Old brass TRV leaking through the cartridge. Cartridge ain't shifting no matter how much penetrating oil and leverage I apply it's just stripping the "nut".

So removed the TRV - except it isn't quite BSP. Literally a mm out - cannot get a new TRV to connect to the extender. It looks identical but it's not - nothing I have matches (tried matching the actual TRV to every male thread I have and it's close but no cigar).

Old thread size?

Failing that - how do you dissolve old plumbers paste. The extender clearly has some old gunk in the threads on the rad connection- and the walls going to give before that moves. Brute force and leverage ain't moving it - so stopped before I strip that nut