The Vermont Gage crest check plug six-step gage is a precision inspection tool designed for verifying the crest truncation (or thread height at the minor diameter) of internal tapered pipe threads, particularly in NPTF (National Pipe Taper Fuel/Dryseal) and ANPT (Aeronautical National Pipe Taper) standards.
Unlike threaded gauges (e.g., L1 or L3), this is a plain (non-threaded) cylindrical plug with six distinct stepped diameters, which allow for checking tolerance zones without relying on thread engagement.Purpose and ImportanceWhat It Checks: The gage inspects the truncation limits of the thread crests (for internal threads, this relates to the minor diameter). Proper truncation ensures the threads achieve the necessary metal-to-metal interference for leak-proof sealing without additional sealants, which is critical in high-pressure or fuel systems where NPTF/ANPT threads are used.
Why Six Steps?: The six steps correspond to overlapping tolerance zones for three pitch diameter conditions (basic, minimum, and maximum). This provides a go/no-go assessment across the full range of acceptable truncation: typically labeled as BTN (Basic Turn Notch), B (Basic), BTP (Basic Turn Plus) for one set, and MN (Minimum), M (Basic Max/Min), MX (Maximum) for the other, though exact labels can vary by manufacturer.
Prepare the Part: Clean the internal threads of the fitting or component to remove debris. Ensure the gage is also clean and calibrated.
Reference L1 Position: First, use an L1 threaded plug gauge to determine the pitch diameter at the hand-tight plane. Note where the fitting's face aligns with the L1's notches (e.g., minimum, basic, or maximum position, often ±1 turn from basic).
Insert the Six-Step Plug: Align and insert the crest check plug into the internal thread opening by hand (no wrenching). It should enter smoothly until it stops at the thread crests.
Evaluate Based on L1 Reference:Cross-reference the stopping point with the L1's position. For example:If the L1 was at the maximum notch, the six-step gage should engage between the "MX" (Maximum) and "MXT" (Maximum Truncation) steps.
If at basic, it should fall between "B" and "BT" steps.
If at minimum, between "MN" and "MNT".
The part passes if it enters the appropriate "go" step but stops before the "no-go" step for that zone. If it doesn't align (e.g., too loose or tight), the truncation is out of spec.
Complete Inspection: For full NPTF compliance, pair this with a root check (if needed) and L3 gauge. Vermont Gage often sells these as sets with L1/L3 plugs.