Correct filler neck caps?
 

I have found that my FD is fitted with what looks like a 0.9bar pressure cap on the filler neck, and the AST has a 19psi "racing" cap from the previous owner. The car has been this way for awhile and seems to run pretty cool based on the cooling fan cycling pattern.

But, I have been reading that the filler neck cap should not have a pressure function, and just have a flat seal. Is this correct? The next question is what is a good cap for the aftermarket AST (it has a larger, normally-sized cap)? The current cap has a pressure-relief lever but I'd like to get away from that if possible.

AST should have the 0.9 Cap.
Filler neck should be flat but you can put a Greddy/HKS/whatever for looks if you wish. If you delete the AST you need to have a 0.9 Cap on the filler neck.

Thanks. It looks like I have pressure caps on both sides now for some reason.

19psi is a lot, maybe switch it back to a lower pressure cap on the AST. Doesn't matter what cap is on the filler neck as long as it seal, it can be a pressure cap or flat cap.

You can get the "flat cap" new from Mazda if you'd like one. As stated, the pressure cap is just fine there, doesn't hurt anything.

Dale

Thanks for all the info. I did order a flat cap from Malloy, and I am changing the AST to a 13psi cap.