Radiator Cap
 

I have a radiator unknown year, I have a shell that is smooth on top and painted at the bottom. Ok problem the radiator neck comes up flush with the top of the shell. I bought a 30/31 cap and there isn’t enough room between the shell and the neck for the cap to fit. Where am I going wrong. Wrong shell for the radiator or wrong cap. I had another radiator that had a taller neck but sold it

Re: Radiator Cap
 

Likely a reproduction shell. I would want to look at several other things such as crank hole alignment, hood alignment, the side straps on the radiator, etc. You didn't say who manufactured the cap either. Someone knowledgeable who can visually inspect items like this greatly reduces the diagnostic time, as certain things can be spotted without measuring or disassembling anything.