ball bearings

Because the best ball bearings in the early 20th century were made by German manufacturers, ball bearings have usually been sized in millimeters, even those made in the United States–but the balls themselves were frequently sized in inches. The Anti-Friction Bearing Manufacturers Assn. adopted a classification of ball bearings according to their construction, with each type identified by a symbol (such as "BA"). Symbols that include the letter "I" identify types that are nominally sized in inches; of ten types, only two have an "I."

Embedded in the manufacturer's model number for most ball bearings is a three-digit code that gives its dimensions in millimeters. The first digit refers to the series. There are four principal series:

The table below shows the sizes up through a bore of 50 mm, but much bigger sizes are defined. In the medium series, for example, a 356 bearing takes a shaft with a diameter of 280 mm–more than eleven inches.

Designation	Bore	Series 100 (x = 1)		Series 200 (x = 2)		Series 300 (x = 3)		Series 400 (x = 4)
Designation	Bore	OD	Width	OD	Width	OD	Width	OD	Width
x00	10	26	8	30	9	35	11	—	—
x01	12	28	8	32	10	37	12	—	—
x02	15	32	9	35	11	42	13	—	—
x03	17	35	10	40	12	47	14	62	17
x04	20	42	12	47	14	52	15	72	19
x05	25	47	12	52	15	62	17	80	21
x06	30	55	13	62	16	72	19	90	23
x07	35	62	14	72	17	80	21	100	25
x08	40	68	15	80	18	90	23	110	27
x09	45	75	16	85	19	100	25	120	29
x10	50	80	16	90	20	110	27	130	31