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There are many kinds of bearings.... This discussion has to do with ball bearings similar to the ones used in the wheel hubs of motorcycles. Ball bearings are made in three basic styles... "Open", "Shielded" and "Sealed". An "Open" bearing has nothing on either side to seal any lubricant into the bearing, a "Shielded" bearing has a metal shield on either side of the bearing and a "Sealed" bearing has a rubber oil seal that seals lubricant in the bearing... If you are going to use a bearing in something such as a motorcycle transmission where it would be constantly bathed in transmission lube and there is no way for dirt to get in than an "open" bearing would be used...If the bearing is used someplace where it would be exposed to dirt a "sealed" bearing would be used...  

Bearing manufactures also make standardized sizes... Numbers are assigned to these standardized sizes. They call this the "bearing trade number"  You will find that most ball bearings are made to these standard sizes and in many cases you can get a bearing that will work at a local bearing shop for much less than the buying the same bearing at the local motorcycle dealership....  The big difference is that different manufactures use different prefixes and suffixes to designate manufacturing tolerances, speed ratings or  weather the bearing is shielded or open...

For example....One of the bearings in a Kawasaki H2 front wheel has a part number of 601B6203 ... Kawasaki's standard part number for bearings is 601B plus the "bearing trade number"  (601BXXXX  where XXXX is the "bearing trade number") Kawasaki uses "open" bearings in their front wheel so In other words a Kawasaki Part number 601B6203 would be a number "bearing trade number" 6203 bearing that is open (no seals or shields)....  A wheel bearing doesn't need to be a high speed bearing  so you can get a suitable 6203 bearing for $10 or less ... Kawasaki charges a bit more for the same bearing....

Some Yamaha part numbers can be deciphered as well.... for example a Yamaha part number 933306-30309-00 is a 6303 bearing...

While this isn't a complete list of the trade numbers, here are many of the metric bearing trade number's and the their sizes:

Bearing trade number Inside Diameter Outside Diameter Thickness
627 7 22 7
608 8 22 7
629 9 26 8
6800 10 19 5
6900 10 22 6
6000 10 26 8
6200 10 30 9
6801 12 21 5
6901 12 24 6
6001 12 28 8
6201 12 32 10
6301 12 37 12
6802 15 24 5
6902 15 28 7
6002 15 32 9
6202 15 35 11
6302 15 42 13
6803 17 26 5
6903 17 30 7
6003 17 35 10
6203 17 40 12
6303 17 47 14
6804 20 32 7
6904 20 37 9
6204 20 47 14
6304 20 52 15
6805 25 37 7
6905 25 42 9
6205 25 52 15
6305 25 62 17
6806 30 42 7
6906 30 47 9
6206 30 62 16
6306 30 72 19
6907 35 55 10
6007 35 62 14
6207 35 72 17
6307 35 80 21
6908 40 62 12
6208 40 80 18
6308 40 90 23
6209 45 85 19
6309 45 100 25
6210 50 90 20
6310 50 110 27
6211 55 100 21
6311 55 120 29
6212 60 110 22
6312 60 130 31