Make sure that the cotter pins are not all the way through the crank arm. Usually, they should go all the way through without sticking out. I always check this carefully and add ensurence with a washer that has a slightly larger hole diameter that the outside diameter of the pin.
When I press the cotter into place, I snug both crank arms slightly and then look to ensure that they are parallel to one another. If satisfied, I start the super snug up process...
Snug up the cotter with the press a lot then give it three taps or hits with a steel hammer, forcing the pin further into its fit. Now, snug up the press a wee bit (it will most likely turn easily at first) and tap again. Snug the press, tighten the press and tap again. I continue to do this until I get no more turn when I go to re-snug the press.
When hitting the press head, do not be shy. Give it a decent three smacks with a steel hammer. You would be wise to support the crank assemble to prevent the impact from damaging bottom bracket bearings, and/or races. This is my home made press. The head on the screw has mushroomed even more that it appears in this picture...
It is a slow process but does work well when done correctly. Now, make sure the round of the cotter is not sticking past the crank arm fit. If no, install the cotter pin nut and washer. If yes, install a washer with a hole larger than the cotter round, once again ensuring the the round does not stick out.
Finally go for a ride but stick close to home. After the ride, remove the cotter nut and use the press again to ensure that the cotter has not slipped or worked its way into a better fit. I know this sounds silly but this final check is important. If the press does turn, go through the snug and tap procedure again.
Now you know my secret about installing cottered cranks...
