Cottered cranks are the older technology. They're usually found on fairly inexpensive bicycles, but many older bicycles of top quality have cottered cranks. They can be quite satisfactory in use, but are harder to work on than the newer cotterless cranks. There is a special tool for installing and removing the cotters, called a cotter press. This is strictly a shop tool: it weighs several pounds and is quite expensive. Bicycle shops that do a great deal of work on cottered bottom brackets might consider investing in a Park Tools cotter press; I've tested it several times and found it satisfactory.
[Update, September, 2013: the Park cotter pin press has been discontinued but one is available from Bikesmith -- John Allen.]
Fortunately, the most common tool to remove cotters is one found in just about every household: the hammer. To drive out a cotter that's been properly installed requires one or two very hard hammer blows. This can be tricky, because unless you take careful aim, you can severely damage other parts of your bike. One common mistake is trying to hammer the cotters out by tapping very lightly, for fear of missing, on the threaded end of the cotter. After a few light blows, you'll realize the cotter hasn't budged, and more forceful blows are in order.
A short time later, you'll notice that the threaded end of the cotter has come to resemble a mushroom, and it will never see a nut again! (Most books tell you to loosen the nut but leave it on, so that if the threads are damaged, unscrewing the nut will correct the damage. This is fine in theory, but in practice there is usually not enough length on the threaded end of the cotter to do this.) Some experts also instruct you to put a block of wood over the cotter's end, so the hammer blows won't damage the threads, but the compressibility of wood also blunts the full force of the hammer, meaning a solidly packed cotter just won't give.
A better way to support the end of the crank is to use a short length of pipe, braced between the floor and the underside of the crank at the bottom bracket. This transfers the impact directly to the floor. (If you care about your floor, put a strong block of wood under the pipe.) The pipe surrounds the end of the cotter, leaving room for the cotter to move, but it gives excellent support to the end of the axle. The length of pipe depends on how the bicycle is being held. At home, I usually use one of the inexpensive "Y" shaped stands that go under the bottom bracket and hook over the down tube. With this type of stand, I find that a 13 1/2 inch long pipe does the job nicely. If you are working with the bicycle standing on both wheels, something more like 12 inches would be in order. If you use a higher stand, you will need a longer pipe. If your stand works by hooking under the top tube, or, like mine, under the saddle and handlebar stem, you will need different lengths of pipe to work on different-sized bicycles. That's why I use the "Y" stand for this job: one size pipe fits all bikes.
If you have a professional-type stand that clamps onto the seat tube, you can adjust the height of the bicycle by choosing where to grip the seat tube, so with that type of stand you could also use a single length of pipe for many different frame sizes. The pipe that I use is 3/4-inch cast iron gas pipe. (Pipe is usually measured by the inside diameter, unlike bicycle tubing.) The diameter of pipe is not critical; almost any scrap will do if it is the right length.
As the cotter will no longer be protruding from the crank, you will need to use a punch (drift) that can go into the cotter hole to drive out the remains of the cotter. My favorite punch for this purpose is an old pedal spindle with the thin end ground flat. These spindles are made from the same superior kind of steel used in good tools. They are thick at the end you hit, but they taper down nicely to a size that will fit into the cotter hole of the crank.
Do not try to hold the punch in your hand, because if you miss, you could do yourself a serious injury; even if you don't miss, the closeness of your hand will inhibit you from hitting the punch as hard as you must. The punch should be held with a pair of pliers, preferably locking pliers such as Vise-Grips. With the punch, 95 percent of cotters should come out without need for more heroic measures. If yours is among the five percent that still won't budge, there are two more things to try. One is to drill the cotter out.
Drill all the way through the cotter lengthwise. Expect that the bit may need resharpening if it encounters the axle, which is of very hard steel.
In almost all cases, this will relieve enough pressure that the remains of the cotter can be easily driven out with a punch. You could also try twisting the head of the cotter with a vise-grip pliers. If the other crank is in place, you could stand on both pedals.
If that fails (and I have never known it to fail), your second option is to heat the crank with a propane torch. This expands the crank, including the hole that the cotter is in. Once the crank is well heated, the cotter will come out easily with a punch. This measure is used as a last resort, since overheating the crank will weaken it. Do not heat the crank so hot that it changes color, much less becomes red-hot.
If you have been unable to remove both cotters without damaging them, you will have to replace one or both of them. This may be a problem because of lack of standardization. There is a limited amount of standardization in the diameters of cotters (usually all cotters used in a given country will be of the same diameter) but very little standardization of the flat face. Neither the angle nor the depth of the flat face is standardized. If possible, you should try to obtain replacement cotters made or distributed by the manufacturer of your bicycle. If not, you will probably have to file new cotters to fit, a touchy job at best.
[Spindles also, in case you have to replace one. French cottered spindles have a diameter of 15.5 mm (0.610") and most others, 0.625" -- John Allen]
You will need a vise, securely attached to a solid bench, and a good file. The most useful single file for bicycle work is the ten-inch half-round bastard, but any flat file of similar size will do, as long as it is made for use on steel.
Make sure that the file has a handle (you should never use a file without a handle of some sort). A Vise-Grip-type locking plier makes a good temporary handle if you do not have anything better.
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 -- though you should not have to use as heavy hammer blows.. If you try to tighten the cotters just by tightening the nuts, you will not be able to get the cotters tight enough and you may strip the threads. Grease the cotters -- both the part that fits into the crank, and 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.
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Last Updated: by John Allen