What kind of BB? does it have lockrings that run up against the shell? Something like a Phil would not really need facing.
The facing operation doesn't really effect the threads it just ensures the sides are parallel. Chasing the shell threads involves specific thread cuttters.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk