Problems with gears after re-attaching front chainring I had to remove the pedal crank arm  together with the front chain set when fixing a bottom bracket problem, but now that I've re-attached it, gear changes in my front derailleur are completely off. 

I think this is because the distance between the chain set and the frame changed slightly after I installed the crank arm back on. I used a crank extractor to take it off, is there another tool I need to use to make it as tight as it was before? Or is the new gap a normal occurrence and I just go ahead re-adjusting the gears?
I had the same issue (taking cranks off, pulling / cleaning bottom bracket, replacing it only to find that the front derailleur didn't match up no matter how much I adjusted it in and out) - it turns out that the cranks weren't tightened fully, allowing / causing the chain ring to stick out slightly farther than tolerances allowed for. The solution was to crank it down using a torque wrench - find the torque specification for the crank bolts (mine was 40 ft/lbs amazingly) and crank them down. You'll be surprised at how much difference this can make.