I have discovered that my radiator cap doesn't reach far enough down to seal the lower ridge of the filler neck!!! It's a new cap (the spring is fine), but too short a reach!! Once the engine is hot, coolant easily flows into the catch can. Okay, it's a '73 F-100 with 3 speed manual and no A/C. Apparently at some point the radiator was swapped out for a bigger one. It is a Ford radiator (has a Ford part number tag) and fits the old bracket fine (no mods were needed to mount it), and is for an automatic (has the auto tranny cooling connections on the bottom). I am assuming that this radiator has a deeper fill neck, and uses a different cap. Anyone have any idea what cap to use?? Thanks, Polkat