I have a BD Motobecane Messenger, SS. In April I had to have the crank set replaced because the one of the holes for the pedal was not threaded correctly, and ended up getting stripped.

After the crank set was replaced I had no problems for about two months. Recently the bolt holding the crank arm (non-drive side) to the BB will not remain tightened, loosening up every 20-30 miles or so, causing the BB to chatter.

Now, even after tightening the crank arm bolt, the BB will continue to creak. Is it ruined? Why won't the bolt remain tightened?

I obviously am not mechanically inclined yet, so any help would be appreciated.

I suspect that your problem is more related to the crank. If you rode for a while with a slightly loose crank, the movement would have worn the area where the spindle engages it. Once that happens the crank can move slightly forward and back with each revolution, creaking and working the bolt loose.

In all likelihood the BB is OK, but the aluminum arm is toast. For future reference, loose cranks are like dental decay, unless you respond and deal with it immediately it'll progress beyond repair.

I agree. The hole for the square-taper crank has become slightly distorted, and as a result will never properly tighten down.

When a new crank arm is mounted it sometimes will need to be retightened after a break-in period. If one does not do so, and then in addition ignored warning signs such as clicking or creaking when pedaling and the crank comes loose even one time the crank is generally ruined and will need to be replaced. Also I have never seen crank arm stripped due to not being threaded properly. Though I won't deny that possibility, you may need to pay more attention to routine maintenance and perhaps your riding style. It is very unusual to have two such similar problems in a short span of time.

Crank arm was not stripped, the threading for the pedal was not right, and eventually was stripped. I needed a new crank arms initially because i was unable to properly put the pedals on.

I also had no warning signs the first time the crank arm came loose, there was no creaking or clicking. Ever since then, I obviously have had noise.

Thank you for the quick replies and advice.