I have been looking for a factory radiator cap for the Mustang, which was a Motorcraft RS-64 (metal, 16psi). I can find them, but they are in the $70-$100+ range. One was $140! For a radiator cap, that is NOT happening. They are rare for some reason. A RS-90 cap would work, but they are now plastic. No thank you. I found another cap, RS-62 (metal), but it is 13psi. I can find them all day long for under $20. What are the risks of using a 13psi cap vs 16psi? I read the boiling point changes, but i drive short distances (maybe 15miles at most, rarely), and dont hammer it (speed bug died years ago), so i can only assume i would be fine.

So, 13lb ok, or no? For those that have gone from 16 to 13 first hand. Actual experience.