L3 Gauge:
Used for internal threads only. Similar to L2 but for the internal side, checking the thread beyond the L1 engagement.
General Rule: Threads are acceptable if the L1 is within tolerance (typically ±1 turn from flush), and the L2/L3 aligns closely with the L1 position (often within ±0.5 turns for tighter classes like NPTF).
Follow with the L3 Plug Gauge:
With the L1 position as reference, insert the L3 plug gauge.
It should engage to a depth that matches the L1 within ±0.5 turns (or per spec). This verifies the taper and thread beyond the initial engagement.
Additional TipsGauge Types: These come in "go/no-go" styles or with steps (e.g., 4-step or 6-step plain plugs for minor/major diameter checks, often used alongside L1/L2/L3 for full inspection).
Standards: Follow ANSI B1.20.1 for NPT or B1.20.5 for NPTF.
Common Errors: Over-tightening can damage threads or give false readings. If threads are coated (e.g., plated), adjust tolerances accordingly.
If this is for a specific application or class (e.g., NPT vs. NPTF), more details could help refine the instructions.