Originally Posted by
speni
Job done. I drilled a screwdriver "slot" into the bolt, and managed to unscrew it.
I spoke to the guy who had the bike before me, and he said he regularly had to tighten this nut, and maybe had done it too harshly, and it has sheared, hence the crank arm is still OK.
I don't know why this bolt regularly needs tightening up, maybe in the future I will need a new BB.
The bolt regularly backs out because the crankarm's hole is completely buggered. It squirms on the spindle and wiggles the bolt loose. Screwing on the bolt tighter doesn't do anything to solve the problem (except to break the bolt). Take a closer look the inside of the square-taper hole on the crank and you'll see that it's NOT a perfectly square taper. most likely it's flower-shaped with rounded corners. THAT is why it's wiggling on the bottom-bracket spindle and loosens the bolt.
The bottom-bracket is fine in this case. Hardened steel cannot be damaged by soft aluminium. You just need a new crank and bolt. Install to proper torque the FIRST time. You have only one chance to install the crarnkarm correctly. If it loosens just once, you've pretty much destroyed it. No amount of re-tightening that bolt will ever keep it on. Just do a search here for "loose crankarm" and research more deeply into this issue.