How to Identify Threads in the Shop
For an ordinary machine screw or bolt, identify two things first: the outside thread diameter and the thread spacing. In 1/4-20, 1/4 means a nominal 1/4-inch outside diameter and 20 means 20 threads per inch. In M10 × 1.5, 10 means a nominal 10 mm outside diameter and 1.5 means 1.5 mm between adjacent crests.
In this guide
Read the size before you reach for a tool
The first number tells you how wide the threads are. The next number tells you how closely the threads are spaced. Length is a separate measurement.
Each field describes a different part of the fastener specification.
20 TPI describes the spacing along the bolt. The counting example below starts at crest zero.
Enlarged teaching view. Head/neck proportions, thread roots, and threaded length are illustrative. Form choices teach measurement; they do not confirm that a combination is manufactured. The size name does not specify a grade or coating. “Plated” is a finish, not the base material.
Measure the spacing
EXAMPLE1/4-20 UNC
NOMINAL OUTSIDE DIAMETER1/4 in = 0.250 in = 6.35 mm
THREAD COUNT20 threads per inch
From crest 0 to crest 20: 20 spaces span 1 inch. There are 21 visible crests when both endpoints are included.
Zero can be any undamaged, fully formed crest. Count along the length of the bolt. The drawing scrolls sideways on a small screen.
Compare a bolt at nominal size on this screen
Use a modern U.S. penny (19.05 mm diameter). Hold it gently beside the screen or against a screen protector. Adjust the circle until its outside edge matches the coin. Do not press metal against an unprotected screen.
Not calibrated. The enlarged teaching drawing is not actual size.
Only nominal outside diameter, axial thread spacing, and selected length are scaled. Head, neck, threaded length, and crest/root shapes are illustrative. This narrows candidates; it does not establish thread class, wear, fit, grade, or approval to install. Confirm with a ruler or caliper and thread gauge. Recheck the penny after moving to another display. Zoom, resizing, page reload, and leaving this tab reset the scale.
A crest is the top of a thread ridge. A root is the valley between ridges. Measure along the bolt to check spacing; measure across the bolt to check diameter. The drawing is enlarged for teaching. It is not a ruler you can hold a part against.
| Marking | Diameter | Thread spacing | Length, if shown |
|---|---|---|---|
| 1/4-20 | Nominal 1/4 in, or 0.250 in | 20 threads per inch (TPI) | Not specified |
| 1/4-20 × 1 in | Nominal 1/4 in | 20 TPI | 1 in |
| M10 × 1.5 | Nominal 10 mm | 1.5 mm crest to crest | Not specified |
| M10 × 1.5 × 30 | Nominal 10 mm | 1.5 mm crest to crest | 30 mm |
For a hex-head bolt or socket cap screw, length is normally measured from the bearing face under the head to the tip. A countersunk flat-head screw or flat-head plow bolt includes the head. A carriage bolt is measured under the round head, including its square neck. Change the bolt type in the example to see where the measurement starts. Confirm special or domed head styles against the product drawing.
A thread callout alone does not specify head style, drive, grade, material, coating, or quantity. The optional penny-calibrated comparison scales nominal diameter, pitch, and selected length. Its head, neck, and thread roots are illustrative; use measuring tools to confirm a candidate.
Choose a measuring tool, then see how to use it
A folding pitch gauge checks thread spacing. A caliper measures diameter and length. Use both to narrow an ordinary bolt identification. Request either tool from FORGE below; ask for help if you do not know which set or measuring range you need.
Hover, focus or tap an instruction to highlight the tool. Original teaching drawings; not to scale or a specific model offered for sale.
Folding thread pitch gauge
Find the spacing: threads per inch or millimeters between crests.
A pitch leaf checks spacing and profile contact. It does not certify size, tolerance class or remaining thread condition. A smooth feeler-gauge blade measures a gap; it is a different tool.
Digital caliper
Measure outside thread diameter and fastener length.
Small upper jaws measure an inside opening, not the matching bolt diameter. For length, place the outside jaws at the correct start and tip. Display resolution is not the same as measurement accuracy.
Other options: steel rule and female-thread plug gauge
Steel rule
Check length or estimate pitch when you have no pitch gauge.
A rule is a rough check. Short, fine or damaged threads need a suitable gauge to separate close alternatives.
Thread plug gauge
Check a female thread against a known specification.
Ask a qualified inspector to select and interpret this gauge when the thread is unknown. Tapered pipe threads need their own gauges and inspection rules.
These links open a request with the tool name filled in. FORGE will confirm the specification, price and availability. No order is placed by opening a link. Use “Open form directly” if the website’s embedded form does not load.
For mixed inch and metric work, ask for both TPI and millimeter-pitch leaves covering the sizes you handle. Confirm the thread family too: a 60° metric/Unified set does not replace a 55° Whitworth set or an ACME gauge. For a caliper, tell FORGE the largest part you need to measure and whether your job requires calibration documentation.
You do not need to buy an inspection tool before asking for help. Photos, markings, the old package or a known mating part can start an identification request. The illustrated tools are examples; FORGE will confirm the model and availability in its reply.
Measure across the outer crests, not across the head
Close the outside jaws of a caliper lightly across opposite thread crests. This is the major diameter: the largest diameter of the external thread.
Keep the caliper square to the bolt. Check an intact, fully formed section, not the tapered starting tip. A wrench measures the hex head across its flats; that measurement is not the thread diameter.
Nominal means the size name. A nominal 1/4-inch thread is identified with 0.250 in (6.35 mm), but the actual outside measurement can be slightly smaller because of its tolerance, wear, or manufacturing condition. Do not reject or accept a part using a nominal number as a precision inspection limit.
A nut or tapped hole has internal threads. Its opening is closer to the small internal diameter, so an inside-caliper reading does not directly give the matching bolt's major diameter. Use an identified thread plug gauge, the drawing, or an undamaged known mating part as evidence. A casual fit check cannot establish tolerance class.
Small US machine screws may use number sizes. For example, #10-24 means a #10 diameter with 24 TPI; it does not mean 10 mm. Keep the # symbol in the request.
Inch threads: start at zero and count one-inch spacing
Align the zero mark with any full crest. Count successive crests as 1, 2, 3 and so on along the bolt. On a 20 TPI thread, crest 20 is one inch from crest 0.
The count is the number of equal thread spaces you cross. From crest 0 to crest 20 there are 20 spaces and 21 visible crests when both endpoints are included. Counting the first crest as 1 is the familiar off-by-one mistake.
Any complete one-inch segment of an undamaged, uniform thread contains the same number of thread intervals. Starting on a crest makes the endpoints easier to read. Keep the ruler parallel with the bolt's axis; do not measure diagonally around the helical ridge.
For a shorter sample, count spaces across a known length: TPI = spaces ÷ length in inches. Ten spaces across 1/2 inch gives 10 ÷ 0.5 = 20 TPI. Count ten spaces from crest 0 to crest 10, not ten crests including the first. Short-span ruler checks magnify measurement error; use a pitch gauge to confirm close alternatives.
A 20 TPI thread has a pitch of 1/20 in = 0.050 in = 1.27 mm. The unit conversion describes its spacing. It does not turn it into a standard metric thread.
Metric threads: measure crest to crest
Metric pitch is the distance, in millimeters, between corresponding points on adjacent threads, measured parallel to the bolt axis.
On M10 × 1.5, one crest-to-crest space is 1.5 mm. For a clearer ruler check, measure several spaces and divide: 15 mm across 10 spaces gives 1.5 mm pitch. Those 10 spaces run from crest 0 to crest 10 and include 11 crests.
Lay a metric pitch-gauge leaf along several full threads. The correct leaf follows the thread shape without progressively opening gaps or rocking. A leaf touching one or two peaks is weak evidence. Check both the diameter and the pitch before naming the thread.
A larger TPI number means finer inch threads. A smaller millimeter pitch means finer metric threads. At the same nominal diameter, 1/4-28 is finer than 1/4-20; M10 × 1.25 is finer than M10 × 1.5.
Close-looking sizes still need separate labels
Diameter and pitch must agree with the same thread system. A similar diameter or one successful turn does not prove a match.
| Candidate | Nominal outside diameter | Axial spacing | What to remember |
|---|---|---|---|
| 1/4-20 UNC | 6.35 mm | 1.27 mm (20 TPI) | Common inch coarse thread |
| 1/4-28 UNF | 6.35 mm | About 0.907 mm (28 TPI) | Same diameter name, different pitch |
| M6 × 1 | 6 mm | 1 mm | Neither 1/4-20 nor 1/4-28 |
| M10 × 1.5 | 10 mm | 1.5 mm | Metric diameter and metric pitch |
UNC means Unified National Coarse; UNF means Unified National Fine. Coarse and fine describe a series within a diameter. Neither word supplies the diameter. Keep the complete size when ordering.
Pipe fittings are a separate branch. A 1/4 NPT fitting is not a 1/4-inch bolt. Its nominal pipe size is not its measured outside thread diameter. Tapered, left-hand, multi-start, damaged, and proprietary threads need the relevant drawing or gauge. For NPT and British pipe threads, use the pipe comparison linked below.
Female threads: useful checks when you have no gauge
A female thread is inside a nut, coupling, or tapped hole. Start with its records or an identified matching male part. A gentle hand fit can narrow the candidate; it cannot certify the thread.
Try known hardware first. Use an undamaged bolt or male fitting whose complete thread designation is known from its label or measurement. Work on a loose part or safely isolated equipment. Align it squarely and turn by fingers only. Check smooth engagement over several full threads where the available depth allows, then remove it. Stop at unexpected resistance; do not use a wrench, force a fit, or use a tap to identify the hole.
Record the candidate and what happened: “Labeled M10 × 1.5 bolt turns smoothly for five turns by hand; hole depth not yet confirmed.” One successful turn is weak evidence. A loose or worn part can accept the wrong candidate. A bolt can also stop against the bottom of a blind hole, where the hole ends, rather than because its pitch is wrong. A hand fit does not establish thread class, remaining strength, or a pressure-tight connection.
| Available evidence | What it can help establish | What remains open |
|---|---|---|
| Original mating part, package, drawing or purchase record | A candidate thread and equipment identity | Wear, modification and exact replacement requirements |
| Positively identified male sample, fingers only | Whether that candidate engages smoothly | Tolerance class, seal, strength and close-looking alternatives |
| Inside-caliper reading and a clear photo beside a scale | Approximate opening size and visible condition | Nominal bolt diameter, pitch and hidden thread shape |
| No tools or no trustworthy sample | A photo/sketch and machine model can start a FORGE identification request | The thread stays unknown until evidence resolves it |
Play-Doh is not a recommended measuring method. It can distort while being removed and leave material behind. Its softness changes with drying and added water, so an impression is not dependable evidence of pitch or thread angle. Do not pack it into a blind hole or fluid passage. A specialist may instead choose a measurement-grade replica compound, after checking material compatibility, removal and cleanliness, then inspect the replica with suitable optics.
Try a FORGE proof print before an expensive order
FORGE provides 3D-printed proof samples for fit checks. A labeled candidate can help check access, length, clearance and initial thread engagement before committing to an expensive fastener.
Ask for a trial sample around a specific question: “Does this candidate reach and engage the female thread without the shoulder hitting first?” Send the mating part or its photos, known measurements, candidate thread, usable depth, and any drawing or CAD file. Include the cost or delay the check should help avoid. FORGE can review whether a printed sample is useful for that particular thread.
The sample should keep the intended thread designation and any deliberate print clearance documented. Check its actual dimensions before trying it; design a way to grip and remove it. Use only gentle hand engagement on a loose or safely isolated part. Stop if it binds, shaves material, bends or begins to break. Do not use the sample to carry a load, tighten an assembly, or test a pressure seal.
Printing errors, layer texture, post-processing and flexible plastic can create a false pass or false failure. Enlarging clearance until a print screws in does not prove that a correctly specified metal fastener will fit. A fine, small, damaged or tolerance-sensitive thread may need a real sample or calibrated plug gauge instead. Confirm the metal fastener specification before ordering; the proof print is supporting evidence, not a certified GO/NO-GO gauge.
Turn your measurements into a request
Send the measured evidence and keep unknown details marked unknown. You can ask for identification before asking for a replacement.
Example: “Please identify this hex-head bolt. Outside thread diameter is about 0.247 in. A 20 TPI gauge fits several full threads. Length under the head is 1 in. Head has three radial marks plus a maker symbol; photos attached. Finish unknown. Needed for a conveyor bracket, quantity 25. Please confirm the specification before quoting.”
That request preserves observations without declaring the part certified Grade 5 or approving a substitute. If you have no tools, send clear side and head photos, a photo beside a scale, the old label or invoice, and the machine make/model. Ask the technician for the drawing or OEM part number when available.
Only clear loose dirt enough to see the measurement points; preparation is a supporting step. Preserve a failed part and its markings. Stop a trial fit at unexpected resistance. Never use a wrench to force an identification. Pressure, lifting, structural, vehicle, and other controlled joints need the responsible technical owner's replacement requirements.
A short detour: thread angles, ACME and unusual forms
Ordinary ISO metric and Unified bolt threads use a 60° included angle between their flanks, the sloping sides of the thread. An 82° callout usually describes a countersunk head or its seat, not this thread angle.
| Thread family | Recognition clue | What to confirm |
|---|---|---|
| Whitworth BSW/BSF; British pipe BSP | 55° profile | Bolt and pipe families still need their own size, pitch and standard |
| ACME; Stub ACME | Standard 29° trapezoidal profile; Stub has less thread depth | Exact form, class, pitch and lead; common on motion screws |
| Metric trapezoidal, marked Tr | Standard 30° profile | Similar-looking ACME nuts are not interchangeable |
| Buttress | Unequal flanks for directional thrust | Exact standard and loading direction; angles vary by design |
| Square, round/knuckle or proprietary | A different profile or an OEM designation | Drawing or specialist identification, not a visual guess |
Left-hand and multi-start describe other features, not a single thread profile. On a multi-start screw, lead is the travel in one turn; pitch is the distance between adjacent threads. Lead equals pitch times the number of starts. Record both before replacing a motion screw.
A specialist uses an optical comparator (profile projector) or measuring microscope to inspect an aligned, enlarged profile and its flank angles. An internal thread may need a suitable replica or dedicated internal measurement equipment. Thread micrometers or the three-wire method help measure external pitch diameter, an important fit dimension; they do not identify every feature alone. Calibrated plug gauges check a female thread against a specified standard. Ask for the identified form, dimensions and inspection basis, rather than “it looks close.”
Frequently Asked Questions
Does 1/4-20 mean the bolt is 20 inches long?
No. It means nominal 1/4-inch thread diameter and 20 threads per inch. Length is added separately, for example 1/4-20 × 1 in.
Why can I see 21 crests on a 20 TPI drawing?
The first crest is zero. Twenty equal spaces run from crest 0 to crest 20. Both end crests are shown.
Is M10 the same as a 10 mm wrench size?
No. M10 describes the nominal outside thread diameter. The head size depends on the fastener form and standard.
Can a photo positively identify a thread?
A photo can narrow the search. A scale, diameter reading, pitch gauge, markings, and equipment record make the identification stronger. Similar thread forms may still require proper gauging.
Can a known bolt identify a female thread without a gauge?
It can support a candidate if the bolt is positively identified and turns smoothly by hand over useful engagement. Stop at resistance. Wear, depth and similar thread forms can mislead; a hand fit does not certify the thread.
Can FORGE print a sample before I order the real fastener?
Yes. Ask FORGE about a labeled proof sample for a specific fit or clearance question. Print accuracy and plastic flexibility limit what it proves. Confirm the final metal thread specification with appropriate measurements or gauging.
Sources and Review Boundaries
Published by GO4FORGE. Revised 2026-10-29. Editorial method. These notes help narrow sourcing evidence and selection risk; they do not replace the responsible site authority, OEM procedure, code requirement, or qualified engineering/safety review.
- GO4FORGE product knowledge files Internal product-knowledge references distilled into public decision guides.
- GO4FORGE RFQ evidence model Shop-floor evidence patterns for quotes, substitutions, and sourcing review.
- Mitutoyo: thread pitch gauge identification Stamped pitch leaves and separate Metric, Unified and Whitworth families.
- Starrett: using a screw pitch gauge Manufacturer demonstration of a folding screw pitch gauge.
- Mitutoyo: caliper types and operation Caliper measurement functions and the difference between incremental and absolute zero systems. Follow the model instructions.
- Bolt Depot: size notation Diameter, thread count or pitch, and length notation.
- Bolt Depot: thread count and pitch Axial thread spacing and pitch-gauge measurement.
- Earnest Machine: plow bolt measurement Plow-bolt length includes the head; confirm the actual head style and drawing.
- Portland Bolt: bolt length Under-head versus countersunk length; carriage-bolt length includes the square neck.
- U.S. Mint: penny specifications Modern U.S. penny diameter: 19.05 mm, used for manual screen-scale calibration.
- Swagelok: thread and end connection identification Pipe-thread identification is a separate task from ordinary bolt sizing.
- Formlabs: printed threads and threaded inserts Print process, surface finish and thread geometry affect a printed fit; a print is not a calibrated thread gauge.
- Hasbro: Play-Doh moisture and softness Consumer modeling compound changes with drying and added water. This is not a dimensional-replication specification.
- Flexbar: measurement-grade replica material Purpose-made internal/external replicas can be inspected with optical comparators; suitability and removal need review.
- Thomson: ACME and trapezoidal thread forms Standard ACME and Stub ACME forms, 29-degree versus 30-degree trapezoidal profiles, and product-specific variants.
- Mitutoyo: optical profile and thread measurement Optical comparators measure enlarged profiles, angles and thread pitch with the correct setup.
- Mitutoyo: dimensional measurement reference Thread micrometers, measuring wires and the limits of individual measurements.
- Accu: imperial countersunk head angle An 82-degree countersink describes the head/seat geometry, a separate feature from thread-flank angle.
- SAE J429: mechanical and material requirements for inch fasteners
- ASTM A563: carbon and alloy steel nuts
- ASTM F3125: high-strength structural bolts
- ASTM F1941/F1941M: electrodeposited coatings on mechanical fasteners
- IFI Fastener Standards reference family
- ISO 10218 industrial robot safety reference
- ISO/TS 15066 collaborative robot reference
- OEM integrator documentation and application notes
Related Guides
Identify a part before requesting a replacement
Still unsure what the part is? Start with the markings or the job it needs to do. Ask FORGE about this topic.