If Swiss bottom bracket shell threads are chased as though they are BCS threads, what is the result? Is the Swiss threading changed to BCS?

Thanks.

I'm not a machinist, but I see chasing as a way to clean threads that is not intended to cut new ones. Tapping is a way of cutting new threads. Some people use a tap to try to chase a thread. If the tap does not fit and you "chase" the thread with it, you will have cut new threads.

Thanks Road Fan. That helps.

A veteran bike mechanic friend retapped a Swiss Motobecane bb shell gor me. Took about a half hour, two passes with the campy tool, lots of cutting oil, some cleanup. Not a big deal, made it easy to fit a modern bb.

There is only a small difference between Swiss and English.

I've forced an English BB into well-greased Swiss threads, had to work the cups in with much back-and-forth action from a big wrench on a splined cup tool on both sides, but it finally bottomed in reassuringly.
I backed off 1/4 turn for every 1/2 turn advanced. Fairly physical work, this.
Saved me a big headache but took about ten minutes each side, working carefully to prevent the tool from slipping and damaging the splines in the cup.
It might be best to secure the wrench/tool using a bolt into (and/or thru) the bb spindle to keep anything from slipping, but in my case I just pushed in against the wrench and tool and applied precise motion as well as I could.

Depending how long is the threaded portion of the cups, will determine how much thread interference results from the slight pitch variation.

I was able to thread in the English bb cups a couple of turns before any interference resistance was felt.

To apply force to the adjustable cup means that the cup must not be turned with just a pin spanner, it would break the pins I'm sure, but once I worked in the splined cups they can now also be removed without tremendous force. I put in a Shimano cartridge bb so will likely never have to replace it.

I think you'll be OK. Half the time British BB's thread in with a little effort.

Something to keep in mind, regarding this "British works OK in Swiss-threaded" thing, is that British-thread cups will be slightly undersized (both 1.370 and 1.375 are under 35 mm), and the pitch is different of course: 24 TPI for British and 25.4 TPI (1 mm) for Swiss. That means in 8 threads, you'll be "off" by about half a thread. To me that doesn't seem like a good approach, but YMMV as always.

There are alloy Swiss cups available now for IRD (JIS taper, symmetrical spindle) cartridge bearing bottom bracket units. A much less expensive alternative to Phil Wood.

Yes. I think you're right. I installed a Shimano b.b. easily and all seems well. I was just concerned that I might be fooling myself and the b.b. would eventually work itself loose.

Useful information, Charles. Thank you.

A tap is the proper tool for chasing threads... but the tap has to fit.

This comes from a guy who has drilled, tapped and chased threads for a living outside of doing the same on bicycles for a very long time.

Rather than buggering the existing threads, why not consider a threadless cartridge instead?

FWIW, I have Swiss taps if you need to chase the threads.