Bearing part numbers follow a standardized coding system where each digit, letter, and suffix tells you something specific about the bearing’s type, size, and features. A number like 6205-2RS, for example, tells an experienced reader the bearing type (deep groove ball), the size series, the bore diameter, and that it has rubber contact seals on both sides. Once you understand the structure, you can decode almost any bearing number at a glance.
The Basic Number Structure
Most standard bearing designations use a core number of three to five digits, sometimes preceded by a prefix and almost always followed by one or more suffixes. The core number breaks down into three parts, read left to right: bearing type, dimension series, and bore size.
The first digit (or letter) identifies the bearing type. The most common codes you’ll encounter are:
- 6 = deep groove ball bearing (the most widely used type)
- 7 = angular contact ball bearing
- N = cylindrical roller bearing
- 3 = tapered roller bearing
- 2 = self-aligning ball bearing
- 1 = self-aligning ball bearing (alternate series)
The next two digits form the dimension series, which defines the bearing’s cross-sectional profile. The first of these two digits indicates the width (or height), and the second indicates the outer diameter. In practice, most people encounter this as the “series” portion of the number. Within deep groove ball bearings, for instance, a 60-series bearing has a lighter, thinner cross section than a 62-series, which is thinner than a 63-series. A higher series number generally means a physically larger bearing for the same bore size, which translates to greater load capacity.
How to Read the Bore Size
The last two digits of the core number indicate the bore diameter, which is the inner hole size. For bearings with a bore of 20 mm and above, you multiply the last two digits by 5 to get the bore in millimeters. So a 6205 has a bore of 05 × 5 = 25 mm. A 6310 has a bore of 10 × 5 = 50 mm.
Below 20 mm, the system uses fixed codes instead of the multiply-by-five rule:
- 00 = 10 mm bore
- 01 = 12 mm bore
- 02 = 15 mm bore
- 03 = 17 mm bore
From 04 onward (20 mm and up), the multiply-by-five rule applies consistently. This small exception trips people up, so it’s worth memorizing those four codes.
Putting the Core Number Together
Let’s decode a few complete examples. A bearing stamped 6308: the 6 means deep groove ball bearing, the 3 indicates the 03 diameter series (a heavier cross section), and 08 × 5 = 40 mm bore. A bearing stamped 7205: the 7 means angular contact ball bearing, the 2 is the 02 diameter series (medium cross section), and 05 × 5 = 25 mm bore.
Compare a 6005, 6205, and 6305. All three fit a 25 mm shaft. But the 6005 is the lightest and thinnest, the 6205 is a medium-duty option, and the 6305 is the heaviest with the largest outer diameter. The series number is how manufacturers scale load capacity without changing the bore.
Seal and Shield Suffixes
The suffixes after the core number describe the bearing’s features, and the most common ones you’ll see relate to seals and shields. These tell you how the bearing is protected from contamination.
- Z = metal shield on one side
- ZZ (or 2Z) = metal shields on both sides
- RS = rubber contact seal on one side
- 2RS (or 2RS1) = rubber contact seals on both sides
- RZ (or 2RZ) = non-contact rubber seal on one or both sides
Shields are thin metal plates that block debris but don’t physically touch the inner ring, so they create less friction. Seals are rubber and press against the inner ring for tighter protection, but they add slightly more drag. For most general applications like electric motors, conveyor rollers, and wheel hubs, 2RS or ZZ bearings are the standard choice. If you’re replacing a bearing, match whatever suffix the original had.
Different manufacturers use slightly different suffix conventions. SKF uses 2RS1, 2RSH, and 2RSL to distinguish between seal materials and friction levels. NSK uses DDU for double rubber seals. NTN uses LLU. Despite the different letters, these all describe the same basic concept: contact seals on both sides. When cross-referencing between brands, focus on whether the bearing has seals or shields and whether they’re on one side or both.
Internal Clearance Codes
Internal clearance is the tiny gap between the rolling elements and the races. It matters because when a bearing is pressed onto a shaft and heats up during operation, that gap shrinks. Choosing the right clearance class ensures the bearing still has room to function properly under real conditions.
The clearance codes, from tightest to loosest:
- C2 = smaller than normal clearance
- CN (or no marking) = normal clearance
- C3 = greater than normal clearance
- C4 = greater than C3
If a bearing number has no clearance suffix, it’s normal clearance (CN). C3 is the most commonly specified upgrade, typically used when higher operating temperatures are expected or when the bearing has a tight interference fit on the shaft. A bearing stamped 6205-2RS/C3 is a sealed deep groove ball bearing with a 25 mm bore and greater-than-normal internal clearance.
Precision and Tolerance Classes
Bearings are manufactured to different levels of dimensional precision. The two systems you’ll encounter are ABEC (used primarily in North America) and ISO/DIN classes (used internationally). They map to each other directly:
- ABEC 1 = ISO P0 (normal precision, standard for most applications)
- ABEC 3 = ISO P6
- ABEC 5 = ISO P5
- ABEC 7 = ISO P4
- ABEC 9 = ISO P2 (highest precision)
The numbering runs in opposite directions, which is confusing at first. Higher ABEC numbers mean more precision, while lower ISO class numbers mean more precision. Most bearings you’ll buy off the shelf are ABEC 1 / P0. Higher precision classes are used in machine tool spindles, aerospace, and medical equipment where runout tolerances are extremely tight. Unless your application specifically calls for a higher precision class, standard ABEC 1 bearings are appropriate.
Tapered Roller Bearing Numbers
Tapered roller bearings use a slightly different system because they come in two separable parts: the cone (inner ring with rollers) and the cup (outer ring). A designation like 32208 follows the same general logic, where 3 indicates a tapered roller type, 22 is the dimension series, and 08 × 5 = 40 mm bore.
Where tapered bearings get more complex is in matched sets. Suffixes like DB, DF, and DT indicate how pairs of bearings are matched for installation. DB means back-to-back mounting, DF means face-to-face, and DT means tandem (both facing the same direction). A suffix like T followed by a number indicates the total width of the matched pair. If you’re ordering replacement tapered roller bearings, note whether the original was sold as a set, since the cone and cup are precision-matched and shouldn’t be mixed between different production runs.
What to Do When the Number Is Worn Off
If a bearing’s stamped number is unreadable, you can identify it by measuring three dimensions: the inner diameter (bore), the outer diameter, and the width. Use a caliper for accuracy. Measure the outer diameter at its widest point, the inner diameter at its narrowest point, and the width between the two ring faces.
With those three measurements in millimeters, you can look up the bearing in any manufacturer’s catalog or online cross-reference tool. The combination of bore, outer diameter, and width uniquely identifies the bearing series and size. For example, if you measure 25 mm bore, 52 mm outer diameter, and 15 mm width, that matches a 6205. A 25 mm bore with 62 mm OD and 17 mm width matches a 6305, the heavier-duty version for the same shaft size.
Common Full Designation Examples
Seeing complete part numbers decoded helps solidify the system. Here are a few real-world examples:
- 6203-2RS/C3: Deep groove ball bearing (6), 02 diameter series, 17 mm bore (03 is the exception code), rubber seals both sides, C3 clearance.
- 6310-ZZ: Deep groove ball bearing, 03 diameter series (heavier), 50 mm bore, metal shields both sides, normal clearance.
- 7208 B: Angular contact ball bearing (7), 02 diameter series, 40 mm bore, “B” suffix indicating a 40-degree contact angle.
- 32210: Tapered roller bearing (3), 22 dimension series, 50 mm bore.
The system is consistent enough that once you’ve decoded a handful of bearing numbers, new ones become intuitive. Start with the first digit for the type, work out the bore from the last two digits, note the series in the middle, then read the suffixes for seals, clearance, and any special features. That sequence works for the vast majority of bearings you’ll encounter in maintenance, repair, and purchasing.