Neat pic, thanks!
I have a question - how tight should the threads of the radiator cap be in the filler neck? My car came with a repro, cast radiator cap that spun on smoothly. I found an original stamped cap with really good nickel plate on it, so I removed the repro and installed the original. The original cap's threads are a tight fit in the filler neck, even after running over the threads with my thread chaser and greasing the threads.
The threads shouldn't be so tight that the cap will freeze up in the neck. That can twist the neck loose, esp. a new radiator as the necks are lightly soldered, unlike originals.
Some old original accessory dog bone caps are made of pot metal, and the threads get swollen. I have some that can't be used any longer, just too big and tight now.
Use some lapping compound and get the treads free running is best. And be sure you have a good gasket in the neck, the new neoprene are nice, but right size O-Rings are good at holding a dog bone cap in the right place, stacking O-rings as needed. Steam and droplets of water will escape if a gasket isn't used, and spray will cover your windshield
Thanks, Dan, I'll try the lapping compound.
I sure wish we could see the rest of that fenderless brass speedster!
running without a gasket is a poor man's motometer, Dan.