How to Identify Your Bike Bearing Size

The quickest way to know your bike bearing size is to read the code printed on the bearing’s seal (for example, “6902-2RS” or “6806-LLB”) or to check the component’s technical specifications on the manufacturer’s website. If neither is available, remove the bearing and measure its inner diameter (ID), outer diameter (OD), and width (W) with a caliper; for headsets, use the SHIS standard (e.g., IS42/28.6, IS52/40). Below is a practical guide to identify sizes accurately across hubs, headsets, bottom brackets, pedals, and suspension pivots.

Start by Identifying the Bearing Type and Location

Before sizing, determine whether you’re dealing with cartridge bearings (sealed units with printed codes) or loose-ball/caged systems (individual balls in cups and cones). Knowing the location helps predict likely sizes and standards.

• Hubs: Commonly use sealed cartridge bearings or cup-and-cone loose balls (especially on older or Shimano hubs).
• Headsets: Typically use angular contact cartridge bearings; many follow SHIS sizing (IS, ZS, EC) and angle specs like 36°x45° or 45°x45°.
• Bottom brackets: Either cartridge bearings pressed into cups/shells (BB30/PF30, etc.) or cup-and-cone/loose-ball in older designs; many modern threaded BBs are serviced by replacing the whole cup assembly.
• Pedals and jockey wheels: Often small cartridge bearings or loose balls.
• Suspension pivots: Usually cartridge bearings, sometimes “MAX” (full-complement) types for higher load capacity.

With the system identified, you can choose the right path: read the code, look up specs, or measure.

Three Reliable Ways to Identify Your Bearing Size

These methods range from quickest (reading a code) to most universal (measuring). Use the first approach that applies to your part and tools.

1. Check the bearing’s printed code: Clean the outer seal and look for a series like “6902-2RS,” “6806-LLB,” or headset markings such as “45×45 41.8.”
2. Look up official specs: Search your wheelset, headset, frame, or bottom bracket model on the manufacturer’s site or tech docs; headset sizes are best confirmed via SHIS (e.g., IS42/28.6 upper, IS52/40 lower).
3. Measure ID/OD/Width with a caliper: Remove the bearing; measure the inner race (ID), the outer race (OD), and thickness (W). If the bearing is missing, measure the axle/spindle for ID and the bore/seat for OD.

Most riders can identify size in minutes with option 1 or 2. Measuring is definitive when markings or documentation are absent.

How to Read Common Bearing Codes

Most cartridge bearings use standardized series codes that map to exact metric dimensions. The suffix often indicates seal type (e.g., 2RS = two rubber seals; LLB = light-contact seal).

• 6802 = 15 x 24 x 5 mm
• 6803 = 17 x 26 x 5 mm
• 6805 = 25 x 37 x 7 mm
• 6806 = 30 x 42 x 7 mm (common in BB30/PF30)
• 6901 = 12 x 24 x 6 mm
• 6902 = 15 x 28 x 7 mm (very common in hubs)
• 6903 = 17 x 30 x 7 mm
• 608 = 8 x 22 x 7 mm (small parts, also known from skate bearings)

If your code differs, search the full code online; nearly all major bearing vendors post dimension tables by code.

Measuring Bearing Dimensions Correctly

When codes aren’t visible and specs aren’t available, precise measurement ensures the right replacement.

1. Remove the part safely: For hubs and BBs, note spacers, seals, and orientation; for headsets, support the frame and crown race.
2. Clean thoroughly: Wipe grease and grit so caliper jaws can seat properly.
3. Measure three dimensions: ID (inner hole), OD (outer diameter), and width (thickness). Record in millimeters.
4. If the bearing is missing: Measure the axle/spindle (ID target) and the housing/bore (OD target). The bearing width is typically standard for the series you identify.
5. Match to a series: Compare your numbers to standard series tables (680x/690x/600x). Allow no more than ±0.05 mm error.

Accurate measurements will map cleanly to a known code; if they don’t, re-check for worn seats, adapters, or misread caliper placement.

Headset Sizing and SHIS: What Those Codes Mean

Headset bearings are often angular contact bearings (ACB) with specific diameters and contact angles. The SHIS system describes cup type and diameters, helping you find the right bearing.

• IS42/28.6 (upper): Integrated, 42 mm headtube bore, 28.6 mm steerer; typically a 41.8 mm OD, 45×45 ACB.
• IS52/40 (lower for tapered): Integrated, 52 mm headtube bore, 40 mm crown race; usually 45×45 ACB.
• ZS44 (upper or lower): Zero Stack, 44 mm cup; check whether your bearing sits in a cup and confirm angle (36×45 or 45×45).
• EC (External Cup): Bearing sits outside the head tube; size governed by the cup, not just the bearing.

Look for markings like “45×45 41.8” or “36×45 30.2” on the bearing, which indicate contact angles and key diameters.

Bottom Bracket Notes: Standards and Bearings

Bottom brackets vary widely. Some use direct-press bearings in the frame (BB30/PF30), while many threaded systems house bearings in cups. Often, you replace the entire cup assembly rather than the individual bearing.

• BB30/PF30: Commonly use 6806 bearings (30 x 42 x 7 mm) for 30 mm spindles. Adapters/sleeves may be used for other spindle sizes.
• Threaded external (BSA) for 24 mm spindles (Shimano HTII): Bearings are inside cups; many brands are not user-serviceable as separate bearings.
• SRAM DUB (28.99 mm spindle): Implemented via specific cups; in BB30/PF30 shells, DUB often relies on cups/adapters rather than a unique loose bearing size. Verify with the BB manufacturer.
• Older square-taper/ISIS/JIS: May use loose balls or serviceable cartridges; sizing varies by model and is best confirmed via spec sheets.

Because BB assemblies are highly standardized by brand and model, confirm the exact BB type and follow the manufacturer’s replacement guidance.

Loose-Ball Systems: Ball Size and Count

If you have cup-and-cone hubs, older headsets, or traditional BBs, you may need ball sizes rather than cartridge dimensions.

• Front hub (many cup-and-cone): often 3/16″ balls (common counts: 9–10 per side when uncaged).
• Rear hub (many cup-and-cone): often 1/4″ balls (common counts: 9 per side when uncaged).
• Headsets (older/caged): commonly 5/32″ or 3/16″ balls; verify with calipers.
• Traditional BBs: 1/4″ or 7/32″ balls are common; check model-specific documentation.

Always match both size and ball count to the available race space; overfilling prevents proper adjustment.

Tools and Practical Tips

A few tools make identification and replacement safer and more accurate.

• Vernier/digital caliper (0.01 mm resolution) and a clean work surface.
• Bearing puller and press or appropriate drifts/sockets; avoid hammering on races.
• Cup removers for headsets/BBs where applicable; crown race setting tool for headsets.
• Grease, thread prep, and torque wrench for reassembly per spec.

Use gentle, even pressure to remove and install bearings to avoid damaging bores and seats.

Common Pitfalls to Avoid

These missteps can lead to mis-sizing or premature failures.

• Measuring a worn bore instead of the bearing itself and assuming that’s the OD.
• Mixing imperial and metric units or rounding aggressively.
• Ignoring contact angles in headsets (36×45 vs 45×45) and SHIS details.
• Replacing only one bearing in paired systems when both are worn.
• Pressing on the wrong race during installation, damaging the bearing.

Double-check dimensions and standards before ordering; when in doubt, consult the component’s service manual.

Where to Find Official Sizing Information

Manufacturer documentation is often the fastest route to an exact match.

• Wheel/hub makers: DT Swiss, Shimano, Industry Nine, Hope, etc., publish bearing charts for specific models.
• Headset makers: Cane Creek, FSA, Chris King list SHIS specs and bearing part numbers.
• Frame/BB makers: SRAM, Shimano, Wheels Manufacturing, Praxis, Token provide BB compatibility tables.
• Bearing suppliers: Enduro, SKF, NSK, NTN, and others post full dimension tables by code.

Cross-referencing your model name with official charts usually yields the exact size and seal type in seconds.

Bottom Line

To know your bike bearing size, first look for the printed code on the seal or confirm the component’s specification via the manufacturer. If that fails, remove the bearing and measure ID/OD/Width precisely with a caliper, and for headsets use SHIS to match angles and diameters. This approach works reliably across hubs, headsets, bottom brackets, pedals, and suspension pivots.

Summary

Identify bearing size by: (1) reading the code on the seal, (2) checking official specs (especially SHIS for headsets), or (3) measuring ID/OD/Width with a caliper. Know whether you have cartridge or loose-ball systems, be mindful of headset contact angles and bottom bracket standards, and avoid common measurement mistakes. With a code, a spec sheet, or accurate measurements, you can order the correct replacement with confidence.

How do I know what size bearings I need for my bike?

Bearing Sizing
This requires knowing the ball diameter, which can be measured using an imperial set of verniers for loose ball bearings. Common sizes include 5/32″ for headsets and pedals, 3/16″ for front hubs, and 1/4″ for rear hubs and bottom brackets.

How to measure bike wheel bearings?

And we’re looking for the nominal measurement or the whole number this here is measuring just a little bit under 17. And then we can go ahead and spread out for the outer. Diameter.

How do I know what size main bearings I need?

Measure the bore of the Rod or Main. Then subtract the diameter of the Rod or Main journal on the crankshaft. A little bit of math will tell you what size bearing you need. The Undersize/Oversize terms used for bearings can confuse you.

How to tell what size a bearing is?

To measure a bearing’s size, you need a digital caliper or vernier caliper to measure its inner diameter (ID), outer diameter (OD), and width. The ID is the measurement of the hole in the center, the OD is the widest part of the bearing’s outer edge, and the width is the thickness between the bearing’s two rings. Once you have these three measurements, you can use them to find the correct replacement bearing.
 
This video demonstrates how to measure the inner and outer diameter of a bearing: 53sBlueswift AxlesYouTube · Mar 21, 2025
Steps to Measure a Bearing

1. 1. Clean the Bearing: Ensure the bearing is clean so you can get an accurate reading. 
2. 2. Zero the Caliper: If using a digital caliper, turn it on and press the “zero” button when the jaws are closed. 
3. 3. Measure the Inner Diameter (ID): 
   • Insert the inner jaws (anvils) of the caliper into the bearing’s bore. 
   • Gently open the caliper until the anvils make light contact with the inside of the bearing. 
   • Read and record the measurement. 
4. 4. Measure the Outer Diameter (OD): 
   • Place the bearing between the outer jaws of the caliper. 
   • Gently close the jaws until they lightly touch the outside of the bearing at its widest point. 
   • Read and record the measurement. 
5. 5. Measure the Width (W): 
   • Use the outer jaws of the caliper to measure the distance between the two sides (rings) of the bearing. 
   • Ensure the measurement is taken at a right angle to the sides. 
   • Read and record the measurement. 
6. 6. Verify and Find the Correct Bearing:
   • Repeat the measurements at different points on the bearing to ensure accuracy. 
   • Use these three dimensions (ID, OD, Width) to search online or in a bearing size chart to find your replacement bearing.