I should have clarified. Cotters come in three diameters, right? Using what appears to be the correct diameter fits without a problem - in fact, it fits very snugly. Where I usually have to file is on the angle, in order to have flat against flat - and that's where the problem seems to be for me. Even though everything feels tight, after I put a little torque on the pedals there is enough play for the crank arm to move - to rotate on the spindle slightly, keeping the pedals from being aligned on a perfect 180 degrees. They are slipping about two or three degrees - and that is both visually and physically discernable.

I've installed enough cotters that I figured, well heck, I must have filed off too much. But I've been through four sets of cotters now and I'm beginning to wonder what I'm doing wrong here - what is it I'm doing differently from every other time I've installed cotters.