What size test-cap for an ABS 2" -> 1.5" slip joint adapter

[BlueGoldfish]

Hoping this is an easy question for the gurus here - but I've searched for a while and can't figure it out as my measurements don't seem to map to various thread dimension charts.

I have a standard 2" ABS vent pipe that has a 1-1/2 threaded trap adapter coming out of the wall. I need to cap this off temporarily to perform a fill-test. Is the threaded ring on the trap adapter the same thread as a 2" ABS pipe cap or a 1.5" cap - or is it some other weird size? The actual dimensions across the threads are 1 13/16". I guess I could insert a glue-capped bit of 1.5" pipe into the trap adapter as a fallback...

 

[John Gayewski]

It's regular npt

 

[Sponsor]

 

[Reach4]

Your screw on cap does not have to be ABS.