13CU in. Air Venturi cyl. w/pro V2 regulator (ninja) fill port (foster fitting?)

[PaperPunch1]

So when I fill my 13CU in. cylinder, the fill fitting (check valve) doesn't hold, so I removed it and the O-RING is bad. I need a new O-RING but don't want to spend 21.95 for a whole kit that may include one. I need to know if anyone definitely knows the thread size on the fill port. I tried a different one from a
NAME BRAND NINJA regulator but the size is different and the stem is like a screw head without a threaded screw a (smooth).
The one I need is made by Air Venturi and says NINJA on the regulator part, and it has the 1100 psi output pressure.
Could it be 8mm thread? I know it is NOT 1/8 NPT
I suppose it could be that 1/8 BSSP thread but not sure what that looks like.
Thanks for any info you have folks!!
PaperPunch1

[nervoustrigger]

I have a couple of the Ninja Pro v2 regs and they are fitted with a 1/8” NPT Foster fitting.  Definitely tapered threading that seals by way of PTFE tape so if yours are different, that is mysterious to me.
 
If somehow it were an 8mm, that’s a parallel thread and it would have a sealing washer or O-ring under it.  Does it?
 
Anyway it should be easy enough to simply replace the O-ring on the check valve--the piece that’s like a screw without threads.  Going from memory, I’m pretty sure it’s a -006.  Should be a slight interference fit…for how this seal works, sizing is actually not particularly critical.  Use a 90 durometer (Buna-N or polyurethane) if you can obtain one easily, otherwise a more common 70 durometer is fine.