Gave it to powdercoaters with an old BB in (which had been previously powdercoated in a another frame), however they clearly had better ideas and removed it. Hence the start of the thread is thick and dusted with plastic further in. I have tried to cut with old BB – no joy.

LBS and thread chaser? – £25 ish?

This happened to me, Take it to your LBS, With cake in hand for haggling.

I tried a few things then took it to my LBS, The total cost Powdercoating and LBS was just short of sending the frame to Bob Jacksons, Where they prep and do a better job of finishing. Lesson learnt for me.

Get your old BB, grind the threads off on a few points – at a right angle to the threads.

ie, if these are the threads:

//////////////

Then grind a groove in from left to right.

Grease it up and use it as a tap. It won't be as good as a proper tap, but it'll be close enough, and crucially it will have a) some room for detritus and b) sharp edges.

Nitromors and a wire brush?

DEpends how bad it is. A wire brush would shift it without damaging the threads but it would be a slow job. Nitromers will also work but the risk is accidentally stripping another part of the frame. Personally I'd be off to the LBS to see what I could do if it is anything other than a hack bike.

yep, nitromors and wire brush, just put the nitromors on carefully. take less that 10 mins!

All done, stanley blade around and around the thread – especially the start and wire brush. Had no nitromors to hand. Many thanks for the "this can be sorted cheaply vibe"

the powdercoaters I used left the shot in the frame before the baked it after it had been coated. it was welded to the BB threads and it managed to destroy the bike shops BB tool 😳

Get an old BB cup, with a hacksaw make some vertical cuts in it, about 3 is sufficient, and you have your own tap. half turn in , quarter turn out, at a time…