If it is the type where the nipple goes straight into the rad ? Close valves before trying these suggestions. Try slotting it and use a flat blade screwdriver then renew nipple ( not impossible but needs luck and loads of patience) another way is drill it out, re-tap thread or ( bit of a bodge this but it does work) drill it out and thread tape a small brass screw and use this as a nipple, last one is really only if you are planning on changing the rad soon ( although brass screw could outlast the rad)
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