Threads & Fits
Thread systems explained
A thread designation includes a system and pitch or threads per inch. Keep ISO metric and unified inch designations distinct even when their dimensions look similar.
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NIST describes the standard inch as exactly equivalent to 25.4 mm
. NIST length definition. ASME — unified inch screw threads. Method and limitations.
Compare metric (M) and imperial (UNC, UNF) threads so you can specify, inspect, and assemble without cross-threading. Learn the differences in pitch, TPI, diameters and fit classes, and how to document conversions safely.
Core differences to check
- Pitch vs TPI: metric uses mm pitch; imperial uses threads per inch.
- Flank angle: most modern threads use 60 degrees, but legacy forms differ (e.g., Whitworth 55 degrees).
- Major/minor diameter: same nominal callout can hide different tolerances and allowances.
- Fit classes: metric 6g/6H vs imperial 2A/2B/3A/3B change clearance and torque feel.
Never assume a near diameter match means interchangeability. Always verify pitch and fit class.
Near matches that fail
M6x1.0 vs 1/4-20 UNC: diameters close, pitch off by 0.27 mm per turn.
M8x1.25 vs 5/16-18 UNC: pitch off by about 0.16 mm per turn.
M10x1.5 vs 3/8-16 UNC: pitch off by about 0.09 mm per turn.
UNF vs UNC: different TPI on the same nominal diameter; do not mix in blind holes.
If parts must interface globally, provide both thread specs or supply matched fasteners with the assembly.
Specification and conversion guidance
Converting threads is not just math. Check available standards and pick the closest standard form instead of inventing hybrids.
Anchor on standards
Use ISO metric threads (e.g., M10x1.5) or ANSI/ASME UNC/UNF. Avoid custom pitches unless required and documented.
Call out fit and tolerance
Include class of fit. Metric internal threads often use 6H; external use 6g. UNC/UNF often use 2B/2A for general purpose.
State pitch clearly
For metric, always include pitch (M8x1.25). For imperial, include TPI (5/16-18 UNC). Avoid ambiguous shorthand.
Field practices to prevent damage
- Provide thread gauges in both systems for incoming inspection.
- Color-code bins and kitting labels for metric vs imperial fasteners.
- Train teams to start threads by hand before applying torque; stop if resistance appears early.
- Keep torque specs per thread form and lubrication; do not reuse torque charts across systems.
Documentation checklist
List unit system, diameter, pitch or TPI, thread series, and fit class.
Reference the standard (ISO, ASME) and revision to anchor tolerances.
Provide metric and inch conversions for clearance planning but avoid hybrid threads.
If substituting fasteners, validate strength class and coating compatibility.
Frequently Asked Questions
ISO metric and unified inch threads both use a 60 degree included angle, but their pitch, nominal diameters and tolerance systems can differ. For example, M6x1.0 has a 1.0 mm pitch and 60 degree angle; 1/4-20 UNC has 20 TPI (~1.27 mm pitch) and 60 degree angle. The mismatch causes cross-threading or loose engagement.
State diameter, pitch or TPI, series, class of fit, and unit system. Example: "M10x1.5-6H" or "1/4-20 UNC-2B". Add material and coating if relevant.
TPI = 25.4 / pitch_mm. Pitch_mm = 25.4 / TPI. Round carefully and do not assume equivalence if values do not align with a known standard thread series.
Label fastener kits with the system and series, keep gauges for both metric and imperial, and train teams to start fasteners by hand before torque. Never force a fastener if resistance appears early.
Specify threads with confidence
Convert supporting dimensions with the mm to inches calculator, then keep thread callouts tied to recognized standards to avoid cross-threading and rework.
Sources and calculation notes
A thread designation includes a system and pitch or threads per inch. Keep ISO metric and unified inch designations distinct even when their dimensions look similar.
Method: Divide millimeters by 25.4 to obtain inches. The factor is exact; displayed decimal values are rounded. Fractional inches are rounded to the selected denominator.
Scope: Use the applicable standard and drawing revision for thread form and fit. This guide explains terminology and does not certify component compatibility.
Content maintained by mmtoinches.im. Updated and reference notes reviewed . How we calculate and present measurements. Sources support the stated definitions; examples and calculators are not certification of a product or compliance with a standard.