Metric vs inch bearings

Most of the world's bearings are metric, but inch-series bearings are alive and well — especially in tapered rollers, agricultural and automotive parts, and older or American-designed machinery. Mixing the two systems up is a common cross-reference mistake, because a metric and an inch bearing can look almost the same size and still not interchange.

How to tell them apart

The fastest test is the bore. Metric bearings have round millimetre bores — a 6204 is exactly 20.000 mm. Inch bearings have bores that are round in inches and therefore odd in millimetres: a 3/4" bore is 19.05 mm, a 1" bore is 25.4 mm. If you measure a bore of 19.05, 22.23, 25.4 or 31.75 mm, you are almost certainly holding an inch bearing. A clean 20, 25 or 30 mm points to metric.

The number style is the second clue. Metric ball bearings use the compact designation this site is built around — 6204, 6205, 30204 (see how to read a bearing number). Inch tapered bearings use manufacturer part numbers with letters — an LM11749/LM11710 cup-and-cone set, for example — that encode the size in a different system entirely.

Tapered rollers: two separate worlds

Nowhere does this matter more than tapered roller bearings. The metric series is built to ISO 355 (designations like 30204, 32206); the inch series follows the original Timken system of part numbers. They are not interchangeable — a metric 30205 is not an inch equivalent of anything by coincidence of size, and the cup and cone angles, the e and Y factors and the envelope all differ. The tapered roller guide goes into why the two worlds don't mix.

Are there metric-inch equivalents?

Occasionally a metric and an inch bearing are dimensionally close enough to substitute, but "close" is the operative word — a fraction of a millimetre in bore or outside diameter changes the fit on the shaft and in the housing, which changes the internal clearance and the way the bearing runs. Treat a metric-to-inch "equivalent" as a different bearing to be verified dimension by dimension, never as a drop-in the way two brands of the same metric number are.

Working from a measurement

If the markings are gone, measure the bore, outside diameter and width, then decide which system you are in from the bore: a round millimetre value points to metric and lets you work back to a designation; a round-inch value points to the inch series and a manufacturer part number. The full method is in identify an unmarked bearing. Getting the system right first saves cross-referencing a metric number against an inch part that was never going to fit.

General engineering reference only. Metric and inch series have their own tolerance and rating standards — confirm dimensions and suitability against the manufacturer's catalogue before substituting across systems.