Thread Dimension Reference

BSP (British Standard Pipe) threads use a 55-degree thread angle and come in parallel (BSPP/G) and taper (BSPT/R) forms. NPT (National Pipe Thread) uses a 60-degree thread angle and is always tapered at 1:16. BSP is the international standard (ISO 228/ISO 7-1), while NPT follows ANSI/ASME B1.20.1 and is predominantly used in North America. They are not interchangeable even when nominal sizes appear similar.

The tap drill size equals the major diameter minus the pitch (for metric threads) or the major diameter minus 1/TPI in inches (for imperial threads). For example, an M10x1.5 thread has a tap drill of approximately 8.5 mm (10 - 1.5). The reference table above provides pre-calculated tap drill sizes for all listed thread standards.

UNC stands for Unified National Coarse and UNF stands for Unified National Fine. UNC threads have fewer threads per inch and are the default for general fastening. UNF threads have more threads per inch and offer higher tensile strength, better vibration resistance, and finer adjustment, making them common in aerospace and automotive applications. Both follow ANSI/ASME B1.1.

No. While they share the same major diameter (e.g., M10), their pitches differ, resulting in different minor diameters, pitch diameters, and tap drill sizes. Attempting to mate a fine-pitch bolt with a coarse-pitch nut will cross-thread and damage both components. Always verify the full designation including pitch before selection.

Use BSP (G or R series) in most countries outside North America, and NPT in the United States and Canada. BSP parallel (G) threads require an O-ring or washer for sealing, while BSP taper (R) and NPT threads seal through thread deformation with PTFE tape or pipe dope. Always match the standard used by your existing fittings and equipment.