i'm having a problem w/ the bike i built up the other day. the 5 bolts which attach the crank to the chainring keep loosening themselves up while i'm riding. i did really lay into them w/ the allen rench, so i know they were tight before i started riding. but about halfway through my ride i noticed they were loose. this results in the chainring moving around a little when stopping and starting. and i just worry that they might work themselves off while i'm standing and pedaling and then that would end badly. anyone have any tips on how to get these guys to stay tight? they're bmx bolts i believe, b /c they are not very long(only long enough for 1 ring). thanks for any and all help.