i have a 4 lb cap on my 270 thats in the ashton and 235 thats in the 6100. the rad on the 40 is atmospheric so it doesnt matter what cap you put on it it aint gonna blow nor raise the boiling point. a five psi cap will allow it to reach 224 degrees. since i dont have my steam tables in front of me i cant tell you what 4 psi is. a seven pound cap will put you to 235 degrees.
