Originally Posted by
FBinNY
Most BB shells are tapped simultaneously from both sides, not all the way through from one.
Indeed, that's the only way to tap English and Swiss shells, which have opposite threads on each side.
That means that there's no assurance that the threads are phased as one continuous helix.
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.
Returning to the OP's problem, he should seek out a high-end shop that has been in business for several decades, as newly established shops will likely only want to invest in English thread taps. Older shops may have retained their Italian/French/Swiss/etc tools and thus be able to service the OP's frame.
Another option would be to make a thread chaser from an old Italian thread cup:
