[NigelB]

The M50 325i engine runs with a coolant pressure of 2 bar which I guess is 2 x atmospheric pressure or 30psi. On that basis I can buy radiator caps from a number of sources rated in bar.

If I want to buy a radiator cap rated in psi I can buy anything from 0 psi up to 20 psi, but certainly haven't found anything at 30 psi.

But what is a 0 psi radiator cap. Does it mean 0 psi above the normal 15psi atmospheric pressure (at sea level) ie a 1 bar cap and on that basis can I get a 15psi cap to give me the 2 bar that the M50 operates at (in production vehicles). If that's not what it means, why would you ever need a pressure cap rated below 15psi.

Am I missing something. Should I be sat in the corner with a dunces cap. (No answer required!!) Or is it just confusing.

Nigel