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Some Filing Tips

There are a few pitfalls in filing cotters. You need to remove more metal from the wider end of the flat (near the threads) than from the narrow end of the cotter. Balance the pressure on the file to compensate for this, or you will inadvertently reduce the angle of the taper. If this occurs, your cranks will not come out 180 degrees apart when assembled. One remedy is to put the cotter into the vise with the flat parallel to the tops of the vise jaws. This gives you a reference so that you can see whether you are changing the angle. Tighten the vise securely.

The other pitfall to filing cotters is that most people tend to rock the file as they use it. This will give the "flat" a partially cylindrical surface, and as a result it will fit unevenly against the flat of the bottom bracket axle. It is very important to keep the file horizontal throughout the process.

Good file technique requires lifting the file off of the work for each back stroke. You should also use a file cleaning card, a special brush with very short steel bristles. After every few strokes of the file, clean the file with the file card to prevent a buildup of chips in file teeth.

To reassemble, the cotters must be hammered in just as they were hammered out. If you try to tighten them just by tightening the nuts, you will not be able to get the cotters tight enough and you may strip the threads. The cotters should be hammered, then the nut snugged down, then hammered a bit more, the nut tightened a little more and so on, until further hammering does not produce any more slack to tighten the nut. After 50 miles or so of riding, you should give the cotters a couple of taps and tighten the nuts again. After that they should be secure.

Tool Tips--Cottered Cranks