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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?

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Could you post a picture of your crankset and bottom bracket? There are a couple different things that could be going on depending on what your setup is. Also, what was the bottom bracket problem? That may have affected your chainline too. –  jimirings Jun 22 '12 at 12:05
    
there are slight differences, and the tolerances are such that I would expect to need to tweak the FD, but I don't like the "gap" discription. I second Jim's comment...picture and original symptom. –  Ken Hiatt Jun 22 '12 at 16:51
    
Is this a double or triple chainring on the cranks in front? –  Benzo Jun 22 '12 at 17:13
    
I wasn't allowed to add a photo because of my low rep, so I've put it up on google docs, let me know if you are able to view it: link –  Dr. Greenthumb Jun 23 '12 at 11:44
    
The bottom bracket was creaking, so I took off the cranks and tightened it. The creaking noise seems to be sorted, so hopefully I can go back to cycling when I get the gears fixed. –  Dr. Greenthumb Jun 23 '12 at 11:47
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up vote 2 down vote accepted

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.

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That's most likely the problem. You don't necessarily need a torque wrench though, just an allen wrench with a longer handle. Most allen wrenches in that size don't have a handle long enough to give you the leverage to tighten it up as much as it needs. If you can't find an allen wrench with a longer handle, get a good-sized socket wrench with an allen attachment and you might have some luck. Just crank it on as hard as you can. It's nearly impossible to over-tighten crank bolts. –  jimirings Jun 26 '12 at 17:45
    
@jimirings -- You don't necessarily need a torque wrench though Ah, you're gonna have a couple of people jump on that comment. (But I agree.) –  Daniel R Hicks Jun 26 '12 at 18:23
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Lawndartcatcher, this is exactly what I've ended up doing. I had a big allen wrench so that did the job. Good answer. –  Dr. Greenthumb Jun 26 '12 at 19:28
    
Right - the reason I went with a torque wrench is because a) I love any excuse to buy a new tool and b) I could easily see myself snapping the head right off the bolt if I used my 1/2" socket wrench handle with a hex socket. Glad it helped. The chiba monkeys at the local bike store couldn't figure this one out; I had to call a friend who knows this stuff. –  lawndartcatcher Jun 27 '12 at 14:51
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