I tried adjusting the truss rod on my bass today, I think the neck is too bowed because if i lower the action the frets after the 12th fret start buzzing. so i got a 5mm hexagonal allen key and tried to fit it into the truss rod hole, and it doesnt fit. I shined a torch down there and it looks like everything is ok, i can see that there is a hexagonal hole. I got about 10 allen keys from my bass's previous owner, and about 4 of them are 5mm hex, ive tried them all and they dont work. my question is this, is 5mm hex the right size allen key? if it is the right size key, then any ideas into whats wrong?