I managed to get a handful of nipples from the supplier of the rad via Plumbcentre free of charge even though it was disc. Was just the head that sheared & not the thread so didn't require tapping. You'll be able to pick up a tapping kit from any hardware store. Find a supplier that has a slightly larger nipple than what you have now, phone them & get them posted free of charge then tap the rad to suit the the new nipples.
As for size, I'm not sure. Try tapping various sizes in a piece of metal till you find a good fit for the nipple then cut your rad.
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