A bearing designation is a compressed engineering specification, not a catalogue reference. Split it into prefix, basic designation and suffix, and a code like 6205-2RS-C3 tells you the geometry, the sealing, the clearance and the tolerance class.

By the Juding Engineering technical team · Published 31 October 2026 · 11 min read · Last verified: 2026-10-31

The short answer: read the number from the inside out. Take the middle digits first, because the bore code is arithmetic and the dimension series is a lookup, then work outwards to the suffixes, where the cost, the speed and the fit risk live. This guide is built from our own published data, starting with the 6204 2RS deep groove ball bearing specification, extended across the full bearing range we supply to OEM buyers and distributors, and set against the standards behind our buyer FAQ.

Part One: Split the Designation Into Three Blocks

A complete bearing designation is built in the same order in every major brand, and international standards set the skeleton. According to ISO 15 for radial bearing boundary dimensions and the vocabulary defined in ISO 5593, a designation has three parts.

Source: Juding Engineering published bearing designation reference, read against ISO 15 and ISO 5593.

Because the basic designation is always mandatory and always the same shape, you can read any brand from any country with the same three steps, and the suffixes are the only part you have to look up per brand.

Step one: the type code

The first digit or letter identifies the fundamental design, and that single character decides what the bearing can take.

Source: Juding Engineering published type code table.

Because a 6205 and a 7205 share the same bore and the same basic envelope but carry loads in completely different directions, a misread type code is not a wrong size, it is a bearing that cannot do the job.

Step two: the dimension series

The next two digits are the dimension series: the first is the width series and the second is the diameter series. In 6205 the 2 means the Light series, a standard width with a light outer diameter profile for general-purpose duty. A 3 in the same position, as in 6305, means the Medium series with a thicker cross-section for heavier loads.

Because the dimension series sets the proportion of the envelope rather than the size of the hole, swapping series changes the outer diameter and the load rating while the bore code stays the same, which is exactly what makes a 6205 to 6305 swap tempting and wrong.

Step three: the bore code

The last two digits of the basic designation give the bore, and the rule depends on the size.

• Bore 20 mm and above: multiply the last two digits by 5. In 6205, 05 × 5 = 25 mm.
• Special small bores below 20 mm: 00 = 10 mm, 01 = 12 mm, 02 = 15 mm, 03 = 17 mm.
• Micro bores below 10 mm: the bore is stated directly, so a 608 has an 8 mm bore.

Because the multiplication rule only holds from 20 mm upward, applying it to a 10 mm bore code is one of the most common arithmetic errors in a purchase order, and it is the reason the small-bore table exists.

Part Two: The Suffix Codes, Decoded

Suffixes are where the money and the failures live, and each one answers a different question.

Seals and shields

Source: Juding Engineering published suffix reference.

In 6205-2RS-C3, the 2RS means rubber seals on both sides, which gives the best protection against water and dust ingress of the common options.

Because a metal shield is non-contact and a rubber seal is a contact seal, a shield cannot block water: it only reduces the gap, so in a wet environment the water bypasses it and corrosion starts inside the bearing where nobody looks.

There is a second cost to choosing a shield where a seal was needed, and it is a speed one. A contact seal drags against the inner ring for the life of the bearing, so the same designation with 2RS and with ZZ does not deliver the same permissible speed, and the closure has to be right at the point of ordering, because a bearing that arrives with the wrong closure cannot be converted later without scrapping it.

Internal clearance

Radial internal clearance is the total distance the rings can move relative to each other with the bearing unloaded. Standard clearance is unmarked and is called C0.

The groups run in one direction only. C2 sits just above the basic group, C3 is a further step, and C4 and C5 are progressively looser. The tolerance standard for radial bearings is ISO 492 and the terminology is set in ISO 5593.

C3 is chosen where the operating temperature is high and thermal expansion is significant, where heavy interference fits are used, or where the shaft grows faster than the housing. C3 is essential in those cases. Specifying C3 for a precision, low-vibration application does the opposite and introduces play and noise.

Because the running clearance is the manufactured clearance minus whatever the shaft and housing fit has taken out of it, the group is only meaningful together with the fit, and a clearance code on its own is half a specification.

Precision class

Precision grades are written as P followed by a number, and the hierarchy runs from standard to ultra-precision: P0 is the standard class, then P6, P5, P4 and P2. A P5 suffix indicates high precision, and it is the class normally required for machine tool spindles.

Note the direction of travel. Moving up the list, from P0 towards P2, means a tighter class, not a larger number. Because precision and clearance are separate axes, a precise bearing in the wrong clearance group is still the wrong bearing.

Because the tolerance class controls how closely the manufactured dimensions and the running clearance are held, a precise clearance in a loose tolerance class is still a loose bearing, which is why the two codes must travel together on the order line.

Cage and prefixes

M denotes a machined brass cage, which offers high strength and high temperature capability. TN or TVH indicates a polyamide or nylon cage, which is lighter and quieter. Stamped steel is used on our range up to 400 mm outside diameter.

Prefixes are less common in everyday replacement work, and they describe structural variations or split components. K indicates a roller and cage assembly without the inner or outer rings. NU indicates a cylindrical roller bearing with no inner ring ribs, which allows axial displacement.

Because cage material and prefix both describe a different physical assembly rather than a different specification of the same one, each changes what you have to install, and both have to be recognised on the drawing rather than discovered at the bench.

Part Three: Where SKF, NSK and FAG Write It Differently

The skeleton is the same across the three brands, and the vocabulary is not. SKF, NSK and FAG all follow the ISO structure of type code, dimension series and bore code, and all three use the C0 to C5 clearance groups, the P0 to P2 precision classes and the M, TN and TVH cage codes. What differs is the seal and shield notation, and this is the point where a cross-brand purchase order quietly changes the part.

Source: Brand designation conventions for SKF, NSK and FAG, checked against the ISO structure in ISO 15 and ISO 5593 and against current manufacturer product listings for each brand.

Because the three brands agree on the geometry codes and disagree on the closure codes, a part number transcribed between brands is reliable in its bore and unreliable in its sealing, which is the one attribute that cannot be corrected by changing the supplier.

There is a commercial consequence to the same point. A shielded and a sealed version of the same bearing are different SKUs with different lead times and different minimum order quantities, and our published range treats them as separate products with separate tooling and separate packing, so a suffix error is not a paperwork issue, it is a physical mismatch on arrival.

The one-line correction to keep

Most of the errors in this area come from a single confusion, and it is worth naming it precisely. The letter Z on a bearing label is not a quality grade and it is not a noise grade. It is a shield designation. A buyer who reads Z as a tolerance class and ZZ as a precision level is reading two unrelated systems as one.

Because Z and V are routinely swapped in supplier catalogues, with V1 to V3 offered as vibration grades and Z1 to Z3 as noise grades, which inverts the meaning of both families, a grade specified under the wrong heading is a specification that cannot be enforced in a dispute.

The two ladders also overlap in a way that matters commercially. Vibration classes Z1, Z2, Z3 and Z4 permit velocity limits of 2.8, 1.8, 1.2 and 0.9 mm/s r.m.s. over the 100 Hz to 1000 Hz band, while precision classes V1, V2 and V3 permit 1.2, 0.9 and 0.6 mm/s. V1 is therefore numerically identical to Z3 and V2 is identical to Z4, so specifying V1 instead of Z3 buys tighter dimensional and runout tolerances but not a quieter bearing. Only V3 at 0.6 mm/s is quieter than any Z class.

The measurement method is defined in ISO 15242-1, which specifies measuring methods for the vibration of rotating rolling bearings under established conditions together with calibration of the measuring systems. Acoustic noise in decibels is a separate classification, and the grease noise test symbols BQ+, MQ and NQ in the ISO 21250-1 series grade the grease rather than the assembled bearing.

Because the three measurements are velocity in millimetres per second, sound pressure in decibels and a grease symbol set, they answer three different questions, and quoting one in place of another changes what is being controlled and what is being delivered.

Part Four: Confirm the Decode Against a Dimension

A decoded number is a hypothesis, and a dimension check is the test.

Once you have read the type, the series and the bore, verify the result against the standard boundary dimensions for that series. The 6204 2RS is a useful worked case: the 6 says deep groove ball bearing, the 20 is the Light dimension series, and 04 × 5 = 20 mm bore. The published boundary dimensions are 20 × 47 × 14 mm for bore, outside diameter and width, and they are the ones specified by ISO 15 so that standard products are interchangeable.

Because the boundary dimensions are the part of the designation that is standardised and therefore the part two brands always agree on, a mismatch on bore, outside diameter or width proves the decode is wrong before any tolerance or clearance argument begins.

What inspection has to confirm

Dimensional inspection covers both the basic size parameters and the geometric characteristics, and the items depend on the bearing type, the accuracy class and the application.

Source: Juding Engineering dimensional inspection reference.

The methods are chosen by stage. Routine production checks use fast repeatable gauges such as micrometers, calipers, bore gauges and air gauging, while final inspection and certification use coordinate measuring machines, optical measuring systems, and roundness and profile testers. A single measurement confirms size, but additional checks are needed to evaluate roundness, parallelism, runout and surface conformity.

Because even micron-level deviations affect fit, clearance, vibration, temperature rise, noise and service life, the inspection report has to state the measured parameters, the reference standards, the instrument type and the acceptance criteria, or it is not evidence of anything.

What a bearing line on a purchase order should carry

Five things, and clearance is the first of them. The clearance group code written as the code itself, never as small, medium or large, because those words mean nothing to the factory. The tolerance class alongside it, because clearance precision is only as good as the class holding it. The seal or shield code, which decides lubrication retention and contamination protection. The cage and material, where the duty calls for a specific one. And the lubricant, if the bearing is supplied filled, since that is a manufacturing decision.

Written together, a line looks like a bearing number followed by the codes that matter: a 6204 in a sealed configuration at C3 clearance, to a stated tolerance class, with the vibration grade and the cage agreed, in a stated material, under the BMT trademark, industrially packed and shipped under HS 8482800000. Our published pages carry exactly this structure, so a distributor can build the line from our own data rather than from a catalogue guess.

Because the operating conditions behind the choice are stated once at the top of the order, a question about clearance can be answered with a clearance answer instead of a catalogue paragraph, which is the whole point of decoding the number in the first place.

Frequently Asked Questions

Read the Number Before You Order the Part

The whole skill is three questions in a fixed order: what type, what series, what bore, and then every suffix after the hyphen.

The failure this prevents is not subtle. A buyer reads ZZ as a precision class, orders a shielded bearing for a washdown line, and receives a part that looks correct on the shelf and corrodes inside the housing six months later. Nobody made a mistake. The number was simply never read as the specification it is.

Because the geometry codes are standardised and the suffix vocabulary is not, a part number transcribed between brands is trustworthy on bore and untrustworthy on closure, and closure is the one attribute that cannot be changed after the bearing is sealed in the housing.

Our bearing range covers the 6000 and 6200 families through 6300, 6400, 6800, 6900, 16000, 62200 and 62300 to the NR series, with extra-large bearings from 180 mm to 6300 mm bore and specials made to a customer’s drawing or sample. Each family carries the same published option set: internal clearance C2, C3, C4 and C5; tolerance class ABEC-1, ABEC-3 and ABEC-5; vibration level V3, V2 and V1; closure Open, Z, ZZ, RS, 2RS and 2RZ; cage in stamped steel or solid brass; and material in chrome steel, carbon steel, stainless steel or ceramic. The 6204 2RS product page publishes the specification behind one of them in full, and our buyer FAQ covers the commercial terms that come with it.

Because the same six option axes are published on every product rather than negotiated case by case, a distributor can build a compliant order line from our own data, and an OEM engineer can compare the alternatives on the same axes instead of on a price list.