A coupling nut that shows the rod has reached the middle, one that is closed at one end, one that can be turned with a bar where no spanner fits, or one with a flange to bear on a panel? None is in the DIN 6334 catalogue. Each is one more operation on the hex bar, and we cut them to order from one piece. No drawing needed: a photo with a rule, the measurements in a message, the old part or its part number is enough; we draw it, you approve, we machine. Get a quote →
EKINSUN LTD is a custom parts manufacturer that cuts coupling nuts from hex bar to the thread, length and across-flats you specify, from one piece. The standard coupling nut is a plain hex body threaded through. The five forms below are asked for whenever the nut has to be inspected, sealed, turned without a spanner, or seated on a face.
| Form | What it does | What goes on the drawing |
|---|---|---|
| Sight hole (witness hole) | Shows both rods have reached mid-length; inspector can probe it | Hole diameter, one side or through, at mid-length |
| Blind end (cap coupling nut) | Caps a rod end, seals a threaded hole, gives a stud a stop | Usable thread depth, bottom thickness, whether the end is domed |
| Cross-drilled for a tommy bar | Turned where no spanner fits, without flats | Bar diameter, hole position |
| Round knurled body | Finger-tightened; sleeve nut, standoff, adjuster | Outside diameter, knurl pitch, thread depth |
| Flanged or shouldered | Bears on a panel or a soft material; stops at a set depth | Flange diameter and thickness, shoulder position |
The hole is small enough not to weaken the body and large enough to see a rod end or pass a wire probe. These are the diameters we drill unless the drawing says otherwise; the hole is centred on the length and goes through one flat, or both on request.
| Thread | Across flats | Sight hole | Wall left beside the hole |
|---|---|---|---|
| M6 | 10 mm | 2.5 mm | 2.5 mm each side of the thread |
| M8 | 13 mm | 3 mm | 3.2 mm each side of the thread |
| M10 | 17 mm | 3 mm | 4.3 mm each side of the thread |
| M12 | 19 mm | 4 mm | 4.4 mm each side of the thread |
| M16 | 24 mm | 5 mm | 5.1 mm each side of the thread |
| M20 | 30 mm | 6 mm | 6.4 mm each side of the thread |
| M24 | 36 mm | 6 mm | 7.6 mm each side of the thread |
A blind nut is tapped from one end; the tap or the boring bar stops short of the far end and the bottom stays solid. The usable thread depth is the tapped length less about one pitch of incomplete thread at the bottom. The bottom is left at least one pitch plus 2 mm thick so it does not distort when the rod bottoms out; a domed or chamfered outer end is turned if the nut is visible.
| Thread | Pitch | Minimum bottom thickness | Usable depth in a 3d body |
|---|---|---|---|
| M6 | 1 mm | 3.0 mm | about 14 mm |
| M8 | 1.25 mm | 3.2 mm | about 20 mm |
| M10 | 1.5 mm | 3.5 mm | about 25 mm |
| M12 | 1.75 mm | 3.8 mm | about 30 mm |
| M16 | 2 mm | 4.0 mm | about 42 mm |
| M20 | 2.5 mm | 4.5 mm | about 53 mm |
| M24 | 3 mm | 5.0 mm | about 64 mm |
| Feature | Rule we use | Example, M10 nut |
|---|---|---|
| Cross hole for a tommy bar | Bar about one third of the across-flats; hole at mid-length through two opposite flats | 17 A/F → 5 mm bar |
| Knurled round body | Outside diameter equal to the across-corners of the hex, straight or diamond knurl | 19.6 mm OD |
| Flange | Diameter 1.5 to 2 times the across-flats, thickness 2 to 4 mm | 25–34 mm × 3 mm |
| Shoulder | Half the length threaded above, plain below, to set a depth | 60 mm nut: 30 mm thread, 30 mm plain |
There is no list price for a machined coupling nut; a single piece starts from €65 and is quoted within 12 hours from the callout. One to a few dozen pieces ship in 5 working days, 50 to 500 in 15 working days, delivered DDP to your door with duties in the quote. The first piece is the sample; repeat orders run to the same drawing, and 500-piece and 1,000-piece runs are priced lower per piece. Bar is 304 (1.4301) unless the order says 316 (1.4404), 303 (1.4305), brass CW614N, zinc-plated steel or aluminium 6061; the grade goes on the drawing and the certificate.
| What you need | In the catalogue? | What to do |
|---|---|---|
| Plain hex coupling nut, DIN 6334 | Yes | Buy it there, or ask us — we may have it in stock |
| Rebar coupler with a sight hole | Yes, in steel at rebar sizes | Rebar suppliers; not stainless, not metric fine |
| Stainless coupling nut with a sight hole | No | Drilled to order, MOQ 1 |
| Blind coupling nut, any size | No | Tapped from one end to order |
| Cross-drilled or knurled body | No | Turned and drilled to order |
| Flanged coupling nut in 304 or 316 | No | Turned to order |
What is the hole in the middle of a coupling nut for?
It is a sight hole, also called a witness or inspection hole. It sits at mid-length so an inspector can see that both rods have reached the middle of the nut and the joint has full engagement. Rebar and rod couplers carry them; DIN 6334 nuts do not, so on a stainless nut it is drilled to order.
Can a coupling nut be closed at one end?
Yes. A blind coupling nut is drilled and tapped from one end only, leaving a solid bottom; it caps a rod end, seals a threaded hole, or gives a stud a fixed stop. Specify the usable thread depth and the bottom thickness, at least one pitch plus 2 mm.
How do I tighten a coupling nut with no room for a spanner?
Cross-drill it for a tommy bar, or make it round and knurled for the fingers. A cross hole through the flats at mid-length takes a bar of about a third of the across-flats; a knurled round body is turned to the same outside diameter as the hex.
Can the nut have a flange or a shoulder?
Yes. A flange at one end gives a bearing face on a panel or a soft material; a shoulder in the middle stops the nut at a set depth in a housing. Both are turned on the same bar, and the flange diameter, thickness and face finish go on the drawing.
Can you make these in M2 or M3?
Yes, as far as the geometry allows: a sight hole in an M3 nut is 1 mm, a cross hole for a bar is not practical below M5, and a blind end works at any size. Knurled round bodies are common at M2–M4 as finger-tightened standoffs.
What do a sight hole or a blind end add to the price?
One extra operation each; a single piece still starts from €65 and is quoted within 12 hours. Small quantities ship in 5 working days, DDP to your door.
Sketch it or say it. "M12 coupling nut, 50 long, 19 A/F, 304, 4 mm sight hole" is a complete order. Quote in 12 hours, MOQ 1. Get a quote →
Sight hole, blind end, cross hole, knurl or flange on any size M2 to M24. Quote in 12 hours, MOQ 1.
Send a drawing, sketch, sample or photo — reply within 12 hours. MOQ 1.