We changed to a longer set of cranks (same make), and have tightened the crank as tight as it will go without stripping. There is still a space of approx 2mm between the crank and the lip on the bottom bracket (can't remember whether that gap was there when we took it off. ) But now the front derailleur won't change onto the largest gear, and the adjustment is at its max. We cannot tighten the bottom bracket either. Any ideas, please?