HTHOMETALLYTHE PRACTICAL HOME GUIDE
Original schematic of a folding toothed pitch gauge and an enlarged repeating thread profile

Assembly & measurement / TOOL GUIDE

Thread pitch gauge: record pitch without guessing the fitting

Compare a loose part with a marked leaf, distinguish millimetres from threads per inch, and keep pitch separate from diameter and connection compatibility.

A toothed leaf compares the repeat spacing of a thread. Use it to document an already loose screw or spare fitting before asking for an identified replacement. Unlike a caliper it does not measure diameter; unlike a feeler gauge it does not measure a gap. A matching leaf is a pitch observation, not permission to assemble unknown parts.

A REPEATABLE METHOD

How to use it

  1. Work only with a safely accessible, already loose part on a stable bench. Do not disconnect a gas, water, heating or pressure line to obtain a sample. Photograph existing labels and keep the part with its origin record.
  2. Read the gauge family and leaf units before unfolding a candidate. Metric leaves give pitch in mm; inch leaves give TPI. Manufacturer sets distinguish 60° metric/Unified and 55° Whitworth profiles. A number without its family and units is an incomplete record.
  3. Remove only loose dirt with a suitable soft brush, without cutting, filing or changing the thread. Gently place the leaf along the thread axis and compare several consecutive teeth under good light. Do not force a leaf into a damaged groove or judge by a single contact.
  4. Compare neighbouring available leaves and repeat on another undamaged area. Record the best visible match and any light gaps, rocking or uncertainty. If access is too short, the profile is worn, or two candidates remain plausible, record “unidentified” rather than pick the nearest number.
  5. Write the leaf marking, units, profile family, part photograph and separate diameter measurement with its method. Ask a supplier to confirm the full designation, taper and sealing arrangement against the part documentation. Keep uncertain parts apart; do not use trial tightening to establish compatibility.

INTERPRET THE MEASUREMENT

How to read the result

  • Metric example: a 1.5 mm leaf indicates 1.5 mm pitch, not a 1.5 mm screw diameter. Six crest positions span five intervals: 5 × 1.5 = 7.5 mm. Counting crests as intervals would give the wrong check.
  • Inch example: 16 TPI gives 25.4 ÷ 16 = 1.5875 mm pitch. This differs from 1.5 mm by 0.0875 mm per interval, or 0.4375 mm across five intervals. This arithmetic explains accumulated mismatch; it is not a gauge tolerance or a substitute for a proper leaf.
  • Same pitch is not the same connection. Record diameter separately, but do not treat a pipe’s nominal inch designation as its measured outside diameter. Pitch comparison does not establish thread tolerances, sealing condition, material suitability or pressure rating.

TROUBLESHOOT THE METHOD

Common problems and what to check

Problem

A leaf touches at one end but leaves increasing gaps.

What to do

Check alignment, cleanliness and the correct units/profile. Repeat gently on sound threads. If the mismatch persists, reject that candidate; pressing harder does not improve the match.

Problem

Two leaves seem equally close on a very short thread.

What to do

Do not round the result or select a replacement from that observation. Retain the photograph and uncertain markings for a supplier; a longer sound sample or specialist identification may be needed.

Problem

The leaf matches, but the fitting still cannot be identified.

What to do

Stop at a pitch record. Swagelok notes that some ISO 7/1 and NPT sizes are difficult to distinguish even with these tools. Do not infer interchangeability from a visually similar thread.

STOP BEFORE THE HAZARD

Safety boundary

  • Never loosen a live, pressurised or hot installation for this check. A pitch gauge does not demonstrate isolation or safe recommissioning; use a qualified service professional where the system requires one.
  • Handle sharp threads and pointed leaves carefully, keep fingers away from the folding pivot, and do not use a damaged leaf. Do not reach into moving or energised equipment.
  • Do not cut, chase or force unknown threads to make a match. This comparison is not a GO/NO-GO acceptance inspection and does not authorise reuse of a safety-critical fastener or pressure fitting.

SMART BUYING GUIDE

What to look for when choosing

  • Choose the actual metric or TPI range and profile family needed for documented parts, not the largest leaf count.
  • Look for legible permanent markings, undamaged teeth and leaves that can reach the sample without bending.
  • Keep the gauge clean and dry according to its instructions; use a separate suitable caliper for diameter and retain the part documentation.

Types of tool

Metric pitch leaves

Best for: Comparing spacing stated in millimetres, with the specified profile family.

Inch pitch leaves

Best for: Comparing TPI with the correct Unified or Whitworth family; inch markings alone are not enough.

Common functions

Sort loose workshop parts into documented pitch observations and an unidentified group.Prepare a replacement enquiry with a photo, leaf units and independent diameter measurement.

Worth paying more for

  • Clear units and profile identification on the set.
  • Sound teeth and useful access to the actual sample.

Usually not worth the extra cost

  • Extra leaves outside the tasks you actually encounter.
  • Claims that one matching leaf identifies every fitting or proves a safe seal.

Sources & instructions

Original schematic, not a photograph or a full-size measuring template. Tooth spacing and shapes are illustrative.