I bought a new frame (online) intending to build up the bike. It will be used for recreational rides on gravel/dirt paths mostly.
The frame has an English-threaded bottom bracket shell. I happened to have a new I.R.D. BB manufactured by Tange Seiki in Japan, 68x127, which I'd bought for another bike and never used. The BB shell's threads were clean, I applied a thin layer of Park anti-seize to them with a toothbrush, and then gently threaded the BB in by hand, first the drive side and then the non-drive side. Everything was smooth as butter, no cross-threading, so I torqued them to 40Nm.
On the non-drive side, between 1 and 4 o'clock, the outermost threads of the cup are proud of the BB shell face by 0.56mm (at 2 o'clock) whereas on the 7-10 o'clock side of the shell, the cup sits pretty much flush with the face.
There's bare metal on the face of the BB shell, so I assume the BB shell has already been faced.
Is 0.56mm sufficient imperfection to try to rectify?
Is it possible the axis of the threading is itself askew?
I don't have the bike built yet, so I can't say if this is affecting the pedaling.