Jay, this has come up before; below is one example thread. I replaced my plug, but it was when the replacements were available. I have, sitting in front of me, the adapter bushing whose ID is 1 Inch NPT, to accept a common block heater element, and whose OD is close to 1 1/4 inch BSPP or NPS (there are only four threads on it, so the pitch difference between 11 and 11.5 is negligible). The actual OD of the threads is 1.744, which doesn’t seem to match anything exactly AFAIK.