Question about facing a BB

[msl819]

Pardon my ignorace, but i hear people talking about properly facing a bottom bracket before installation. Can someone explain what this is and how it is done properly? What are the effect of not doing this correctly?

[DfCas]

If it is a threaded bb shell, most facers thread into the shell, and use this as a reference. The facer then removes a small amount of material on the outer surface of the BB shell to make sure the "faces" are plano-parallel.

Benefits are reduced bearing wear and easier installation. External BB's are very dependent on accurate facing for best life and lowest turning resistance.

Hope this helps.

dan

[Dave]

Most modern frames don't need facing, if they are well made from the factory. I've never faced one in the the last 10 years.

When in doubt, I first remove any paint from the face, screw the BB cup in until it contacts a .010 inch feeler gage and then use .008-.012 inch feelers to search for high or low spots. I's like to see the deviation run half that amount, but up to .004 is probably tolerable.

If you're using a Campy UT crank, the shell width is critical. It must be 67.2-68.8mm in width. If facing take it below the low limit, you need a shim on the left side cup.