How to Measure Your Jeep for Custom Long-Arm Control Arms
Here is something a lot of people don't know Core 4x4 offers: if you have an old or worn-out long-arm kit you can no longer get parts for, you don't have to rip off the brackets, recut, and reweld to fix it. Send us a few measurements and we'll build custom replacement arms to fit the brackets already on your Jeep. In Episode 1 of this JK build with the CarBuff Network and RockJock, Spence walks through exactly which measurements we need—and how to take them at home with nothing but a tape measure, a notepad, and your phone.
The Project: Two Jeeps, Two Weeks, One Jamboree
This build kicks off a partnership with the CarBuff Network—an online platform where builders showcase their projects and credit everyone involved—and RockJock. CarBuff's Spencer and his brother Trent (the team behind the SEMA-shown '79 Wagoneer) brought two Jeeps to the Core 4x4 studio with two weeks to get them trail-ready for the Jeep Jamboree in Tillamook, Oregon.
The JKU featured in this episode is going from a 3-inch to a 4-inch setup with a custom long-arm suspension, a high-steer setup, and an anti-rock sway bar. Episode 1 is all about the first step: measuring the existing worn kit so replacement arms can be built to spec.
Custom Arms Without Re-Cutting Your Brackets
Here is the core idea. A long-arm kit relocates your control-arm mounts to brackets welded onto the frame and axle. When the arms in that kit wear out and the manufacturer no longer sells replacements, the usual fix is to tear off those brackets, recut, and reweld an entirely new system—a big, messy job.
The alternative: keep your existing brackets and simply swap in new arms built to match. Because Core 4x4 machines, welds, and powder coats in-house in Orem, Utah, a custom-fit replacement arm can be built in less than a week from a handful of measurements. The company has been building custom arms this way for over 13 years.
The Three Measurements Every Arm Needs
For any straight arm—control arm, track bar, or steering linkage—there are three measurements:
1. Bolt-to-Bolt Length
Measure from the center of one bolt to the center of the other bolt. This is the length that actually matters, because it defines where the arm holds each mount relative to the other. Don't measure end-to-end—that number isn't useful. Every Core 4x4 arm is adjustable, so getting within an eighth to a quarter of an inch is plenty; the arms are built with adjustment range around your measurement (with tolerance given on the short side so you can dial it in).
2. Mount Width
The easiest way to get mount width is to run a tape measure across the inside width of your mounting bracket. These widths are fairly standardized across Jeep, Dodge, and other manufacturers, so getting into the ballpark is usually enough for us to narrow down the exact spec.
3. Bolt Hole Size
The diameter of the through-bolt. Bolt sizes are also standardized across most manufacturers—if you're unsure, the sales team can help you identify it.
With those three numbers, a custom replacement arm can be built to fit. That is the entire measurement list for a straight part.
Measuring Bent Arms
Most suspension setups mix straight arms with bent ones. Bends exist for clearance—around the frame, the differential cover, or other arms—so a replacement has to replicate them. Bent arms need the same three measurements, plus a little help understanding where the bend is.
To locate a bend, measure from the center of a bolt to the center of the bend. There are two kinds of bend to be aware of:
- Upward bends (like on the lower rears) are for ground clearance and are straightforward.
- Side bends (bending inward or outward) are the ones we really need dimensioned, because they change the effective length.
The inside/outside trick: on a side-bent arm, the inside of the bend and the outside of the bend are often different lengths. Measure bolt-to-bolt on both the inside and the outside. Add the two together and split them in half—that average is your true center-to-center (eye-to-eye) starting length, which the bend then adjusts slightly. On this JKU's upper rears, for example, the inside measured 19 inches and the outside 19-3/4, giving a true center of 19-3/8. Sometimes the two sides are identical; sometimes they aren't—so always give us both.
Pictures Make the Difference
Numbers tell us the geometry; pictures tell us the intent. For any setup—bent or straight—photos of the interference points help enormously: tire clearance, differential-cover clearance on an aftermarket diff, or frame and body-mount contact. On this Jeep there was some tire rub at the front of the lower arm, so the plan is to move the bend closer to address it—exactly the kind of detail a photo communicates instantly.
To start a custom arm, email sales@core4x4.com with your measurements and a few pictures of your suspension and any interference points.
Common Jeep JK Measurements
These are the typical reference figures from the video for a Jeep JK long-arm setup. Always verify against your own kit—some manufacturers change the bracket-side dimensions—but if your axle side is factory, these are the usual numbers:
| Location | Typical Mount Width | Notes |
|---|---|---|
| Lower arms (front & rear) | 2-5/8 in. | Lower bolt usually 14mm / 9-16 in. |
| Upper front arms | 2 in. | Slight bend on this JKU's uppers |
| Upper rear arms | 2-3/8 in. | More pronounced side bend; measure inside & outside |
Reference figures from the video for a Jeep JK; your kit may vary. When in doubt, send measurements and photos and the sales team will confirm the spec.
Prefer an Off-the-Shelf Kit?
Custom arms are the answer when you already have a long-arm system with good brackets. If you're building from scratch or replacing a complete kit, Core 4x4 offers off-the-shelf heavy-duty adjustable long-arm kits for the JK/JKU, available as a full kit or split front and rear:
- JK 07-18 12pc Heavy-Duty Adjustable Long Arm Kit (full)
- JK 07-18 6pc Front Long Arm Kit
- JK 07-18 6pc Rear Long Arm Kit
What's Next in the Build
With the measurements recorded, the JK got torn all the way down—including a rear track bar bent into a "smiley face" that's getting replaced regardless. The axles are strapped in place for the weekend, and the JL half of the project heads off for gears, fender flares, liners, bumpers, and a winch before it returns.
Episode 2 moves into the fabrication shop: the machining, bending, and welding of the new custom long arms, and the start of bolting fresh parts onto the JK. Stay tuned.
