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How to Read Bearing Numbers (And Why You Can’t Use Math)

Have you ever been mid-project, realized you need a larger bearing, and thought: “If a 608 bearing has an 8mm hole, then a 12mm version must be a 6012?”

Stop right there. If you try to use math to calculate bearing sizes, you are almost guaranteed to order the wrong part.

In the world of precision engineering, bearing numbers (or “call-outs”) aren’t measurements—they are coded indices. In this guide, we will break down how to read these numbers and provide a “Cheat Sheet” for the most common sizes used in robotics, 3D printing, and DIY engineering.

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The Big Secret: It’s a Code, Not a Measurement

When you look at a bearing number like 608, you aren’t looking at a mathematical formula. You are looking at a “Library Index.”

• The First Digit (The Series): This tells you the “family” of the bearing (e.g., the 6000 series vs. the 6200 series). This determines the general weight and load capacity.
• The Middle Digits (The Width Group): This tells you how thick the bearing is relative to its size.
• The Last Two Digits (The Bore/ID): In the metric system, these digits point to a specific entry in a standardized table. For a 608, that entry translates to an 8mm Inner Diameter.

The Danger: Because these are indices, there is no linear relationship between the numbers. Moving from a “08” to a “01” doesn’t follow a mathematical progression; it simply points to a different row in the master dimension table.

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The “Golden Rule” of Replacing Bearings

If you are increasing the Inner Diameter (ID), you must measure your physical space with a caliper.

As you increase the size of the hole in the middle (ID), the Outer Diameter (OD) and the Width will almost always change. You cannot simply “scale up” a bearing. You must match the ID, OD, and Width to your specific housing or axle.

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Pro Tip: Always use a digital caliper to measure your required Outer Diameter and Width before hitting “Buy.”

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The Bearing Cheat Sheet: Common Metric Sizes

Use this table as a quick reference for the most common “608” and “6200” series bearings used in hobbyist and light industrial applications.

Bearing Call-out (Link)	Inner Diameter (ID)	Outer Diameter (OD)	Width	Common Uses
608	8mm	22mm	7mm	Skates, 3D Printers, Small Motors
6000	10mm	22mm	7mm	Small Robotics, RC Cars
6001	12mm	28mm	8mm	Robotics, Small Axles
6002	15mm	32mm	9mm	Medium Robotics, Tooling
6004	20mm	42mm	12mm	Larger Gearboxes, Heavy Duty
6005	25mm	52mm	15mm	Industrial Motors, Large Shafts
628	8mm	28mm	8mm	High-Load Small Shafts
6200	10mm	30mm	9mm	High-Load Robotics

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Decoding the Suffix: ZZ vs. 2RS

Once you find the right size, you’ll notice letters at the end of the number. These describe the sealing of the bearing, which is vital for the longevity of your machine.

ZZ (or 2Z)

• What it is: Metal shields on both sides.
• Best for: Environments with large dust particles.
• Note: They are NOT waterproof. They allow a tiny bit of lubrication to escape and a tiny bit of moisture to enter.

2RS (or DDU / LLU)

• What it is: Rubber seals on both sides.
• Best for: Almost everything else. They are much better at keeping water, mud, and fine grit out.
• Note: They create slightly more friction than metal shields, but the added protection is usually worth the trade-off.

Open

• What it is: No shields or seals.
• Best for: Inside a gearbox or a machine that is constantly submerged in an oil bath.

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Summary Checklist for Ordering

Before you checkout, verify these four things:

1. Inner Diameter (ID): Does it match your axle?
2. Outer Diameter (OD): Does it fit into your housing?
3. Width (W): Is there enough (or too much) room in your assembly?
4. Seal Type: Do you need 2RS for waterproofing or ZZ for low friction?

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