M3 or #4-40: why they are not interchangeable

M3 and #4-40 machine screws are close enough in nominal diameter to be confused during electronics assembly, but their diameters and pitches are different. A mismatched screw may appear to start in some internally threaded parts; that is not evidence of compatibility, and continuing to turn it can damage the mating thread.

For a broader identification procedure, see the UNC, UNF and metric thread pitch chart.

The numbers

	M3 × 0.5	#4-40 UNC
Major diameter	3.000 mm	2.845 mm (0.1120″)
Pitch	0.500 mm	0.635 mm (40 TPI)
Threads per 10 mm	20	15.75
Tap drill (75%)	2.5 mm	2.26 mm (#43)
Clearance, close fit	3.2 mm	2.95 mm (#32)

The nominal major diameters differ by 0.155 mm, while the pitches differ by 0.135 mm. These differences are difficult to judge reliably by eye. Whether a mismatched screw begins to engage depends on the internal-thread dimensions, tolerance class, coating, wear and condition of both parts.

What actually happens when you force it

Because the pitches do not match, interference increases as engagement continues. Applying torque after resistance is felt can deform or remove material from the internal or external thread. Softer internally threaded components, including some brass or aluminum standoffs, can be especially vulnerable.

A #4-40 screw in an M3 internal thread may feel loose or bind, depending on tolerances and condition. Even if partial engagement is possible, the thread-flank contact and usable engagement are not equivalent to a correctly matched pair. Do not evaluate compatibility by tightening a questionable combination.

Telling them apart on the bench

Thread pitch gauge. The reliable method, and the reason a $10 gauge belongs in every kit. M3 reads 0.5 mm; #4-40 reads 40 TPI.

Count threads. No gauge? Lay the screw against a steel rule and count crests over 10 mm. Twenty means metric. Sixteen — roughly, you will not get a clean count — means imperial.

Calipers. Useful as a preliminary check, but diameter alone is not conclusive. The nominal values are 3.00 and 2.845 mm, while actual measurements vary with thread tolerance, coating, wear, damaged crests and measuring technique. Confirm the result with a pitch gauge or a known correct mating part.

Match a known part. If a screw already lives in the assembly and turns freely by hand for its full length, buy that.

Where each one shows up

#4-40 is commonly encountered in US-designed electronic enclosures, instrument hardware and D-sub connector hardware. The actual connector or drawing must still be checked because thread choices and accessory hardware vary by manufacturer and application.

M3 is widely used in metric-designed electronics, enclosures, fan mounts and equipment hardware. Its prevalence does not make it a substitute for a nearby Unified thread.

The awkward reality is that a single build often contains both: an imperial enclosure holding a metric board. If you are stocking a bench, stock both and keep them in separate bins — not separate compartments of the same bin.

The adjacent traps

#6-32 and M3.5. Their nominal major diameters are very close—3.505 mm and 3.5 mm—but their pitches differ. Diameter alone therefore cannot establish compatibility.

#4-40 and M2.5. Their nominal diameters are farther apart. Raspberry Pi mounting holes are commonly used with M2.5 hardware; confirm the board model, enclosure and standoff specification rather than assuming the holes themselves are threaded.

The rule

Do not determine a thread by appearance or by partial engagement. Measure the diameter and pitch, or compare it with a verified mating component that turns freely by hand through the required engagement.