What Do the Numbers on Deep Groove Ball Bearings Actually Mean?

The short answer: every digit and letter in a deep groove ball bearing part number encodes specific, standardized information about the bearing's dimensions, internal design, and sealing configuration. Once you understand the system, you can decode any standard bearing number in seconds—without a catalog. For example, a 6205ZZ bearing tells you: it's a deep groove ball bearing (6), medium series (2), 25mm bore (05), with metal shields on both sides (ZZ). That's four pieces of critical information packed into six characters.

This guide decodes the ISO and ABMA bearing numbering system from first principles, walks through real-world examples including the 6310 bearing, 6210 bearing, 6002Z bearing, and 6205ZZ bearing, and explains every suffix and prefix you're likely to encounter when specifying or sourcing bearings for industrial, automotive, or mechanical applications.

The Structure of a Deep Groove Ball Bearing Number

Standard deep groove ball bearing numbers follow a globally recognized system established by the International Organization for Standardization (ISO 15). The full designation consists of three parts arranged in sequence: a prefix (optional), a basic number (always present), and a suffix (optional but common). Understanding each section independently is the key to reading any bearing number with confidence.

Let's now examine each section in depth, starting with the bearing type code—the first digit that tells you immediately what kind of bearing you're dealing with.

The First Digit: Bearing Type Code

The first digit (or digits) of the basic number identifies the bearing type. For the vast majority of standard deep groove ball bearings, this number is 6. This single digit is the most important character in the designation because it immediately classifies the entire bearing family. Here's how common bearing type codes compare:

Since all four bearing examples in this guide—the 6310 bearing, 6210 bearing, 6002Z bearing, and 6205ZZ bearing—begin with 6, they are all confirmed deep groove ball bearings. The number 6 is by far the most common type code encountered in general industrial and commercial applications, which is why deep groove ball bearings account for roughly 80% of all ball bearing sales worldwide.

The Second Digit: Dimension Series and Load Capacity

The second digit of the basic number defines the dimension series—a coded combination of the bearing's width series and diameter series. For deep groove ball bearings, the most common dimension series codes are 0, 2, 3, and 4. This digit directly controls how large and heavy-duty the bearing is for a given bore size.

Think of it this way: two bearings can have identical bore sizes (same last two digits) but completely different outer diameters, widths, and load ratings depending on the dimension series. A higher series number generally means a physically larger bearing with greater load capacity.

This explains a key difference between the 6210 bearing and the 6310 bearing: both have the same bore size (50mm), but the 6310 is a medium series bearing with a significantly larger outer diameter and higher load rating. We'll examine that contrast in detail in the next section.

The Last Two Digits: Bore Size Code

The final two digits of the basic number encode the bore (inner diameter) of the bearing. This is the dimension that determines whether a bearing will fit your shaft. The coding system has two distinct rules depending on the bore size range:

Bore Sizes 20mm and Above: Multiply by 5

For bearings with bore codes 04 and above, the bore diameter in millimeters equals the last two digits multiplied by 5. This is the most commonly used range in industrial applications:

• Bore code 04 = 4 × 5 = 20mm
• Bore code 05 = 5 × 5 = 25mm (as in 6205ZZ)
• Bore code 10 = 10 × 5 = 50mm (as in 6210 and 6310)
• Bore code 20 = 20 × 5 = 100mm

Bore Sizes Below 20mm: Special Codes

For smaller bearings, the multiply-by-5 rule does not apply. Instead, bore codes 00 through 03 correspond to fixed dimensions:

• Bore code 00 = 10mm
• Bore code 01 = 12mm
• Bore code 02 = 15mm (as in the 6002Z bearing)
• Bore code 03 = 17mm

For very small bearings with bores below 10mm (common in miniature applications like dental tools, RC motors, and precision instruments), the bore size is typically written directly in the designation as a fraction or decimal rather than using the coded system—for example, a bearing with a 5mm bore may be designated 625 or 605 where the last digit directly represents millimeters.

Real Example: Decoding the 6205ZZ Bearing

The 6205ZZ bearing is one of the most widely used bearings in the world, appearing in electric motors, household appliances, power tools, HVAC fans, and light industrial equipment. Let's decode it character by character:

• 6 — Deep groove ball bearing
• 2 — Light dimension series (relatively compact for its bore)
• 05 — Bore code 05 × 5 = 25mm inner diameter
• ZZ — Metal shields on both sides (non-contact, grease-lubricated, not washable)

The full dimensional specification of the 6205ZZ bearing is: 25mm bore × 52mm outer diameter × 15mm width. Dynamic load rating: approximately 14.0 kN. Static load rating: approximately 7.8 kN. Maximum speed: approximately 13,000 RPM (grease lubricated). This bearing is the default choice for a 25mm shaft in any standard-duty rotating application.

Real Example: Decoding the 6002Z Bearing

The 6002Z bearing is a small, lightweight bearing commonly found in household appliances, small electric motors, power tools, skateboards, and inline skates. Decoding it:

• 6 — Deep groove ball bearing
• 0 — Extra light dimension series (very compact outer diameter relative to bore)
• 02 — Bore code 02 = 15mm inner diameter (special code, not multiplied by 5)
• Z — Metal shield on one side only

The 6002Z bearing has a 15mm bore × 32mm outer diameter × 9mm width. It is notably narrow and compact, which is why it's chosen for space-constrained applications. Its single-shield (Z) configuration means one side is open, allowing for relubrication or use in applications where the open side faces an oil bath or mist lubrication environment. Dynamic load rating is approximately 5.6 kN—substantially lower than the 6205ZZ, reflecting its smaller size.

A common point of confusion: the 6002Z (one shield) and 6002ZZ (two shields) are both available. The single-Z variant is often used when one side of the bearing is exposed to a controlled lubrication environment, while the double-Z is preferred for sealed, maintenance-free applications. Always verify which variant your application requires before ordering.

Comparing the 6210 Bearing and 6310 Bearing: Same Bore, Very Different Bearings

The comparison between the 6210 bearing and the 6310 bearing perfectly illustrates how the dimension series digit changes everything about a bearing's physical size and performance—even when the bore size is identical. Both bearings have a 50mm bore, yet they are far from interchangeable.

Decoding the 6210 Bearing

• 6 — Deep groove ball bearing
• 2 — Light dimension series
• 10 — Bore code 10 × 5 = 50mm inner diameter
• No suffix — Open bearing, no seals or shields, intended for oil lubrication or external sealing

The 6210 bearing dimensions are: 50mm bore × 90mm OD × 20mm width. Dynamic load rating: 35.1 kN. Maximum speed (grease): approximately 7,500 RPM.

Decoding the 6310 Bearing

• 6 — Deep groove ball bearing
• 3 — Medium dimension series (larger and heavier than series 2)
• 10 — Bore code 10 × 5 = 50mm inner diameter
• No suffix — Open bearing

The 6310 bearing dimensions are: 50mm bore × 110mm OD × 27mm width. Dynamic load rating: 61.8 kN. Maximum speed (grease): approximately 6,300 RPM.

The key takeaway: if you need a 50mm shaft bearing for a high-speed light-duty application (like a centrifugal pump or fan), the 6210 bearing is the right choice—it's lighter, runs faster, and fits in a more compact housing. If you need a 50mm shaft bearing for a heavy-duty, high-load application (like a gearbox output shaft, compressor, or conveyor drive), the 6310 bearing with its 76% higher dynamic load rating is the correct specification. Never substitute one for the other simply because the bore matches.

Bearing Suffixes Decoded: Seals, Shields, Clearance, and Tolerance

Suffixes are appended after the basic number and can dramatically affect how a bearing performs, how it's lubricated, and where it can be used. The suffix is not optional engineering jargon—it encodes critical operational parameters. Ordering a bearing without understanding its suffix can result in premature failure, contamination, or incompatibility with your housing or shaft tolerances.

Sealing and Shielding Suffixes

These are the most commonly encountered suffixes in everyday bearing selection:

Z vs. 2RS: Which Should You Choose?

Metal shields (Z/ZZ) are non-contact, meaning they do not touch the inner ring. This results in lower friction and higher speed capability—a ZZ bearing can typically run 15–20% faster than the same bearing with rubber seals. However, metal shields are not waterproof; they protect against solid contaminants (dust, metal particles) but not liquid ingress.

Rubber seals (RS/2RS) make contact with the inner ring and provide superior protection against moisture and liquid contamination. The trade-off is slightly higher friction (typically 1–3 N·m additional drag at low speeds) and a lower maximum speed rating—usually 10–20% below the equivalent shielded version. For applications in wet, washdown, or outdoor environments, 2RS is almost always the correct choice.

Internal Clearance Suffixes

Internal clearance refers to the total internal free movement of the bearing's balls between the inner and outer rings. Standard clearance (CN or no suffix) is correct for most applications. But operating conditions sometimes require deviation:

• C2 — Less than normal clearance. Used when interference fits are applied to both inner and outer rings, or in precision applications requiring minimal play.
• CN — Normal clearance (default). No suffix required; this is the standard specification for most applications.
• C3 — Greater than normal clearance. The most common non-standard clearance specification. Used in applications with significant temperature rise (electric motors, high-speed applications) where thermal expansion would otherwise preload the bearing and cause premature failure.
• C4 — Much greater than normal clearance. Heavy industrial use, high-temperature environments, or applications with severe interference fits.

The C3 clearance specification is extremely common in electric motor bearings because motors generate significant heat during operation. A bearing specified as 6205ZZC3 or 6205-2RSC3 has the same basic dimensions as the standard 6205ZZ but with larger internal clearance to accommodate thermal expansion—and this distinction matters enormously for motor bearing life.

Tolerance Class Suffixes

Precision tolerance classes define how closely the bearing's dimensions conform to their nominal values—tighter tolerances mean higher precision and higher cost:

• P0 (or no suffix) — Normal precision. Standard for most industrial applications.
• P6 — Higher than normal precision. Used in machine tool applications, moderate-speed spindles.
• P5 — High precision. CNC machine tools, precision gearboxes, measuring instruments.
• P4 — Very high precision. High-speed machine tool spindles, aerospace applications.
• P2 — Ultra-precision. Gyroscopes, precision instruments, highest-speed spindles.

A P5 or P4 bearing costs significantly more than a P0 equivalent—often 3–10× the price depending on size—because of the additional grinding, measurement, and sorting steps required in manufacturing. Never specify a higher precision class than the application requires; you'll pay a premium for tolerance you don't need.

Manufacturer Prefix Codes and Cross-Reference Considerations

While the ISO numbering system is standardized, individual bearing manufacturers sometimes add their own prefix codes or use slightly different suffix conventions for the same underlying specifications. Understanding this helps you cross-reference bearings between brands accurately.

Common Manufacturer Suffix Variations

The same bearing might be listed differently across catalogs from SKF, NSK, FAG, Timken, NTN, and Koyo. For the standard 6205ZZ as an example:

• SKF: 6205-2Z (uses "2Z" instead of "ZZ" for double shield)
• NSK: 6205ZZ (standard notation)
• FAG / Schaeffler: 6205-2RSR (uses RSR for their specific seal design)
• NTN: 6205LLB (uses "LLB" to denote their non-contact rubber seal)
• Koyo: 6205ZZ (standard notation)

The basic dimensions (bore, OD, width) are standardized and will be identical across all manufacturers for the same basic number. Where variations can occur is in internal design details—ball complement (number and size of balls), cage material, grease type, and seal geometry. For critical applications, always specify the brand if a particular manufacturer's design has been validated for your use case.

When Manufacturer Matters—And When It Doesn't

For general-purpose applications—electric motors, fans, pumps, conveyor systems—a bearing from any reputable manufacturer meeting the ISO basic number specification will perform equivalently. The dimensional interchangeability is guaranteed by ISO standards. However, for high-speed spindles, aerospace, medical equipment, or applications with extreme temperature or contamination requirements, the specific manufacturer and their proprietary design details can make a meaningful difference in bearing life and reliability.

Practical Guide: How to Read Any Deep Groove Ball Bearing Number in 30 Seconds

Armed with everything covered in this guide, you can now decode any standard deep groove ball bearing designation quickly and confidently. Here's a step-by-step method you can apply immediately:

1. Identify the bearing type code (first digit). If it starts with 6, it's a deep groove ball bearing. Write this down as your confirmed bearing family.
2. Read the dimension series (second digit). Is it 0 (extra light), 2 (light), 3 (medium), or 4 (heavy)? This tells you the load capacity class relative to the bore size.
3. Decode the bore size (last two digits of the basic number). If the digits are 04 or higher, multiply by 5 to get millimeters. If 00–03, use the fixed code table (10mm, 12mm, 15mm, 17mm).
4. Parse the suffix from left to right. First look for sealing codes (Z, ZZ, RS, 2RS). Then check for clearance codes (C2, C3, C4). Then check for precision codes (P6, P5, P4).
5. Verify against a bearing table. Confirm the outer diameter and width match your housing dimensions using a manufacturer's dimension table or online bearing database (e.g., SKF bearing calculator, NSK bearing guide).

Quick Reference: Four Bearings Decoded Side by Side

Common Mistakes When Selecting Deep Groove Ball Bearings

Even experienced technicians make selection errors that lead to premature bearing failure, system downtime, and unnecessary replacement costs. Here are the most frequent mistakes and how to avoid them:

• Substituting based on bore size alone: As the 6210 vs. 6310 comparison shows, identical bore size does not mean identical bearings. Always verify outer diameter and width against your housing drawings before ordering.
• Ignoring clearance in high-temperature applications: Using a standard CN clearance bearing in an electric motor that runs at elevated temperature is one of the leading causes of early motor bearing failure. Specify C3 clearance for motors and other heat-generating applications.
• Choosing ZZ shields for wet environments: Metal shields are not waterproof. In any application involving water, coolant, or high-pressure washdown, specify 2RS rubber seals instead.
• Over-specifying precision class: Paying for P5 or P4 bearings in a standard conveyor or pump application is a waste of budget. P0 (standard) bearings are correct for the vast majority of non-precision applications.
• Mixing manufacturer suffix conventions: Ordering "6205-2Z" from SKF and "6205ZZ" from NSK will yield dimensionally identical bearings. But confusing "6205-2RS" (rubber seal) with "6205-2Z" (metal shield) from the same catalog will get you the wrong sealing type. Always read the full manufacturer suffix description, not just the code letters.
• Selecting an open bearing where a sealed variant is needed: An open bearing in a contaminated environment will fail dramatically faster than a sealed equivalent. If the original bearing had a ZZ or 2RS suffix, always replace with the same sealing configuration.

A Word on Counterfeit Bearings

The bearing market—particularly online marketplace channels—has a significant counterfeit problem. Counterfeit bearings from major brands like SKF, FAG, and NSK are widely sold with convincing packaging but inferior materials, incorrect heat treatment, and substandard geometric tolerances. A counterfeit bearing can fail in hours rather than years, and in critical applications this can cause equipment damage or safety incidents. Purchase bearings from authorized distributors, verify authenticity codes where available, and be skeptical of prices significantly below market rate for name-brand products.

The Complete Bearing Number Decoder

Deep groove ball bearing part numbers are a compact, internationally standardized language. Once you learn to read it, every number on a bearing box tells you exactly what you're holding—its family, its load capacity class, its bore size, its sealing configuration, its internal clearance, and its precision level. No catalog required.

The four real-world examples in this guide—the 6310 bearing, 6210 bearing, 6002Z bearing, and 6205ZZ bearing—cover the full range of situations you're likely to encounter: from compact small-bore applications to heavy-duty 50mm shaft installations, from single-shielded semi-open designs to fully double-shielded maintenance-free units. Each digit and letter carries meaning, and understanding that meaning is the difference between a confident, correct bearing selection and a costly substitution error.

When in doubt: verify the bore, confirm the OD and width against your housing, check the suffix for sealing and clearance, and purchase from a reputable distributor. These four steps will protect your equipment and your budget on every bearing selection decision.