Identifying a U-joint’s size means measuring two numbers — bearing cap diameter and overall width — and knowing which of those two numbers to read depends on how the retaining clips are set up.
Tools: Use Calipers, Not a Tape Measure
Digital or dial calipers are the right tool here. U-joint series are often only a few thousandths of an inch apart in one or both dimensions, and a tape measure isn’t precise enough to reliably tell them apart. A tape measure can get you into the right ballpark, but don’t trust it to distinguish between two closely-sized series.
Step 1: Measure the Bearing Cap Diameter
Measure straight across the outside diameter of one bearing cap. If it’s a conversion U-joint (built with two different sizes on the same cross), measure both pairs of caps — they may not match.
Step 2: Identify the Clip Style
Check how the bearing caps are retained in the yoke:
- Outside snap rings — the clips sit in grooves on the outside faces of the bearing caps.
- Inside snap rings — the clips sit in grooves machined into the inside edges of the yoke ears.
This determines exactly where you take your next measurement.
Step 3: Measure the Overall Width — the Method Depends on Clip Style
| Clip style | How to measure width |
|---|---|
| Outside snap ring | Fully seat the bearing caps, then measure from the outside face of one cap to the outside face of the opposite cap |
| Inside snap ring | Measure from the inside edge of one snap-ring groove to the inside edge of the opposite groove |
Clean off any dirt or old grease before measuring on either style — buildup on the caps or grooves will throw the reading off.
What the Numbers Tell You
Cap diameter and width together identify the series. As one reference point: a common 1310-series U-joint runs roughly 1.0625″ cap diameter by 3.219″ width. Treat that as an illustrative example, not a conversion formula — verify your specific measurements against a manufacturer’s size chart or catalog rather than assuming they’ll match this example exactly. For the full breakdown across the common series (1310/1330/1350/1410), see U-Joint Series Comparison: 1310 vs. 1330 vs. 1350 vs. 1410
PTO U-Joints Follow the Same Method — With Less Room for Error
Power take-off (PTO) joints on farm and industrial equipment get measured the same way, but the tolerance between adjacent series can be even tighter than on automotive U-joints. This is exactly where a caliper stops being optional — a tape measure genuinely isn’t precise enough to reliably distinguish PTO series from each other.
Checking for Wear, Not Just Identifying Size
If you’re checking an installed U-joint for wear rather than identifying a new one, rotate the driveshaft so the joint sits vertically, zero a dial indicator against a bearing cap, and check movement in two perpendicular directions. One source puts the replace-it threshold at readings over roughly 0.006″ — treat that as a reference figure from a single source rather than a universal spec, since it wasn’t independently confirmed elsewhere in this research.
Related reading: What Is a Universal Joint? · U-Joint Series Comparison: 1310 vs. 1330 vs. 1350 vs. 1410 · Can You Drive With a Bad or Broken U-Joint?
