A cap nut closes the end of a bolt: it hides the thread, protects hands and cables from a sharp end, and keeps water off the threads. Three standards give three heights, and every one of them assumes the bolt stops where the dome starts. A bolt that reaches further, a dome that must be polished, or a thread that is fine or left-hand is a cap nut that gets turned.
EKINSUN LTD is a custom parts manufacturer that turns cap nuts and sleeve nuts to drawing: a deeper thread for a long bolt, a taller or slimmer dome, a polished or plated finish, a fine or left-hand pitch, a size above M30, a material the catalogue skips. The stock ranges below are read from a 2026 catalogue; the height table explains what each standard gives; the negative-space table marks the border.
"Stainless dome nuts M10 for a handrail, but the studs are 4 mm too long and the domes bottom out before the hex touches."
Thread depth under the dome is the number nobody prints on the box.
| Type | Standard | Stock range | Stock materials | Notes |
|---|---|---|---|---|
| High cap nut, one-piece dome | DIN 1587 | M2–M30 | 304 stainless; brass M3–M20; zinc, nickel, black and plain steel M3–M16 | the standard acorn nut: tallest dome, deepest thread |
| Low cap nut | DIN 917 | M3–M16 | 304 | shorter dome where space is tight; less thread depth |
| Dome nyloc nut | DIN 986 | M5–M12 | 304 | prevailing torque and a closed end in one part |
| All-metal dome lock nut | maker series | M5–M12 | 304 | closed end with metal locking; heat and chemicals |
| Fine-pitch cap nut | maker series | M8–M16 | 304 | M8×1, M10×1.25, M12×1.5, M16×1.5 |
| Left-hand cap nut | maker series | M6–M16 | 304 | counter-rotating shafts and rollers |
| Two-piece welded dome nut | maker series | M3–M16 | steel, zinc plated | a hex nut with a pressed cap welded on; cheaper, not for polished work |
| White-capped or coloured protective cap | maker series | M3–M24 caps; M5–M10 white-capped nuts | PE / nylon caps on steel or 304 nuts | cosmetic and finger protection on fences, playgrounds, appliances |
| Nylon cap nut | — | M3–M20 | nylon 66, white and black | insulating; see plastic fasteners |
Every cap nut has a threaded length and a dome cavity above it. The bolt must stop inside the threaded length; if it runs into the dome it either bottoms before the nut seats — the joint is loose and looks tight — or it pushes the dome off a two-piece nut. The rule is simple: bolt protrusion beyond the mating face must be shorter than the nut's thread depth, and DIN 1587 lists that depth per size. In practice:
Read the pitch before you order the cap nut. Lay a steel rule along ten thread crests and divide by ten: 10 mm over ten pitches is a 1 mm pitch (M6 coarse), 12.5 mm is M8 coarse, 15 mm is M10 coarse, 17.5 mm is M12 coarse. A reading that is not a round number of millimetres is an inch thread, and a reading that is shorter than the coarse pitch for that diameter is a fine thread. If diameter, pitch and the size range in the table all agree, buy the stock part. If any of the three is off the shelf, that is the one we make. The full measuring routine, including the inch traps →
| What you need | In the catalogue? | What we make |
|---|---|---|
| DIN 1587 cap nut M2–M30 in 304, or M3–M16 in zinc steel, coarse pitch | Yes | Buy it from stock. No one-off competes with a formed dome nut, and 304 domes are stocked to M30. |
| Brass cap nut M3–M20 | Yes | Stocked; buy it. Nickel or chrome plating over brass on request. |
| A cap nut with a thread deep enough for a bolt that cannot be shortened | No | Turned from bar with the thread depth you need and a standard dome outside; from 1 piece. |
| A taller, slimmer or wider dome than DIN — for a handrail, a lamp, an instrument | No | Turned to the profile; polished, brushed or plated. |
| Fine-pitch or left-hand outside the stock sets, or any pitch above M16 | No | Tapped or single-pointed to your reading. |
| A cap nut above M30, or in 316L / 1.4404, titanium or 42CrMo4 | No | Turned from bar; certificates per batch where the drawing calls for them. |
| A sleeve nut: a long closed nut joining two studs, or capping a long thread | No | Turned; internal thread to the full length you specify. |
| A cap nut in aluminium 6061 or 7075 | Partly | Anodised aluminium cap and cup washers M2–M8 are stocked; aluminium cap nuts are turned on request and not offered for high preload. |
| Material | Designation | Where it fits here |
|---|---|---|
| Stainless 304 | 1.4301 / A2 | the stock cap nut M2–M30; outdoor, handrails, food equipment |
| Stainless 316L | 1.4404 / A4 | turned to order for marine and pool fittings |
| Free-machining stainless | 303 / 1.4305 | custom turned and polished domes |
| Brass | CW614N / C36000 | stock M3–M20; nickel or chrome plated for decorative work |
| Carbon steel | zinc, nickel, black oxide, plain | stock M3–M16; one-piece and welded |
| Alloy steel | 42CrMo4 / 1.7225 | turned cap and sleeve nuts for machine spindles |
| Aluminium 6061 | AlMg1SiCu | turned light cap nuts for low-load, decorative or weight-critical parts |
| Aluminium 7075 | AlZn5.5MgCu | anodised cap and cup washers M2–M8 in stock; turned nuts on request |
| Titanium | Grade 5 Ti6Al4V | turned dome nuts for motorcycle and race parts |
| Nylon 66, PE | PA66, PE | insulating cap nuts M3–M20; protective caps M3–M24 |
Nothing: they are three names for the same closed-end nut. DIN 1587 is the high one-piece form most people mean; DIN 917 is a low cap; a dome nyloc DIN 986 adds a nylon locking ring under the dome.
Shorter than the nut's thread depth, measured from the mating face; DIN 1587 lists the depth per size. If the bolt reaches into the dome the nut bottoms before it seats and the joint is loose. When the bolt cannot be shortened, the cap nut is turned with a deeper thread.
In stock: fine pitch M8–M16 and left-hand M6–M16 in 304. Other sizes, pitches and materials are turned and tapped to order from 1 piece.
Yes. Stock 304 domes are bright-finished; brass domes take nickel or chrome; turned domes in 303 or 316L are polished to the finish you specify, which is one reason decorative cap nuts are made rather than bought.
EKINSUN LTD is a custom parts manufacturer that turns cap nuts, dome nuts and sleeve nuts in 304 and 316L stainless, brass, steel, aluminium and titanium, from M3 to M64, in any pitch, from 1 piece, with a quote within 12 hours.
There is no list price. The drivers are material, thread depth and dome profile, polishing, and quantity. A batch of 304 cap nuts with a deeper thread is a short lathe job; a single polished 316L dome above M30 is priced as the turned and polished part it is. The quote states the drivers.
Thread and pitch, how far the bolt reaches beyond the face, dome height or diameter if it matters, finish, material, quantity. A photo of the stud with a rule works. From 1 piece, quote within 12 hours.
Cap nuts and sleeve nuts turned to thread depth, dome, pitch and material — from 1 piece, quote in 12 hours.