Originally Posted by
Bianchigirll
I don't think this bike had cotter pins.
Everyone..... I wonder if the OP is saying cranks tightened but it is actually the BB coming loose?
It could be a case of the crankarms coming loose.
When a cotterless crankarm is insufficiently tightened and ridden loose, the steel spindle damages the tapered hole in the softer, aluminum crankarm. The damage can reach a point where loose crankarms become a chronic issue.
The most common damage is that the end of the spindle creates a burr in the hole. This is visible and often (about 80% of the time) careful filing of the burr will resolve the problem. The problem is most common with the softer/less dense, melt forged cranksets. I saw it a lot on recently purchased department store bicycles.
However, the hole can also be distorted and this is usually visible to the naked eye only in extreme cases. Distorted holes are rarely repairable, except by a machine shop, and even then refinishing of the hole often results in the removal of enough material to cause the spindle end to protrude beyond the bolt seat.
Regardless, I agree with member Classtime, the first thing that the OP needs to upgrade is his bicycle shop.