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Retainer
A retainer is a clip, usually of sheet-metal or plastic, that holds and separates the bearing balls in a ball bearing. The advantages of retainers are ease of assembly, and economy because the manufacturer can get by with fewer balls, if they are spaced apart by a retainer. Retainers are a good thing for one-piece cranks, due to the ease of assembly issue. Retainers also can make headsets easier to assemble, but on the other hand, using a larger number of loose balls to replace retainers can salvage a headset with indented bearing races, or prolong the life of a new headset. Retainers are used in cartridge bearings, to keep the balls spaced apart following assembly.: A cartridge bearing with both races in one plane could not be assembled if balls went more than halfway around. Retainers are generally not so good for other bicycle bearings.
Conventional bottom brackets with loose balls can hold 11 1/4" balls per side. High quality retainer sets also have 11 balls. Cheaper retainers will only have 9 or even as few as 7 balls, and should not be used.
Sometimes it can be confusing to determine which side of a retainer faces which way, but careful examination of the retainer and the races it fits with will show the answer. The fundamental principle is that the retainer never touches either the cup or cone race, it only touches the balls. If a retainer is installed backwards, the retainer rubs and the bearing can not be adjusted.
