Push the pads apart to retract the piston, then use mole grips on the pin to twist it.
Maybe file two flat side on the pin to help.
I filed the top and bottom of the pin and tried with a variety of tools, but not getting enough grip so the tool just revolves around. Not sure if I should file it up more, don’t want to weaken the pin too much.
Carefully drill it out taking care not to damage the threads in the caliper. When the head is mostly gone the pin should come free. The threads can then be cleaned out carefully.
Seems like a last resort kind of thing, I’m unsure if I’ll be able to leave the threads intact. But I am running out of options now.
Or buy a second hand caliper…
Might have to go that way if nothing else works.
Brake pad pins should only be tightened to 15 to 20 Nm, clearly these are not who fitted them?
If I was a gambling man I’d wager some monkey boy fitted them using a high strength threadlock such as red Loctite 272 or similar. If that’s the case to disassembly apply localised heat to the threads and heat to around 250 °C, disassemble while hot. Localised heat can be applied by holding an Allen key in the pin head with a pair of grips and heating the Allen key shank with a blow torch. When the key shank starts to glow red that’s about 500 °C, not all that heat will transfer to the pin. Hopefully the heat will also cause the Allen key to expand enough to get some purchase on the rounded fastening head.
I was the monkey that tightened it (probably). I have a torque wrench, though I didn’t use for this specific pin since I never used much force to tighten it (it has a little cap that goes on top of it and holds it in place anyway, so seemed redundant). I definitely didn’t use any loctite. I do use copper grease on most bolts, I wonder if I forgot to use it in this instance. Although… I did change my rear tyre not too long ago, so a garage monkey might have been the last one to tighten it…
I tried to blowtorch an allen key which has some purchase inside the pin, but it doesn’t expand enough to grip it.