Female-to-Female Thread Adapters

When a female-to-female adapter is the right part

Three genders of thread adapter exist and they solve different problems. This one solves the case where neither side will accept the other because both are male. A gearbox has a stud sticking out of it; the lifting eye you want on that stud is M20 and the stud is M30. Nothing threads onto anything. A body with an M30 female at one end and an M20 female at the other is the only single part that joins them.

If one of your two sides is a hole rather than a stud, you want the other genders instead: female-to-male reducing adapters for a male stub into a female port, and male-to-male adapters when both sides are holes.

What this part is called

The same body carries a different name in every catalogue, which is one reason a size pair looks unavailable when it is not. In the United States it is usually a female-female coupler, a female to female coupler, or simply a female to female adapter. British and European drawings say reducing coupling, thread adaptor or coupling nut; a German drawing will say Gewindeadapter or Gewindereduzierung. Every one of those names describes the same thing: one body, an internal thread at each end, a hex or round outside.

Two naming traps are worth knowing before you search again. Coupling nut properly means the same thread at both ends — that is DIN 6334 — so searching under it will not surface reducing sizes at all. And in hydraulics and pneumatics a coupler usually means a quick-disconnect fitting, which is a different part that joins hoses rather than threads. If a supplier search under one name came back empty, try the others before concluding the pair cannot be made.

6H, not 6g — the one spec error we see most

Lower-case tolerance letters belong to external threads and upper-case to internal ones. So a female thread specified as 6g does not exist; it should read 6H. This turns up on perhaps one adapter drawing in four, and it matters because a shop that follows the drawing literally has nothing to follow. When a drawing reaches us with 6g on a female end, we cut 6H and write that on the acknowledgement rather than changing it silently. The full working of this on a real job is on the M30×1.5 to M12×1.75 adapter in 316, which also covers why engagement depth and hole depth are two different numbers.

Three numbers decide whether the part works

1. Wall at the large bore. The large tapping drill is the biggest hole in the part, and what is left between it and the hex flats carries the torque. The table gives this for every pair. Below roughly 1.6 mm the hex has to grow or the body goes round with spanner flats.
2. The step between the two bores. Because the two tapping drills differ, there is a shoulder inside the part. That shoulder is a hard stop for anything threaded in from the large end — usually wanted, since it fixes how far the stud goes. The Bore step column gives its width. A stud can only pass straight through when the step is zero, which on a reducing adapter effectively never happens.
3. Engagement against hole floor. One diameter of engagement is our default, so 30 mm of thread on an M30 end. A blind hole needs floor below the last full thread, so the body is always longer than the sum of the two engagements. This is the number people forget when they specify an overall length first.

Size pair table — M6 to M64

Each row is a pair we machine, with the numbers that decide it. The Also written column carries both ways round of naming the same part, because half of the enquiries we get name the small thread first. Coarse-to-coarse pairs are listed at every size step; fine pitches are listed on the large end throughout and on both ends within three size steps, which is where mixed-fine jobs actually occur. A pair that is not listed is not a refusal — it is machined the same way, from the same three numbers.

What the catalogue actually carries

Materials

1.4301 (304) is the default for general and outdoor work and 1.4404 (316L) where salt or chemicals are involved. Where a part is almost entirely thread and the quantity is more than a handful, 1.4305 free-machining stainless cuts the price without changing the fit. Plain steel is used when the adapter is inside a machine and will be greased, and brass where an electrical or instrument fitting wants it.

Aluminium is the one to think twice about. 6061 and 7075 are available and make sense where weight is the point, but a female thread in aluminium galls and strips at a fraction of the torque a stainless one takes, and an adapter is by definition a part that gets spannered. If the joint will be torqued to a figure, we quote stainless and say why.

What one of these costs

Why every part is priced differently

There is no catalogue number to look up, so the quote is built from the part. These are the eight things that move it, roughly in order of how much they move it:

• Machining time — Two bores to tap and a hex to turn. The large bore is the long pole: an M30 end at one diameter of engagement is 30 mm of thread to cut, and the small end adds a tool change rather than much time.
• Material — 1.4305 free-machining runs noticeably faster than 1.4404 on a part that is almost entirely thread, so on these it is the cheapest honest stainless. 1.4404 is quoted when salt or chemicals are in the job, not by default.
• Part dimensions — Bar diameter follows the hex, and the hex follows the larger thread. An M48 body starts from 75 mm bar whatever the small end is, so the large thread sets the stock cost on its own.
• Tolerance — 6H at both ends is standard and included. A spigot or seal face held to H7 on the same part adds a finishing pass.
• Geometry — Blind bores cost more than through bores because the floor has to be cut below the last full thread and chips have to come back out; a through part with a shoulder is the cheaper form when the design allows it.
• Surface finish — As-machined is included. Passivation, bead blast or black oxide are separate operations with their own batch minimums, which is why they hurt most on a quantity of one.
• Quantity — Setup is the same for one piece as for fifty. See the table above - the fall between a one-off and a batch of ten is the largest single step in the price.
• Inspection — Thread gauging with go/no-go plugs at both ends, a dimensional report against your drawing, or a 3.1 certificate to EN 10204 are quoted as line items rather than buried in the unit price.

Which is why the honest answer to “what does a thread adapter cost” is a question back: two thread designations, the length, the material and how many. With those four we quote within 12 hours, and we quote the part you actually need rather than the nearest thing in a catalogue.

The hexagon around these bores has its own page — standard SW series versus the old DIN widths, odd spanner sizes, and where hex bar ends and milling begins: custom hex thread adapters.

Frequently asked questions