Why can metric and UNC/UNF threads with close diameters not be interchanged?

They differ in pitch and flank angle. 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.

What should I include in a thread callout to avoid ambiguity?

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.

How do I convert between pitch (mm) and TPI (threads per inch)?

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.

How can I prevent cross-threading in the field?

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.

Threads & Fits

Thread systems explained

Compare metric (M) and imperial (UNC, UNF) threads so you can specify, inspect, and assemble without cross-threading. Learn the differences in pitch, TPI, flank angle, fit classes, and how to document conversions safely.

Pitch vs TPIFit classesCompatibility risk

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.

Convert mm to inches

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

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.

Open mm to inches Fraction converter