Alu frame back from powder coaters with some overspray into bb threads.
Will try Nitromors and gentle brushing, but if this does not work, then should it be a quick, easy and cheap job to do?
Or could I do it myself with a tap or is it a die?!?
It's a tap. Best to get your LBS to do it, although I have used an old steel cupped bottom bracket (shimano square taper) to do it once.
as above i used an old stainless steel bb to do one the other week and it was fine
Cut some slots in a steel BB cup, and your good to go. Nice and steady does it.
Really nickc? I'd have thought without a taper and a way of aligning the axes (plural of axis) there's be a real risk of cross threading.
Note - I am into my bodging!
1/4 section of an old steel cup & gentle working should do it.
I bought a Rock Lobster from Merlin once & had to send it back due to overspray on the b/b threads. It was bad though, totally un-masked during powdering.

