Depending on the thread it can get burn by overheating so things like 40 and 32 tpi its usually best to start the taper tap into the hole when the bush is still in the lathe and then complete after soldering. On a bigger model with BSP threads not such an issue.

You can use fully formed bronze bushes without issue – I use hundreds and have yet to have one fail. If you were to use brass then there is a high chance that the whole bush will melt and the threads dont like the heat regardless so dont go there.

I know that at least one of the professional boiler makers didn’t drill right through the bush and therefore can only put one to two turns of thread into the bush. When the boiler had been hydraulically tested the bush would be drilled through with a pistol drill and the tapping done then. Saves messing around with making blanking plugs, so I was told.

just to add….if im doing a decent sized loco boiler the bushes are 90% drilled, soldered in and drilled and tapped cold. The amount of heat that goes into a 5" gauge boiler is unreal and controlling it is difficult. On the small stuff which I do most (3.5inch dia to 6inch dia) I have cnc bronze bushes made. Sometimes silver solder will creep up and over the face and into the threads but chasing them out is easy enough.