Hmm, looks like that may be a 3/8" cap. I found one on the web that says it's a 3/8" cap, but 5/8 OD and 9/16 ID. I guess 3/8" is the ID of the line or something like that.
I assume the correct cap would be a compression (concave taper/bevelled) cap (therefore not NPT or obviously not flare).
or should it be like this?
__________________
Steve
'08 Boxster RS60 Spyder #0099/1960
- never named a car before, but this is Charlotte.
'88 targa
SOLD 2004 - gone but not forgotten