Technical reference
Thread standards & tolerances
A reference for the thread callouts on a micro screw drawing — what they mean, which one to choose, and how much engagement the joint actually needs.
Standards
Thread systems in use
ISO metric is the global default. North American designs still use unified threads, and Japan-origin designs often specify JIS — which is compatible with ISO for common coarse sizes.
| System | Standard | Designation example | Where used | Notes |
|---|---|---|---|---|
| ISO metric coarse | ISO 68-1 / ISO 965-1 | M1.6 × 0.35 – 6g | Global default | Pitch is implied when omitted |
| ISO metric fine | ISO 261 | M2.0 × 0.25 – 6g | Thin walls, adjustment | Shallower thread, finer adjustment |
| ANSI unified coarse | ASME B1.1 | No. 0-80 UNC | North America | Numbered sizes below 1/4 inch |
| ANSI unified fine | ASME B1.1 | No. 2-56 UNF | North America | Finer pitch than UNC |
| JIS metric | JIS B 0205 | M1.6 × 0.35 | Japan-origin designs | Compatible with ISO for common sizes |
| Self-tapping (plastic) | — | Ø1.6 × 0.4 | Plastic bosses | Specified by body diameter, not thread |
| Sheet metal screw | ISO 7049 / 7050 | ST3.5 × 9.5 | Thin sheet | ST prefix, coarse thread |
Tolerance classes
External and internal thread classes
The class defines how much clearance exists between the screw and the mating thread. It is a functional decision, not a quality grade.
| Class | Applies to | Position | Fit | Typical use |
|---|---|---|---|---|
| 6g | External (screw) | General | Clearance | Default for almost all screws |
| 6f | External (screw) | Tighter | Close clearance | Located joints, plated mating thread |
| 6e | External (screw) | Larger clearance | Free | Rapid assembly, dirty environments |
| 4h | Internal (nut) | Close | Close | Precision instruments |
| 6H | Internal (nut) | General | Standard | Tapped housings and inserts |
| 6G | Internal (nut) | Larger | Free | Coated or plated internal threads |
6g is the default for commercial screws. Tighter is not better — it raises cost, reduces tolerance to coating and does not improve clamp load.
Dimensions
Metric coarse thread data, M0.5 – M6
Reference dimensions for the sizes we produce most. Values are nominal; the drawing and the gauge govern production.
| Size | Coarse pitch (mm) | 6g major Ø max / min | Tapping drill (mm) | Close fit hole (mm) | Min. engagement (mm) |
|---|---|---|---|---|---|
| M0.5 | 0.125 | 0.500 / 0.467 | 0.35 | 0.45 | 0.8 |
| M0.6 | 0.150 | 0.600 / 0.567 | 0.45 | 0.55 | 0.9 |
| M0.8 | 0.200 | 0.800 / 0.760 | 0.65 | 0.75 | 1.2 |
| M1.0 | 0.250 | 1.000 / 0.953 | 0.75 | 0.85 | 1.5 |
| M1.2 | 0.250 | 1.200 / 1.153 | 0.95 | 1.05 | 1.8 |
| M1.4 | 0.300 | 1.400 / 1.347 | 1.10 | 1.20 | 2.1 |
| M1.6 | 0.350 | 1.600 / 1.540 | 1.25 | 1.35 | 2.4 |
| M2.0 | 0.400 | 2.000 / 1.933 | 1.60 | 1.70 | 3.0 |
| M2.5 | 0.450 | 2.500 / 2.427 | 2.05 | 2.10 | 3.8 |
| M3.0 | 0.500 | 3.000 / 2.919 | 2.50 | 2.60 | 4.5 |
| M4.0 | 0.700 | 4.000 / 3.901 | 3.30 | 3.40 | 6.0 |
| M5.0 | 0.800 | 5.000 / 4.888 | 4.20 | 4.30 | 7.5 |
| M6.0 | 1.000 | 6.000 / 5.868 | 5.00 | 5.20 | 9.0 |
Tapping drill values target approximately 75% thread engagement in steel. Aluminium and brass usually take a slightly smaller drill; plastics are specified by body diameter rather than thread.
Engagement
Minimum thread engagement by material
Engagement is usually the limiting factor in a micro joint — not the screw strength. A high-strength screw in a 1 × d aluminium thread will strip the aluminium long before the screw yields.
| Material | Minimum engagement | Recommended | Notes |
|---|---|---|---|
| Steel (tapped) | 1.0 × d | 1.5 × d | Full strength at 1.5 d |
| Aluminium | 1.5 × d | 2.0 × d | Lower shear strength; inserts preferred above M3 |
| Brass | 1.2 × d | 1.8 × d | Good machinability, moderate strength |
| Plastic (thread-forming) | 2.0 × d | 2.5 × d | Engagement limits strip torque |
| Plastic (insert) | Insert length | Insert length + 0.5 mm | Insert governs, not the screw |
| Cast magnesium | 1.5 × d | 2.2 × d | Brittle; avoid over-torque |
Verification
How a thread is actually checked
A thread callout is only meaningful if it can be verified. At micro sizes the gauge itself becomes a precision instrument, and coating thickness has to be separated from thread tolerance.
01
GO / NO-GO ring gauges
The GO gauge must pass fully; the NO-GO must not enter more than two turns. This checks the maximum material condition without measuring every element.
02
Optical thread measurement
Optical comparators and toolmaker microscopes measure pitch, flank angle and root radius on first article, where a gauge only tells you pass or fail.
03
Coating thickness separation
Coating adds to the effective diameter. Gauge before coating for thread tolerance, then measure coating separately by XRF at the thread root. Do not conflate the two.
04
Functional assembly test
Drive the screw into the actual mating part at the specified torque. A thread that gauges perfectly can still fail functionally if the mating material or engagement length is wrong.
Drawing checklist
What a complete thread callout contains
- Nominal diameter and pitch (e.g. M1.6 × 0.35)
- Tolerance class (6g or 6f)
- Thread length and whether it is full or partial
- Thread direction if not right-hand
- Material and condition
- Finish and coating thickness
- Whether gauging is before or after coating
- Mating material and minimum engagement
Questions
Thread FAQ
Common questions from design and quality teams.
What does 6g actually mean?
Should I specify 6f for a precision joint?
How much thread engagement do I need?
Do you supply JIS or ANSI threads?
How is the thread verified?
Send the drawing, we will check the callout
Thread class, coating thickness and engagement interact. If one of them is wrong, we will tell you before quoting rather than after the first inspection.