Hey, I'm relatively new to bike mechanics, and generally figuring it out by doing things wrong and then trying to fix them.
I overhauled my bottom bracket about three months ago, and just last night my left crank came loose while I was riding. I'm still suspicious that someone unscrewed the bolt, because I park my bike the street, delivered to someone's door around the corner, came back, and immediately noticed the crank was wobbly. I got a socket wrench and tightened it down, but less than five minutes later the crank was loose again. I figured I hadn't tightened it properly, and tried again. Same thing a few minutes later.
I took off my cranks today and put the bolts into the spindles without cranks and wiggled them around--the left side seems no more loose than the right, so I don't think I stripped the bolt or the inside of the spindle. The inside of the crank has wear, and slightly more on the left crank than on the right, so I'm thinking that may be my problem. Also, the washer inside the left crank won't come out anymore. It's been pretty worn for a while, but this is the first time I've been unable to get it out of the crank.
I think that the crank is probably just worn enough on the inside that it's working itself loose. Any other thoughts? Thanks.