They go "tappy-tappy-tap", nothing happens, then they try hitting a bit harder, then harder still, until the end of the cotter is mushroomed over or bends. Most people are scared to hit a bicycle with a hammer, and are afraid they will miss if they swing too hard. Claw hammers are designed to drive nails without bending them, and that is very similar to what you want to do to a cotter. To be sure, a really rigid support can blemish the crank - but these days, most cottered-crank replacements will be with new, lighter, more trouble-free cotterless cranks. The cotter shown, already mushroomed by several failed attempts, let go with one hammer blow when resting on this support.You could also rest the assembly on a bench vise or an actual blacksmith's anvil, if you have one. (Use a left crank so there won't be a chainwheel or crank spider in the way of your hammer.) The crank in the photo is resting on a steel fencepost which is filled with concrete and set in concrete at its base. One neat trick is to place the head of the cotter inside another cottered crank, used as an anvil as shown in the photo below. The more rigid the support, the better your luck is likely to be. You could damage a wooden floor, and as the pipe indents the floor, it won't provide as good protection to the bearings. It is best to rest the pipe on a concrete floor or to place a thick, steel pad under the end of the pipe. The pipe will transmit the impact of the hammer to the floor, so as not to damage the bearing cups or axle cones. Wedge it in position so that the non-threaded end of the cotter can fit inside of it. You need a short length of pipe, long enough to go from the underside of the crank to the floor. The hammer and pipe system is best if you don't have a press, or if the press fails. Var also makes a cotter pin press that uses a cam and long handles like those of a bolt cutter. Park makes the best one I know of, it is basically a super-heavy-duty "C" clamp. The first thing to try, if you have one available, is a cotter pin press. Recalcitrant crank cotters are among the most challenging bicycle repair problems.
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