If you're unsure which joint you need, the fastest way to figure it out is this: measure your column's lower output shaft (confirm it's DD and what diameter), then look up — or measure — the input shaft spec on your steering box or rack. The u-joint you need bridges those two specs. If both ends are 3/4" DD, straightforward. If one end is DD and the other is splined, you need a hybrid. We carry both.
Still not sure? Reach out directly. Call the Don at 805-272-6924 or send a message through our Contact page. We're builders, not a call center, and we're happy to talk through your shaft run before you order.
Ready to shop? Browse the full steering universal joint lineup at Don Dotta Solutions — Suspension & Steering.
Related reading from the DDS blog:
