Bicycle Mechanics - Installing new crankset

So my left crankarm has been coming loose. I looked up the problem and decided that I probably needed a new crankarm (rounded taper in the arm), but since I was already buying stuff- I also decided to go ahead with a whole new crankset with 165 mm length cranks and a larger chainring. I ride fixed gear.

Here come the questions: 1.) Should I consider changing out the bottom bracket also? 2.) I remember reading a debate somewhere about whether to tighten the bolt on a new crankarm to a specific torque or whether to just tighten it as much as possible. I'm sure my crankarm came loose because of lousy installation on the part of the idiots at my LBS. What should I do to ensure that I don't have this problem again? 3.) Is there anything else I should be considering here? I'm kind of nervous as changing the crankset seems to be basically like an engine swap in a car. I want to do everything right.

what kind of cranks did you have? What is the new crank set you're getting? Do you know the chainline of your current rear set-up? Was the chainline pretty good before?

Changing cranks isn't a big deal at all, in m experience. Square-taper BBs can be a pain, though, depending on the condition of the threads in the shell. You can always get it tapped, if need be, although you might want to find a new lbs if you have no confidence in the current one...

-rob

oh, a description of the current BB would be helpful, too. Model, spindle type/length, etc...

I have a 2010 Trek Soho S. On the Trek site it says that the cranks are Bontrager Satellite Nebula. The length was 170 mm. It says 26.8 in the section that says "Bottom bracket" on this site.

I ordered the Origin 8 track black crankset. This: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300473749716&ssPageName=ADME:L:OC:US:1123#ht_1443wt_930

The main thing I'm concerned with is the tension that should be applied on installing the crank this time. Apply as much force as necessary or go for a specific amount of torque? If I regularly tighten my cranks, should I not have this problem again?

The main thing I'm concerned with is the tension that should be applied on installing the crank this time. Apply as much force as necessary or go for a specific amount of torque? If I regularly tighten my cranks, should I not have this problem again?

Torque specs here: http://parktool.com/repair/readhowto.asp?id=88 . A square-taper crank bolt requires 25-35 foot-pounds of torque. Grease the threads on the bolt. Whether you grease the tapers on the spindle is up to you- there's opinions on both sides. Rechecking the torque after riding a week is a good thing, IMO, but after that they should not come loose.

if you tighten it all the way, krapes, you risk stripping the bolt or the threads in the spindle. As jeff suggests, go for about 35 ft-lb. And, yes, do recheck it after a few rides. I'd recheck periodically, just in case...

.. A square-taper crank bolt requires 25-35 foot-pounds of torque. Grease the threads on the bolt. Whether you grease the tapers on the spindle is up to you- there's opinions on both sides. Rechecking the torque after riding a week is a good thing, IMO, but after that they should not come loose.

+1 Proper torque is very important. 30 lb-ft cannot be attained with a multi-wrench or an L allen wrench. Rather, it's best to do with 3/8 drive allen bit socket and a 15" torque wrench. Assuming you install with the recommended torque and the arms are in good shape (square holes still square), they should not inadvertently loosen. And good to grease the threads. I grease the tapers, but as mentioned, it's one of those third rail topics on BF (along with "WD-40 as chain lube").