Two size systems
Metric bearings have boundary dimensions in millimetres set by ISO 15 for radial bearings and ISO 355 for tapered roller bearings. Their designations follow a structure of type, series and bore code, so the dimensions can be derived from the number. Inch bearings have dimensions in fractions or decimals of an inch, and their designations are usually not derived from the size. The number is a catalogue identifier, and the same size is often found through a table rather than by calculation.
The conversion between the two units is exact: 1 inch is 25.4 mm. The problem is not the arithmetic, it is that inch dimensions rarely land on a round metric number. A 1 inch bore is 25.4 mm, which is 0.4 mm away from a 25 mm bore. That gap is far larger than the bearing tolerance, so a shaft made for one will not accept the other.
How close the near-matches are
| Inch bore | Exact in mm | Nearest metric bore | Difference |
|---|---|---|---|
| 1/2 in | 12.700 | 12 mm | 0.700 mm |
| 5/8 in | 15.875 | 15 or 17 mm | 0.875 or 1.125 mm |
| 3/4 in | 19.050 | 20 mm | 0.950 mm |
| 1 in | 25.400 | 25 mm | 0.400 mm |
| 1-1/4 in | 31.750 | 30 or 35 mm | 1.750 or 3.250 mm |
| 1-1/2 in | 38.100 | 40 mm | 1.900 mm |
Even the closest case, 1 inch against 25 mm, differs by 0.4 mm. A metric bearing forced onto a 1 inch shaft will be loose, and an inch bearing on a 25 mm shaft will not go on. The same logic holds for outside diameter and width. Do not accept a near-match as a replacement unless the shaft and housing are being changed on purpose.
Where inch bearings still appear
Inch-series parts are most common in tapered roller bearings, where they were established long before metric standards. They are still found in vehicle wheel hubs, trailer axles, agricultural implements and older machinery from the Americas, as well as in inch-dimension ball bearings used in industrial and farm equipment. For tapered roller bearings the product line covers both metric and inch types; see the tapered roller bearing page. The market segments that use inch parts most are described on the automotive aftermarket and agriculture and conveyor pages.
Inch tapered roller bearings are normally supplied as a cone (the inner ring with rollers) and a cup (the outer ring), each with its own part number. The two must be matched as a set. Metric tapered bearings, by contrast, are usually identified by a single designation such as 30205.
How to cross-check by measured dimensions
Because inch numbers are catalogue identifiers, the safe route to a replacement is dimensional, not by numbers on a competitor list. This site does not give brand cross-references. Instead, follow these steps:
- Measure the bore, outside diameter and width with a caliper at two angles, as in how to measure a bearing. For a tapered set, measure the cone bore and width, and the cup outside diameter and width, separately.
- Test for inch. Divide each reading by 25.4. If the bore comes out near a simple fraction of an inch, such as 0.75 or 1.25, the part is inch-series.
- Record the markings on every face, even if partial, and photograph them.
- Send the dimensions with the application and, for tapered sets, say whether the cone and cup are both needed.
- Verify on samples. The desk checks the dimensions with the partner plant and a sample is fitted before a bulk order is placed.
Dimensional matching confirms the envelope. It does not confirm load capacity, internal geometry, or quality of the steel. Those come from the drawing and the datasheet for the specific bearing. For tapered sets in particular, the cone and cup contact angle and the matching of the pair determine performance, so the full set must be specified, not just the outside size.
Mixed equipment and mixed stock
Many machines contain both systems. A tractor or trailer may have inch tapered hub bearings next to metric deep groove bearings in an alternator or pump, and imported equipment often mixes the two without any label. This is a common cause of wrong orders in MRO and aftermarket stock rooms: a technician reads a worn number, assumes metric, and orders a part that is almost right. The remedy is procedural. Keep a measured record for each machine position, and let the record, not memory or the printed number alone, drive reordering. Where the equipment is still under production, ask the original maker for the drawing.
Tolerances are different too
Inch bearings follow their own tolerance conventions. Standard inch tapered roller bearings are commonly built to ABMA classes, which are separate from the ISO 492 classes used for metric radial bearings. The names and numbers do not map one to one, so do not compare a metric P6 with an inch class by number. When specifying, state the tolerance class by its own system and ask the supplier to confirm it in writing. The comparison of metric classes is covered in bearing precision classes.
Practical advice for buyers
A buyer who sells both inch and metric should keep two separate lists and never mix them in the same table. In stock lists, add the nominal dimensions beside every designation, in the unit of origin and in millimetres. Check that bore, outside diameter and width all agree before you reorder from an old order sheet, because a single wrong digit in an inch number can lead to a different size entirely.
For metric designations, bore codes let you derive the size from the number. For inch parts there is no such shortcut, which is why measurement and a signed drawing matter more. The general designation system is in the designation guide.
Sourcing inch parts through the desk
The desk works with partner plants that make both metric and inch-dimension bearings. For an inch program, the usual path is drawing or measurements, then a sample, then an agreed inspection plan, then the order. Typical reference MOQ for standard stock sizes is 100-500 pieces per designation and samples take roughly 3-7 working days for stock items; non-stock inch sets may take longer. These are indicative ranges and are confirmed in writing for each program. Send measurements and photos through the quote form.