L2 Gauge: Used for external threads only. It checks the effective thread length beyond the L1 point, ensuring proper wrench-tight engagement.
Follow with the L2 Ring Gauge:
Leave the L1 ring in place as a reference.
Screw the L2 ring gauge onto the pipe (wrench-tight if specified for the class).
Compare positions: The L2 should stop at a position relative to the L1 (typically within ±0.5 turns for acceptance). This checks the taper and effective length.
For NPTF (dryseal) threads, tolerances are tighter, and you may need to reference specific class standards (e.g., Class 1 or 2).
If outside this range, the thread fails.