How to Measure Coupling Bore Size

Coupling bore size is the inside diameter of the coupling hub where your shaft inserts, and it needs to match your shaft diameter within the manufacturer’s stated tolerance — not just be “close enough.” Measure the shaft’s actual diameter with calipers at multiple points, not the coupling’s existing bore, and always check keyway width and depth separately since a bore can be correct while the keyway isn’t.

Get this wrong and you’ll end up with a coupling that either won’t slide onto the shaft or fits so loosely it slips under load. Here’s how to measure it properly.

What You’ll Need

• Digital or dial calipers (a tape measure isn’t accurate enough for this)
• The existing coupling or shaft you’re matching, if replacing a worn part
• A keyway gauge or a second set of calipers for keyway width/depth
• The manufacturer’s spec sheet for the coupling you’re considering, if available

Step 1: Measure the Shaft, Not the Old Coupling

If you’re replacing a coupling, it’s tempting to measure the bore of the worn-out part instead of the shaft itself. Don’t — years of use can wear a bore slightly oversize, and you’ll carry that error into your new part. Measure the shaft diameter directly with calipers, at two or three points along its length, since shafts can have minor variation from wear or manufacturing tolerance.

Step 2: Take the Measurement in Two Directions

Measure the shaft diameter, then rotate 90 degrees and measure again. If the two readings differ noticeably, the shaft may be worn oval — a common issue on older equipment — and you should note the larger reading as your working diameter, since that’s what needs to fit.

Step 3: Check the Keyway Separately

Bore diameter and keyway dimensions are two different specs, and a coupling can have the right bore but the wrong keyway width or depth. Measure the keyway’s width across the slot and its depth from the shaft’s outer diameter down to the keyway floor. Standard keyway sizes are tied to shaft diameter ranges, but manufacturing tolerances mean it’s worth confirming rather than assuming.

Step 4: Compare Against the Manufacturer’s Tolerance, Not Just the Nominal Size

Every coupling bore is manufactured to a tolerance range around its nominal size — for example, a “1 inch” bore might actually be machined to +0.0005″/-0.0000″ or similar, depending on whether it’s meant to be a slip fit or an interference fit. If the manufacturer publishes this tolerance, compare your measured shaft diameter against the actual range, not just the rounded nominal number stamped on the spec sheet.

Step 5: Check Fit Type Requirements

Some couplings require a specific fit type — clearance (slip) fit for set-screw or clamp-style hubs, or a tighter interference fit for some keyless or press-fit designs. Confirm which fit type your coupling needs before finalizing your measurement, since the acceptable tolerance range differs significantly between the two.

Common Mistakes That Cause a Bad Fit

Measuring the coupling instead of the shaft. Worn bores give you the wrong number. Always measure the shaft.

Ignoring keyway dimensions. A bore can be dead-on and the coupling still won’t seat correctly if the keyway width is off.

Rounding to the nearest common size. A shaft measuring 0.998″ isn’t automatically a “1 inch” shaft if the coupling’s tolerance doesn’t allow for that gap — check the actual spec, especially on precision or high-speed applications.

Not accounting for shaft wear. On older equipment, a shaft can wear undersize at the point where the old coupling sat. Measure at a fresh, unworn section if one is accessible.

Assuming both sides of a coupling match. Many couplings — especially jaw, disc, and Quadraflex styles — are sold or manufactured with different bore sizes on the driving and driven hub. Measure both shafts independently.

When You’re Between Two Sizes

If your shaft measures between two standard bore sizes, don’t round up and rely on the coupling’s tolerance to take up the slack — an oversized bore on an undersized shaft creates play that leads to premature wear, vibration, and eventual failure. Check whether the manufacturer offers a bushing or sleeve adapter for that in-between size, or size down and consider a keyless locking bushing that clamps directly to the shaft instead of relying on a fixed bore match.

Once you’ve got your shaft diameter and keyway dimensions confirmed, browse our coupling selection filtered by bore size, or check our shaft collars and keyless bushings if you’re dealing with a non-standard or worn shaft that needs an adapter solution.