1/4-28 UNF, #8-32 and M6: Why the Nearest Metric Size Is Not a Substitute

Short answer: Imperial and metric threads both use a 60 degree flank angle, and that is where the similarity ends. 1/4-28 UNF counts 28 threads in an inch; M6 is specified by a pitch in millimetres. The two pitch systems are defined differently, the tolerance classes come from different standards, and a fastener made to the nearest metric size will not go into an imperial tapped hole. If a drawing says 1/4-28 UNF or #8-32, the part gets made in that system, and the gauges have to match it.

What the two designations actually tell you

An imperial designation names the nominal diameter and the number of threads per inch. In 1/4-28 UNF the shank is a quarter of an inch and there are 28 crests along one inch of thread; UNF marks it as the fine series. In #8-32 the number sign plus 8 is a gauge number for the diameter, and 32 is again threads per inch. A metric designation works the other way round: M6 x 1.00 gives a 6 mm nominal diameter and states the pitch directly, 1.00 mm from one crest to the next.

An inch is 25.4 mm, so an imperial pitch rarely converts to a round millimetre value. That is the practical problem. Put a metric fastener into an imperial hole and the first turn or two may feel fine, because the crests still meet. A few turns in, the accumulated difference in pitch usually binds the part or strips the first engaged threads. The same happens the other way round. Nothing about the finish, the material or the head shape rescues it; the pitch decides.

The tolerance classes are a second, separate difference

Even within one system a thread is not a single dimension but a band. Imperial threads carry classes such as 2A for external and 2B for internal threads; metric threads use designations such as 6g and 6H. The bands are defined by different standards and do not translate into one another. So a drawing that says 1/4-28 UNF-2B is specifying both the pitch system and how much clearance the tapped hole is allowed, and a supplier quoting “metric equivalent” has quietly dropped half of that instruction.

What it changes on our side of the quote

Material and heading are rarely the issue. The thread is. A rolled imperial thread needs dies cut for that pitch, and a tapped imperial hole needs the matching tap. Both are ordinary items, but they are specific: a 1/4-28 die set does not make #8-32. Inspection is the part buyers tend to overlook. Go and no-go gauges are per thread and per class, so an imperial order has to be checked with imperial gauges. We keep that in the quote rather than discovering it at first-article inspection.

• Write the full designation, including the class if the mating part is already fixed: 1/4-28 UNF-2A rather than “1/4 fine”.
• Say what it mates with — a tapped hole in your own part, a nut you buy, or an insert. That decides which side carries the clearance.
• Do not ask for the metric equivalent unless you are willing to change the mating part too. There is no equivalent, only a replacement for both halves.
• Ask how the thread will be verified. For an imperial thread the answer should name imperial gauges, not a calliper reading of the outside diameter.

Imperial fine threads we actually run

This is not a theoretical distinction for us. Current and recent jobs include 1/4-28 UNF nylon insert nuts, stainless knurled thumb screws in #10-32, pan head security screws in #10-24, and long turned parts threaded 1/4-28 at one end. They sit beside metric work in the same workshop and are kept apart by their own tooling and their own gauges, which is the whole point: the two systems are run as two systems.

Where a part has a fine thread, the thread is usually also the feature that decides the process order. We have written separately about rolling a fine thread before or after heat treatment, using parts with exactly these imperial threads as the example.

If your drawing is in one system and your stock is in the other

It happens, usually on an assembly that was designed in one market and is now built in another. The decision is a design decision, not a sourcing one: either the drawing changes and both halves move to metric, or the imperial thread stays and is bought as an imperial part. What does not work is mixing the two and hoping the clearance absorbs it. If you are redrawing anyway, tell us which features have to stay as they are, and we will quote both ways so the comparison is on the table before anything is cut.

Send the drawing or a photo with the thread callout through our RFQ form and say which standard it follows. If the callout is unclear, say so — that is a cheaper question to answer now than after the first samples.

Related: PZ2 on the drawing cannot be made as PH2 · custom fasteners made to your drawing

How this was written: the thread comparison comes from our internal English-Chinese terminology notes for new staff, reviewed in July 2026. The parts shown are our own, from the product list. No customer, industry or drawing dimension is named.