I am going to buy a 86-88 cap assembly w/ overflow tube to eliminate AST.

http://www.mazdatrix.com/b8.htm

Which cap does it come with (pressure or flat) and which one do I need?

 

Should be pressure, call ray tell him yiyr want the fd ast delete he will send you exactly what you need.

 

it comes with a pressure cap, as that's how its setup on the early FC's

 

yeah, the mazdra trix filler neck comes with the cap
you can also get a cap for any FD from your local autoparts store, they should all come with a 16lbs spring cap stock

 

Cool, thanks guys I just wasn't sure because the one I have is w/o overflow tube has flat cap.

 

You have a 2-piece coolant fill neck right? ie has a pair of 10mm head M6 bolts.

You're deleting the AST (which has a pressurized cap), this is why the S4 piece comes with it's own pressurized cap. With a non-pressure cap your boiling point would be too low.

 

Yes, it is 2 piece with 2 x 10mm bolts and has non-pressure cap.

Glad the S4 comes with own pressure cap. Thanks for the info.