Radiator Cap

[Screwdriver]

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