Thread Measuring Wires — Best-Size Wire Diameters for UN Screw Threads (4 – 80 TPI)
Best-size wire diameters for measuring UN screw thread pitch diameters by the three-wire method, per ISO 16239 — one wire diameter per threads-per-inch value from 4 to 80 TPI, transcribed row-for-row from the collected chart.
Last updated: 2026-11
Specifications
The UN companion to the metric wire chart: the same three-wire method, with best-size wires allocated by threads per inch instead of pitch. The collected chart covers the whole practical unified range — 80 TPI down to 4 TPI — and because wire diameter tracks pitch (0.577×P), the diameters run from 0.183 mm wires on 80 TPI up to 3.666 mm on 4 TPI. Reading the pairs the way the shop floor uses them: 20 TPI (1/4-20 UNC, 1/2-20 UNF territory) takes 0.733 mm wires, 13 TPI takes 1.128, 8 TPI takes 1.833 — pick the row for the thread's TPI, not its diameter, since every TPI value shares one wire size across UNC, UNF and the constant-pitch series alike. That is the chart's quiet efficiency: a single matched wire set covers the unified system, where the metric chart needs one wire per pitch but the pitches repeat across sizes too. As on the metric page, best-size wires cancel flank-angle error from the pitch-diameter reading; using whatever wire is in the drawer introduces it. The TPI column header carries the source's decimal comma ("per 25,4 mm") — reproduced as printed. Wire tolerance class and calibration are not tabulated — per RFQ.
| Standard | ISO 16239 — metric series wires for measuring screw threads, applied to UN threads; three-wire pitch-diameter measurement |
|---|---|
| Size range | Threads per 25.4 mm: 4 – 80 TPI, 14 wire/TPI pairs printed (two pairs per source row) |
| Allocation basis | One wire diameter per TPI value, shared across UNC / UNF / constant-pitch UN series — wire size follows the thread's TPI, not its major diameter |
| Example pairings | 80 TPI → wires 0.183 mm; 20 TPI → 0.733 mm; 13 TPI → 1.128 mm; 8 TPI → 1.833 mm; 4 TPI → 3.666 mm |
| Wire tolerance / calibration | Not tabulated by the collected chart — per RFQ (order matched triples per TPI; state wire class and calibration needs) |
| Nominal diameter of wires (mm) | Threads per 25,4 mm | Nominal diameter of wires (mm) | Threads per 25,4 mm |
|---|---|---|---|
| 0.183 | 80 | 0.917 | 16 |
| 0.204 | 72 | 1.047 | 14 |
| 0.229 | 64 | 1.128 | 13 |
| 0.262 | 56 | 1.222 | 12 |
| 0.306 | 48 | 1.275 | 11.5 |
| 0.333 | 44 | 1.333 | 11 |
| 0.367 | 40 | 1.466 | 10 |
| 0.407 | 36 | 1.629 | 9 |
| 0.458 | 32 | 1.833 | 8 |
| 0.524 | 28 | 2.095 | 7 |
| 0.543 | 27 | 2.444 | 6 |
| 0.611 | 24 | 2.933 | 5 |
| 0.733 | 20 | 3.259 | 4.5 |
| 0.815 | 18 | 3.666 | 4 |
- Row 1 (0.183): The source header carries the decimal comma of the original ("25,4 mm" per 1 inch) — the column labels reproduce it as printed.
Materials
- Measuring wires are through-hardened steel; specify wire set class and calibration certificate requirements at RFQ
Ordering & documentation
Documents
Shipments are backed by Mill Test Certificate (MTC), Dimensional Report, Salt Spray Test Report on request, with full batch traceability from raw material to finished product. Quality management follows the owner-confirmed compliance list: ISO 9001:2015, RoHS, REACH. Exact document sets are confirmed on the order.
Quotation
- The fastener family (Thread Measuring Wires — Best-Size Wire Diameters for UN Screw Threads (4 – 80 TPI)) plus the standard designation from your drawing — DIN / ISO / JIS / ASTM / ANSI, or a drawing for custom parts.
- Size (diameter and length), thread type (metric or inch), property class or grade, and coating or finish.
- Quantity per size and your target delivery date.
- Destination country, and any required documents (for example a mill test certificate) so the quote includes them.
Sample & supply
Minimum order quantity is 10,000 pieces. Stock items run 3–7 days. Quantities or variants outside the standard range are confirmed at quotation.
Storage & shipping
Parts are packed for export and shipped factory-direct to buyers in Europe, Asia, the Americas and the Middle East. Store fasteners dry and protected until installation, and keep coated parts in their packaging until use; specific packing and preservation requirements can be stated on the order and confirmed at quotation.
Frequently asked questions
One wire set for every UN thread — how does that work, and what does the measurement give me that a ring gage does not?
The wire size depends only on the pitch, and in the unified system pitch is expressed as threads per inch — so every thread with the same TPI, whether it is UNC, UNF or a constant-pitch UN size, takes the same wires. The collected chart lines those up: 20 TPI (1/4-20, 1/2-20, any 20-pitch thread) takes 0.733 mm wires; 28 TPI takes 0.524; 13 TPI takes 1.128. A shop holding a few matched triples can therefore gage the whole unified system, picking rows by TPI alone — the chart runs 80 down to 4 TPI, which covers from instrument threads to large-diameter studding. What the measurement buys over a ring gage is a number: ring gages say pass/fail at a worst-case limit, while the three-wire reading gives the actual pitch diameter, which is what you need to chart a threading process, diagnose a lathe cutting oversize, or accept a ground thread against its 2A/3A limits (see this catalog's ASME B1.1 limiting-dimension pages for those numbers). Two disciplines keep it honest: use the best-size wire for the TPI (any other size folds flank-angle error into the reading), and keep the triple matched and clean. The source header prints its TPI column "per 25,4 mm" with the original's decimal comma — same unit, 25.4 mm. Wire class and calibration practice — per RFQ.
What is the MOQ and lead time for threaded parts and gaging hardware on this standard?
The minimum order quantity is 10,000 pieces. Stock items ship in 3–7 days; non-stock items are confirmed at quotation. Confirm the exact schedule against quantity and size mix at quotation.
Related knowledge
- Thread-Forming vs Thread-Cutting vs Thread-Rolling Self-Tapping Screws: What the Evidence Settles on Base Material
- Screw Family Selection by Base Material: Wood, Machine and Self-Tapping Threads, Engagement and Coating
- Hex Bolts vs Socket Head Cap Screws: What the Evidence Settles on Strength, Clearance and Cost
Request a Quote
Get factory-direct pricing from the manufacturer.
What to include in your inquiry
- ✓ Standard and property class (e.g. DIN 912, ISO 4762, class 8.8) — or a drawing for custom parts.
- ✓ Sizes, lengths and thread type (metric or inch).
- ✓ Quantity per size and your target delivery date.
- ✓ Destination country, plus any coating or finish requirement.
- ✓ Any documents you need with the shipment (e.g. mill test certificate) so the quote includes them.
- ✓ Your inquiry goes straight to the factory team by email and gets a reply within 24 hours.
- ✓ Factory-direct pricing
- ✓ Standard and custom specifications
- ✓ Factory-direct export to buyers in Europe, Asia, the Americas and the Middle East
- ✓ Full fastener family support