Peter & Dave, Based on Dave’s measurements, the nominal cap size is 2 inches; the next sizes are out of range at 2-1/4 and 1-15/16 inches. Peter, can you please measure (micrometer preferred) the outside diameter; top-to-top of the threads, in 4 locations around your new “dogbone” and check thread pitch with a thread gauge? Dave, could you also measure the threads pitch on your cap? Within the standardized UN thread series, the 2 inch size has several threads counts, each with different inside diameters and fits, which determines how tight the threads will be.
If both Peter’s new cap and Dave’s existing radiator cap have the same external thread diameter and pitch, then it may be possible that Peter’s new cap can be used on his car without trouble. Based on Dave’s neck inside measurement of 1.97 inches, I expect either 6 or 8 threads per inch, but please check and let me know (by email?). End of lecture.