Do you mean a 1/8 NPT?
The threads you see after you remove the pressure port dust cap is a -4AN. There is some debate whether this is a real -4AN configuration or not.
If you mean what size threads is the pressure port to fuel rail, I don't know but it's definitelty NOT a NPT thread. It's a straight thread that seals to the rail with an aluminum washer.
You'll need a hose that has -4AN fittings (female) on both ends so you can hook up to the pressure port fitting (you'll need to remove the Scharader valve from the inside of the pressure port first). You'll need an adapter that will adapt the other end of the hose to your gauge; a -4AN male to 1/8 NPT (male/female depending on your gauge).
Autometer sells a gauge hook up kit with the hose and adapter fittings.
http://www.autometer.com/cat_accesso...st.aspx?pid=18
Note: AN fittings are not the same as the flared fittings you get at the hardware store.
