One of the challenges (perhaps the biggest) is getting whatever type of thread chaser to engage the initial few threads as the original were done. Of the more then a few thread repairs I've done it's the couple/four threads at the shell's face that are the ones needing the corrections, don't remember when on a used bike I had to clean up the threads beyond these first few. (Well, this excludes the few that needed more threads inside the shell to allow deeper cups to fully thread in, but that's not a repair job. It's modifying to fit other then OEM parts, a different job motivation IMO).

Why is this, the first few threads, an important detail? Because if these first few threads are not coaxial to the shell any "corrections" will only better allow the cup to cross thread and then the deeper ones will be damaged. Especially if the deeper threads are also crossed (and this is from an assembly gone bad) when those first few are properly cleaned up and the cup is run in 5-6mm it will pretty much every time act as a chaser for those deeper threads. Cross threaded cups are far less common then wallowed out threads from either seriously loose/backed out cups (and the bike is continued to be ridden) or from a buggered up insertion.

Some of us (and I overlap with this tiny number only slightly) can retain the coaxial orientation while also applying enough force to move/cut metal. This is far easier with the slotted across it's threads cup then with a single faced tool like a tap. Sure the tap will remove the crud but to actually move metal that's a lot harder to do, most here have never had the chance to found this out. A slotted cup offers a reference surface, the cup's outer face, to try to keep parallel to the shell's face. A cup lock ring threaded onto the cup (pretty much RH threaded rings exist, but also pretty much most thread damage is found on the shell's LH side) will bring this reference face closer the that of the shell, making the visual easier to track.

A very big reason why a proper tap set made for BB shells is so nice to use is that they will be mounted on a pilot shaft. Thus both taps are kept pretty much coaxial at the start of any chasing. Thus the first few threads are cleaned up/chased coaxially. I consider this to be important.

So for mere cleaning the previously mentioned methods are fine and far less expensive then a proper bike intended and piloted tap set is. But for correcting damaged threads I'll want a true BB tool (pun intended true as in proper taps held true to each other).

I advise many budding frame builders in other forums and one question that repeats (sometimes it seems too often repeated) is that of tool needs. I suggest that only when one wants to control all aspects of their product will a BB tap set be a good value. Otherwise I suggest to partner with a shop/other builder who has them already. But in the building of a frame with a lugged/socketed shell the filler often flows through the sockets and into the threads so actual metal removal is the need. Andy