How to Measure & Buy a Replacement Bearing

Many ball bearings come in standard sizes. If the bearing you require is just an ordinary ‘standard type’, you can measure the size, (Bore x Outside Diameter x Width), or find the bearing size in a parts manual. With this information, you can easily buy an ‘off the shelf’ ball bearing to replace one that’s worn out.

How to Select a Bearing?

What Measurements or Information do I Need to Buy a Bearing –
– Choosing the Right Bearing

As long as it’s not some weird and wonderful, specialised type of bearing, the information below is all you need to select and buy a bearing:


Make sure you measure the bearing in the correct format. Is it a metric or an imperial (inches) bearing? You can measure the bearing in either format, but in some cases, there may be a slight difference in size (dependant on the tolerances you’re working to?).

Information Required:


Measuring The Size of Bearing:

Use the following measurements, when finding the bearing size:

Bore (or hole diameter) x Outside Diameter x Width

or…    Bore Ø x OD x W

How to Measure a Bearing

The Type of Bearing:

There are generally four types of bearing…

Types of Bearing Available:

  • Ball Bearing.
  • Roller Bearing.
  • Thrust Ball Bearing.
  • Thrust Roller Bearing.

The basic requirement is to replace the bearing with the same type. Going by the names, you can generally select the ball bearing type:

  • a ball is in the shape of a ball,
  • a roller is the shape of a garden roller, or a baked bean tin on its side.
  • and a thrust bearing is where the load on the bearing is from the side.

The Type of Bearing Seal:

Bearings will often contain a seal to prevent the loss of lubricant and to prevent the ingress of dirt and debris. There are generally, two types of seal:

An Open Bearing:

This is where the bearing is open to the elements. It is therefore usually installed in a sealed protected environment or housing.

A Rubber Sealed Bearing:

This is where the open bearing raceways are covered by a rubber seal to prevent the ingress of dirt and moisture.

These will have the abbreviation of: RS or 2RS


The rubber seal is generally thought to provide a better seal against ingress of dirt and moisture (compared to a steel seal).

A Steel Sealed Bearing:

This is where the open bearing raceways are covered by a steel crimped seal to prevent the ingress of dirt and moisture.

These will have the abbreviation of: ZZ or 2ZZ

The Bearing Material:

Chromium Steel:

Chromium Steel has increased wear and corrosion resistance over plain steel.

Stainless Steel:

Stainless steel has increased wear and corrosion resistance over chromium steel (and are approximately twice the cost).


Ceramics are used in the manufacture of the ball bearings. They are much harder than steel and provide high wear resistance (and are approximately four times the cost).

The Clearance:

The clearance (‘C’ rating) is the amount of play in the bearing. This sometimes needed due to heat expansion.

For precision (critical alignment) or where no great heat is generated, a ‘C’ rating is not required. E.g. a bicycle does not require a ‘C’ rated bearing.

The larger the number, the bigger the clearance: C3, C4, C5.

What’s Lubricant is the Bearing Filled With:

This isn’t always stated, but you may have a personal preference?

  • Oil.
  • Grease.
  • Nothing?

Why Should I buy a Bearing Direct from a Bearing Supplier?

There are various types of bearings, and they come in a multitude of shapes and sizes. When you buy a bearing, sometimes it’s easier just to specify “I want a bearing for a widget machine” and the jobs done. But is it actually the easiest route to buying a bearing?

Positives (buying from the widget machine supplier):
  • If the bearing is for a high-end product, where safety is involved, or there is a high cost for machinery downtime. It is best to get the OEM (or widget machinery suppliers) bearing for peace of mind.
  • You don’t have to measure the bearing (Bore x Outside Diameter x Width), or find the size in a manual.
Negatives (buying from the widget machine supplier):
  • Additional cost – If you buy from the original supplier, there are added markup costs (they will also buy in their bearings from an external supplier).
  • The OEM supplier may be no-where near you if you need a bearing in a hurry.

“Buying direct, cutting out the middle man will always be because of cost or the speed of supply”. I would guess that 99.9% of manufacturer’s buy-in their bearings.

Therefore, why not go directly to a bearing supplier (see the positives and negatives above)?

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