Perenially loose cottered crank
Hi, I have a 70s road bike that I'm trying to restore and get bike to riding condition. I have everything fixed except the left crank is loose. This is a cottered crank and I have replaced the original cotter pin, installed a new one, and after 5 minutes of riding it would get loose again. Then I would tighten and it would loosen in 5 seconds of riding. Both of these pins had a nice U shaped gash in them on the flat side after I removed them. I put a 3rd one in and didn't tighten it as much as I had the 2nd, and it worked for a day but it is starting to loosen already.
The crank itself looks ok so I'm wondering if anyone has any suggestions. I'm a novice to bike repair but I've enjoyed fixing these problems.