Metric and Inch bearings

Bearings come in both in inch and metric variations. It is important to carefully check when measuring your bearings to ensure that you take accurate measurements to ensure that you get the correct bearing.

Some inch sized bearings have a very similar shaft size to those of metric bearings. Particular care needs to be paid with these sizes I was trying to fit the wrong sized bear into your shaft could cause damage or at the very least waste the fitters time and mean that you have to get an alternative bearing. On this page we have created a table off standard inch and metric sizes and we have converted the inch sizes to millimetres so that you can use this in your bearing identification to try and ensure that you get the correct bearings first time around.

You can see from the table above how similar some items are and how close the measurements can be between inch sized bearings and metric bearings. Here we are going to explain some of the more common mistakes that people make when purchasing bearings. Whilst some metric part numbers are very close in size to some inch sized part numbers we are going to focus on the most popular mistakes with the most popular bearing sizes

One inch bearings such as UCF205-16 and 25 millimetre metric bearings such as UCF205 are regularly mistaken for each other with the inch sized bearings being 25.4 millimetres leaving a difference between the two of just 0.4 millimetres. it is important to make sure that your caliper's start at zero when you are measuring to help prevent incorrectly identifying your bearings.

Another potential opportunity for error is with a 2” bearing so something like a UCF210-32 and a 50MM bearing. 2” is 50.80mm. Sometimes when measuring quickly it’s easy to get to 50mm and think that is right but if you tried to put a 50mm bearing onto a 2” shaft it certainly wouldn’t fit. The part number for the 50mm variant of this item would be UCF210. You can see with that the only difference between the part numbers on both items is the -32 at the end of the inch sized bearing and that is because inch sized bearings are measured in sixteenths of an inch. Therefore using fractions, you would have 32/16 and 32 divided by 16 is 2 which is where the 2 inch comes from with the part number.

Other inch to metric crossovers are :

30mm and 1.3/16”. The 1.3/16” bearings are not very popular to it’s highly likely that your bearing would be 30mm but just keep in mind that 1.3/16” is 30.1625mm and double check before selecting your bearing.
35mm and 1.3/8”. Bearings such as RLS11 and UC207-22 are 1.3/8” and are 34.925mm which is very close to 35mm.
40mm and 1.9/16” (39.6875mm)
45mm and 1.3/4” (44.45mm) This is quite a popular item in both metric and inch sizes so additional care needs to be taken.
65mm and 2.9/16” (65.0875mm)
70mm and 2.3/4” (69.85mm)

If you are in any doubt on your measurements then try measuring the shaft directly to see if that gives a clearer indication. Or it that fails then have a look at the bearings to see if there are any markings on them. Inch sized bearings will have different markings to metric bearings.

For example

A 1” standard insert bearing will be marked UC205-16 an a metric bearing of 25mm will be marked UC205. Inch sized bearings are measured mostly in sixteenths of an inch and the bearings will be labelled as such.

A 1.9/16” insert bearing for example would be a UC208-25. It is one full inch and then 9/16 of an inch. 16/16 + 9/16 = 25/16 hence the -25 on the end of the bearing part number.