Hi, i own a 7 year old fuji supreme hybrid bike. for the first 5 years everything was well, but 2 years ago, my left crank arm started falling off. I went to multiple bike repair shops, none could make it last more than 3 months. The last place i visited replaced both cranks, the bolt, the threaded rod that links both cranks and the front gear set, the new set is different as instead of a square hole, it is more like a pattern with many teeth to prevent widening the hole if it gets loose again, and the bolt is a 7 mm alem (hexagonal hollow). He even used blue locktight as well, in april or may, and i've been going much much easier on acceleration and started downshifting more on complete stops, but now it's loosened again last week. I pedaled using only the right side as soon as i noticed, and took it to be re-tightened, now a week again it's loose, which probably means the hole is deformed AGAIN.
I'm tired of being robbed by bike stores that look like they don't know what they're doing. The bike when new last me 5 years without crank problems even though i weight 230lbs, never, ever let the rear shift go out of the smallest gear, and pedaled with my full strenght more often than not. Now even when replacing the whole thing and paying attention to be easy on my bike it keeps falling off.
I've been thinking of getting an electric impact drill and new cranks, would that help? Or be damageful to my bike?
edit: i don't care what tool i have to buy, i just don't want to have to worry about this problem ever again, even if it means soldering the damn thing shut