helpless

I had my bottom bracket faced and chased by a shop. My BB is a standard square spindle cartridge BB. The guy only faced the drive side. I watched him do it, and he explained why you don't need to face the non-drive side, but his explanation didn't make sense to me. He said the non-drive side cup just held the spindle in place. But don't you want the cup to hold the spindle in place at a 90 degree angle to the other face?

AEO

the none drive side doesn't matter because it's just a plastic cup and that plastic cup is already perpendicular to the threads.

even the better cartridge BB use a holding cup that simply recesses into the BB instead of stopping at the face of the BB shell.

Mr. Fly

The BB shell preparation is really two activities: facing the shell so that the thin lip is perpendicular to the axis of the BB spindle, and also chasing the BB shell threads.

The chasing of the shell threads is done no matter what kind of BB you have, as this activity ensures that the BB cups can engage the shell correctly and easily. Hopefully, the threads on both sides of the BB shell is already aligned (share the same BB spindle axis) at this point; if they're not, there's not much you can do to fix that so you're FUBAR.

However, the facing of the shell is only needed if there is a shoulder on the BB cup that will nudge up against the BB shell and the facing will ensure that this is squared up. Most cartridge-type BBs from the square-taper era has a right (drive) side BB cup that have this shoulder so the right side BB shell needs to be faced. These BBs also typically have a left side BB cup that does not have a shoulder so facing the left side of the BB shell will net you nothing.
There are some types of BBs (e.g., Phil Wood) that does not require facing at all since these have the "shoulderless" right and left cups. Others, like the Shimano Dura-Ace BB-7700, require both sides to be faced as both cups have "shoulders". The most current BB cups (e.g., Shimano's Hollowtech 2 and Campagnolo's Ultra-Torque) require facing both sides of the BB shell because both cups nudge up against the BB shell.

helpless

Thanks for the explanation Mr. Fly. I installed the BB and I discovered what you explained. The left side(non-drive side) cup had no lip, so there was nothing that could contact the face of the BB, so facing the left side of the BB would have done nothing.

My left side cup was metal--not plastic. In fact, the directions said to "snug" the right side part(the big cartridge with both spindles sticking out of it) against the BB face, then tighten the left side cup to 34-41 N-m. I don't know how tight 'snug' is, so I screwed the right side in by hand until it contacted the BB face, then I tightened it slightly with a wrench.

What should I do about the unfortunate chips of paint around the edges of the BB shell? I imagine I need to get some paint on there so rust doesn't cause more paint to flake off. Facing and chasing sure seems like a primitive way to install a BB.

Mr. Fly

That should be enough. The right cup's lip is a displacement guide, in that it serves as a reference stop for the intended chainline. The tightening of the left cup to spec should ensure that the system is "bound" together correctly, as the engineer designed (hopefully there's engineering, LOL).


If you have a steel frame, a few dabs of touchup paint wouldn't hurt.

Facing and chasing the BB shell isn't really that bad. Getting the parts to specification -- ensuring "squareness" of mating surfaces, threads clean and dimensionally correct -- is pretty standard in machine work. More conscientious manufacturers perform these at the factory/shop but the suite of frame prep work can be done with the correct tools in a couple of beers' worth of time (assuming the frame isn't messed up to begin with).