Technical Foundation
What Is a Screw Pitch Gauge?
A screw pitch gauge (also called a thread pitch gauge) is a folding set of thin steel blades, each ground with a precise thread profile at a specific pitch or TPI (threads per inch). To identify an unknown fastener, blades are held against the thread crest until one seats flush with no light showing through the root — that blade's marked value is the thread's pitch. Gauges are made to a 60-degree included angle for ISO metric and UN/UNC/UNF threads, or 55-degree for BSW/BSP (Whitworth-form) threads, since the two families use different thread angles and a 60-degree blade will never seat correctly against a 55-degree thread even at a matching pitch.
Blade sets are labeled either in millimeters of pitch (metric) or threads per inch (imperial), and a full set typically spans the common commercial range — roughly 0.4mm to 7mm pitch, or 4 to 42 TPI. Combination gauges fold both metric and imperial blade sets into a single body, useful when the origin of an unmarked fastener is unknown. The gauge only confirms pitch and thread angle; major diameter still needs to be measured separately with calipers to fully specify the thread (e.g., M8x1.25 or 3/8-16 UNC).
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Thread Angle | 60° (ISO metric, UN/UNC/UNF) vs. 55° (BSW/BSP) | A blade cut to the wrong angle will not seat flush even at the correct pitch, giving a false reading |
| Pitch Range | Metric ~0.4-7mm; imperial ~4-42 TPI | Determines which fastener sizes the set can identify without gaps |
| Blade Count | Typically 17 to 60 blades per set | More blades means finer pitch coverage and fewer ambiguous partial matches |
| Blade Material | Hardened stainless or chrome-vanadium steel | Resists wear from repeated seating against fastener threads, keeping profiles accurate over time |
| Marking System | Pitch in mm or TPI stamped on each blade | Read directly off the blade once it seats, without needing a reference chart |
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Applications
Where Screw Pitch Gauges Get Used
Pitch gauges are reached for anywhere a fastener's thread standard is unknown or unmarked and must be confirmed before ordering or repair.
Machine Shops & Fabrication
Quickly identifying the thread standard and pitch of an unmarked bolt or tapped hole before sourcing a replacement fastener.
Automotive & Fleet Maintenance
Matching pitch on stripped or worn threads before selecting a repair insert or replacement bolt.
MRO & Facilities Maintenance
Field identification of legacy or unlabelled fastener threads on aging equipment during repair.
Plumbing & Pipe Fitting
Distinguishing BSP from NPT-adjacent threads on fittings where thread form is not visually obvious.
Quality Control & Inspection
Verifying incoming fastener batches match the specified thread standard before acceptance.
Tool Cribs & Inventory Management
Sorting and re-labelling mixed bins of unmarked fasteners by thread pitch and standard.
FAQ
Screw Pitch Gauges Questions, Answered
How do I use a screw pitch gauge?
Fold out a blade and press its toothed edge against the thread crest of the fastener, rocking it gently until one blade seats flush with the thread profile and no light is visible through the root. The pitch or TPI value stamped on that blade is the thread's pitch. Confirm the thread angle family (60° metric/UN vs. 55° BSW/BSP) is correct first, since a blade from the wrong family can appear to seat closely but not fully.
What's the difference between a 60-degree and 55-degree pitch gauge?
The angle refers to the included angle of the V-shaped thread profile ground into each blade. ISO metric threads and UN/UNC/UNF (American) threads use a 60-degree profile, while British Standard Whitworth (BSW) and British Standard Pipe (BSP) threads use a 55-degree profile. Using the wrong angle set will not seat correctly against the thread crest and can produce a false or ambiguous pitch reading.
Can a pitch gauge tell me the full thread specification of a bolt?
No — a pitch gauge only confirms the thread's pitch (or TPI) and, by which blade family fits, its general angle standard. To fully specify a thread (for example M8x1.25 or 3/8-16 UNC), you also need the major diameter, measured separately with calipers or a micrometer, plus knowledge of whether it's an internal or external thread.
Why does my bolt seem to almost fit two different blades?
Adjacent blades in a set are often close in pitch, and a worn or damaged thread can seat imperfectly against several blades. Check for light showing through the root — the correct blade should show no gap along the full engaged length. If two blades from different standards (metric vs. imperial) both seem close, measure the major diameter to disambiguate, since coincidentally similar pitches exist across the two systems.
What pitch range do I need for general shop use?
A general-purpose combination gauge covering roughly 0.4mm to 7mm pitch (metric) and 4 to 42 TPI (imperial) handles the vast majority of common fasteners from small instrument screws up to structural bolts. Specialized work — very fine-pitch instrument screws or coarse pipe threads — may call for a dedicated set outside that range.
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