NPT taps for left-hand threads
Left-hand NPT threads are rare, and for exactly that reason hard to source. They sit on fittings and manifolds where the direction of rotation in service would loosen a right-hand thread, often in gas and refrigeration equipment. VÖLKEL makes short machine taps for them in form C from HSS-G, from NPT 1/8 x 27 LH to NPT 1.1/2 x 11.5 LH.
What separates NPT from G
NPT is a tapered pipe thread to the American standard. The seal is made by the flanks wedging as the joint is tightened, usually helped by a sealant such as PTFE tape or hemp. The flank angle is 60°. The British G thread to BSP is parallel and has 55°. Diameters and thread counts differ slightly, so the two cannot be screwed together. The article Thread types explained covers this in depth, the direct comparison is in NPT vs. BSPT.
Cutting a taper
Because the thread is tapered, torque rises with every turn. The tap is not driven to a stop but to the prescribed engagement depth, which follows from the standard. The hole is countersunk to the taper beforehand, or drilled to the pilot diameter specified for NPT.
Form C and left-hand
Form C has a chamfer of two to three threads and right-hand spiral flutes that move the chips back out of the hole. In the left-hand version the tool turns anticlockwise. Mark workpiece and tool clearly, so nobody later starts with the wrong direction.
Right-hand NPT taps and the NPTF and NPS versions are under short machine taps, machine versions under machine taps, matching dies for external threads under round dies.
