Yep, all the stuff the other guys already mentioned. They need to match, and sometimes the threads don't stick out far enough.
Plus, many of the new cotters are very poorly made. They're just chopped off a metal rod, and likely forged to sort of the right shape before threading. Often the taper isn't flat, and sometimes it's narrower than I'd like. A few even have a bulge at the threaded end of the taper that prevents it going into the crank. Careful filing can make it flatter and wider while keeping the same angle. A wider face on the taper is better as it spreads the pedaling force out a bit more.
Steve Birmingham Lowell, Massachusetts USA
Message: 15 Date: Thu, 13 May 2010 09:45:09 -0700 From: Chuck Hoefer <paccoastcycles@sbcglobal.net> Subject: Re: [CR] Crank cotter filing To: beandk@rcn.com Cc: CR List <classicrendezvous@bikelist.org> Message-ID: <4BEC2C95.8040907@sbcglobal.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
David Bean wrote:
> I have never understood why filing crank cotters is so widely recommended.
> Two new crank cotters (of the same mfr and type, of course) must have the
> same bevel. Using a file on that bevel seems likely to render it less
flat
> than it started out; therefore more likely to come loose in service. What
> am I missing here?
>
> David Bean
> Arlington, MA USA
> beandk at are-see-enn dot com
>
> _______________________________________________
>
> Filing a crank cotter is done when necessary to adjust the amount of
threads showing past the crank arm cotter hole. In other words, some do not
protrude enough. It is ideal to start with two cotters of the same
manufacture.
>
Chuck Hoefer
Vista, California USA