Originally Posted by
JohnDThompson
That's what piloted taps are for, and the pro-quality taps from Campagnolo, VAR, Park and so on are piloted. At Trek we had a pneumatically operated machine machine to chase the threads on the BB shells after brazing; the taps were piloted so it can be done by machine.
There is a difference between threads which are on a common axis and threads with a common lead.
To understand the difference assemble 2 nuts onto a bolt until they touch. Then back one off a full turn making a gap equal to the screws pitch, line up the hexes and put the assembly in a vice but not too tight. Carefully back the bolt out of one and tap it so as to push the other nut half the distance to the first. If you worked carefully the nuts will still be perfectly lined up with a gap equal to 1/2 the pitch.
Now tighten the bolt again, and you'll see that it has to push the 2nd nut back to the original separation in order for the screws helical thread to line up with that of the nut. It isn't a problem of the shaft being aligned, but the helix being in phase.
Now imagine if after changing the distance between the nuts, we tightened the vise, and ran a tap down using the first as a guide. The crest of the tap would enter the second nut 180° out of phase and methodically destroy all the thread.
Experienced machinist retaping a hole whose first threads are buggered start by counterboring out the damaged area so the tap is free to float and pick up the lead of the remaining deeper thread.
If you still don't see the issue, I suggest you drill a hole on a block of scrap, tap 1/2 way through from one side, then flip the block and tap half way through fro the other. Now try to run a screw down the hole and the problem will be obvious.
BTW- Italian, and French bottom brackets are like that block you tapped from both sides, because that's how they were made.