REM, so I tried measuring the plug attachment and it didn't fit any of the threads I had on the measuring tool. Turns out spark plug threads are a bit off from regular bolt threads. I imagine this allows the spark plug to get a tighter seal the more you screw it in? All the attachments I needed came with the pressure gauge I got from Harbor Freight, and in the contents they just list it as  1/4”-18 (male) x 1/8” - 27 (female) and the part that screws into that is listed as 1/8”-27 (male) x 7/32” (barbed). Look at the picture on their website to get a better idea, i posted the link earlier.