Verify NPTF thread with L1 and L2 Gage 2

Absolutely. The L1 checks the pitch diameter of the threads, which will change by 4x the plating thickness after plating. The L2 is probably not as critical, but the minor diameter will also change some due to plating buildup on the crest of the threads. The unplated thread form must be modified to account for the thickness of the plating. Although I'm not as familiar with pipe threads, on a standard screw thread the pitch diameter is generally the only dimension adjusted for plating thickness. The minor and major diameters do not change as significantly as the pitch.

Yes. In the past I have used the gages both before and after plating. When we used them before plating, we first developed a correlation between the change in L1 and L2 reading before plating and after plating. The amount of change per plating thickness should be the same for both L1 and L2, because both check the functional surfaces of the threads and should not touch the thread crests. (Different gages are needed to check the thread crest diameter) Then we machined the threads so the L1 and L2 gages would engage by an "extra" amount before plating, something like 1/2 turn or 1 turn (I haven't worked with this in a while, so I don't remember the exact correlation to plating thickness).

We always checked again after plating to be sure.