NPT uses a 60° included angle with flat-truncated roots and crests. BSPT uses a 55° Whitworth angle with rounded roots and crests. Both use a 1:16 taper ratio, but their pitches (TPI) differ for many sizes (e.g. 1/2" NPT has 14 TPI, whereas 1/2" BSPT has 14 TPI, but 1" NPT has 11.5 TPI while 1" BSPT has 11 TPI).

NPT threads are flat truncated at roots and crests. As a result, when the flanks engage under torque, small helical gaps remain between the roots and crests. Without sealants, pressure will leak along this path. NPTF (Dryseal) threads are designed with tighter truncation parameters so that the crests physically crush into the roots of the mating part before flank contact, creating a metal-to-metal seal.

The L1 gauge checks the hand-tight engagement depth. It verifies that the pitch diameter is within tolerance at the gauge plane. For a plug gauge screwed into a female thread, the notch on the gauge must align flush with the pipe end face within \(\pm 1\) turn.

Tapping torque increases exponentially with thread height percentage. A 75% thread is standard for mild steel. However, reducing thread height to 60% in stainless steel or titanium reduces tapping torque by roughly 40% while losing less than 5% of ultimate pull-out strength. Tapping at 100% thread height is bad practice because it increases tool breakage risks without structural benefits.

BSPP is a parallel straight thread (ISO 228). Since there is no taper, the pitch diameter is uniform along the length. Flank compression is not generated as the fitting is tightened. The thread serves purely to carry mechanical force. Sealing must be provided externally via O-rings, copper washers, or bonded gaskets compressed against a flat seat face.

Thread galling (cold welding) occurs when stainless steel, titanium, or aluminum threads are tightened under high pressure. The oxide film is sheared off, and friction fuses the contacting metal surfaces together, seizing the assembly. It is prevented by: (1) using nickel-based anti-seize lubrication, (2) mating dissimilar alloys (e.g. 316 male with Nitronic 60 female), or (3) slowing assembly rotation speeds to prevent heat buildup.

The pitch diameter is the theoretical diameter where thread tooth width and groove width are equal. For tapered threads, because the diameter changes continuously along the length, standards define the pitch diameter at specific planes: E0 is the pitch diameter at the small end of the pipe, and E1 is the pitch diameter at the gauge plane, located at length L1 from the end of the pipe.

In tapered connections, longer thread engagement increases hoop stresses in the female fitting exponentially as it is tightened further. If the engaged length exceeds the effective thread limit (L2), the thicker section of the male taper will enter the thin female boss, leading to stress cracks, yielding, or thread stripping.

When you drill a straight hole for a tapered tap, the tap must cut a large amount of metal at the top of the hole to form the taper. This uneven cutting force causes chatter, poor thread quality, and tap breakage. Taper reaming creates a 1:16 pre-tapered hole, so each tooth of the tap cuts an equal amount of material, ensuring premium thread finish and long tool life.

It is necessary in high-pressure installations (e.g. hydraulic manifolds, chemical injection lines operating at \(P > 3000\text{ psi}\)). The internal fluid pressure acts on the inner cross-sectional area of the pipe, creating an axial force pushing the pipe out. The engaged thread shear area must safely balance this force. If the material allowable shear stress is exceeded, the threads will strip, leading to a sudden, hazardous blowout.