Join Date: Apr 2010
Location: Sunnyvale, CA USA
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Originally Posted by dain
if i have the square BB can i change to the external BB or would it matter. By the way i will probably have a real bike mechanic do it even though i am a machinist and have a very good mechanical backround.
I wouldn't have a "real" mechanic do it or expect him to get it right even though I'm a professional computer guy who traditionally wouldn't be trusted with a screw driver.
There's _very_ little you need to know.
1. Bottom bracket threads vary, although the vast majority are British / ISO at 1.370 or 1.375" x 24 threads per inch. You need to get the correct part. You also need to note the non-drive side cup thread direction which is left-handled (counter-clock-wise to tighten).
2. External bearing bottom brackets depend on the shell end faces being square. Quality frames seem to come machined that way. If it's shiny metal and not paint it was faced. In the (unlikely?) event you still have paint there you might take it to a shop to get it faced because in a lifetime you're unlikely to save $300 in labor charges to offset the tool cost to do it yourself. Width may be important for bearing life. Use your calipers to measure and validate it's in spec. If you lack a pair ~$20 will get an idiot proof digital set from Harbor Freight.
3. Use anti-seize so you can remove the bottom bracket after it's weathered for the decade you might get out of it.
4. Follow whatever directions come with the bottom bracket and/or crankset regarding wave washers, crush washers, O-ring seals, and whatever else is required.
5. Some click-type torque wrenches don't click when turned anti-clockwise. Tighten the non-drive side crank about like the right side if this is the case for you and you're using a normal bottom bracket.
6. Use a torque wrench to install the crank bolts). Let me re-iterate: Use a torque wrench to install the crank bolt(s). Torque to the high end of the spec. Modern non-square bottom brackets often have the splines bottom against a shoulder and they may not get there at the low end. This is less critical now that we don't have splines on the drive side which carries the ring spider with non-square mounting affecting front derailleur operation although you don't want issues where things get loose and you destroy the left crank arm.
I wouldn't expect a "real" mechanic to get this right. Too many are either ignorant (bad) or just don't care (worse). As a person who can read and follow directions you'll do as well when they're doing their job and much better when they're not.
Last edited by Drew Eckhardt; 07-12-12 at 09:49 AM.