Radiator Cap: Pressure or No Pressure?

BigSky
BigSky Senior Contributor
I’m assuming the original radiator cap on my 1937 Hudson was a no pressure cap but I cannot find anything listed anywhere on it.  

One of my 37’s has a 4lb cap & the other a 7lb cap.  I’ve read that the 7lb cap could create problems (ie: blow the radiator) if the car had a no pressure cap originally.  Does anyone have a recommendation on pressure & also have a good part number for one?