What "interchangeable" really means

A 6204 from SKF, NSK, FAG, NTN and KOYO all share the same boundary dimensions — 20 × 47 × 14 mm — because every maker builds them to the ISO 15 standard. That makes the mounting envelope interchangeable: any of them will physically fit the same shaft and housing. But "fits" and "works the same" are two different claims, and confusing them is the most common and most expensive mistake in bearing replacement.

Dimensional interchange vs functional interchange

Dimensional interchange means the bore, outer diameter and width match, so the part drops into the same space. This is what a cross-reference table (like the ones on this site) tells you, and for the boundary dimensions it is reliable — they are fixed by an international standard.

Functional interchange means the replacement will carry the same load, run at the same temperature, hold the same shaft position and last as long. This is not guaranteed by matching dimensions, because everything that determines it is encoded in suffixes that vary by brand — or in internal design that the number doesn't show at all.

So the honest rule is: the base number tells you it fits; the suffix tells you if it works. Here are the five things to check.

1. Internal radial clearance (CN, C3, C4)

Clearance is the tiny amount a bearing's rings can move relative to each other before it is mounted. It is not a defect — it is chosen. A press fit onto the shaft expands the inner ring and eats into clearance; heat during running does the same. If you replace a 6204 C3 (extra clearance) with a standard 6204 (normal clearance, "CN") on a shaft with an interference fit, the running clearance can go negative and the bearing preloads itself and overheats or seizes. Same size, wrong part. Always match the clearance class. See the clearance guide.

2. Precision class (P0, P6, P5, P4)

Precision (ISO 492, older ABEC) controls the tolerance on the dimensions and the running accuracy. Standard P0 is fine for general machinery; a spindle bearing might need P5 or P4. A higher-precision bearing will fit in place of a standard one, but a standard bearing cannot deliver the running accuracy a machine tool was designed around. Match the precision, especially on anything that has to run true.

3. Seals and shields

An open bearing, a shielded bearing (metal, ZZ/2Z) and a contact-sealed bearing (rubber, 2RS) are three different parts with the same base number. Shields protect against large debris but add little friction; contact seals keep grease in and dirt out but raise the running torque and lower the top speed. Worse, the suffix letters differ by brand — SKF's 2RSH, NSK's DDU and NTN's LLU all mean "double contact seal" but read completely differently. See the seal suffix guide.

4. Cage type and material

The cage that separates the rolling elements can be stamped steel, machined brass or moulded polyamide (nylon). It doesn't change the fit, but it changes the limiting speed, the temperature ceiling and the behaviour under shock and vibration. A polyamide cage is quiet and light but has a temperature limit; a brass cage suits high temperature and heavy load. On a hot or fast application, the cage is not a detail.

5. Load ratings and internal design

Two bearings with identical d × D × B can have different numbers of rolling elements, different internal geometry and therefore different dynamic (Cr) and static (C0r) load ratings — an "E" or reinforced design can carry noticeably more than the standard version of the same number. Load ratings are a safety-critical selection value, which is why this site deliberately does not publish them: read them from the manufacturer's catalogue and make sure the replacement is rated for your application, not just sized for it.

The type letter can override everything

Finally, some "same-size" bearings are not even the same kind. A 6206 (deep groove) and a 7206 (angular contact) are both 30 × 62 × 16 mm, but the angular contact takes thrust in one direction and must be paired and preloaded. An NU206 and an N206 share dimensions but carry axial load through different rings. Matching the size letter — not just the number — is the last check.

A practical checklist

If all five match, a cross-brand swap of the same base number is normally safe. If any differ, the two parts are dimensionally interchangeable but not functionally equivalent — and that distinction is exactly what a good cross-reference should make clear.

General reference only. Verify clearance, precision, seals and load ratings against the manufacturer's specification before substituting any bearing.