Cap nuts (domed / acorn) · DIN 1587 high type, size range and product grades, width across flats, dome height, thread depth and bolt protrusion, material and finish
A cap nut (hexagon domed cap nut or acorn nut) is a hexagon nut with a closed, domed end, made by cold forging or machining from steel, stainless steel or brass and plated, polished or plain, that covers the end of a bolt or stud to protect it and people from sharp threads and to give a finished appearance. Ask three things first: which standard, size and height type you need, whether the dome is deep enough for the bolt end, and which material and finish suit the environment.
Hexagon domed cap nuts and acorn nuts in steel, stainless steel and brass, to DIN 1587 or equivalent. Low-type cap nuts, plastic caps, flange nuts, coupling nuts and blind rivet nuts are outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
DIN 1587 (07/2014) defines hexagon domed cap nuts, high type, from nominal thread diameter M4 to M24 in product grades A and B. Its dimension table gives, for M6: dome diameter dk max 9.5 mm, nominal height h 12 mm, width across flats s 10 mm, width across corners e min 11.05 mm and thread depth t 7.71–8.29 mm; for M8: dk max 12.5 mm, h 15 mm, s 13 mm, e min 14.38 mm and t 10.7–11.4 mm. [1]
Editorial note: older editions and other standards give different heights for the same thread size, so state the standard and edition (for example DIN 1587:2014 high type) rather than 'M8 cap nut' alone.
Editorial calculation: an M8 cap nut needs a 13 mm spanner and stands 15 mm above its bearing face, so allow at least that much clearance above the joint for the nut and tool. [1]
Give the supplier the standard and edition, thread size and pitch, product grade, material (steel property class, stainless grade, brass), finish (zinc, nickel, chrome, plain), dome closed or welded type, required thread depth, packaging and quantities.
02
Check thread depth and bolt protrusion
The thread depth t is the usable depth inside the dome: 10.7–11.4 mm for an M8 and 7.71–8.29 mm for an M6 in DIN 1587. [1]
Editorial calculation: with an M8 cap nut, the bolt end must not protrude more than about 10.7 mm beyond the bearing face of the nut (the minimum t), or it will bottom against the dome before the joint is clamped; a washer under the nut reduces the available protrusion by its thickness. Choose bolt length so protrusion stays inside t with the thinnest and thickest clamped parts. [1]
Editorial note: for outdoor use specify stainless steel or a suitable plating; polished chrome or nickel suits decorative use but ask for corrosion test data.
03
Compare high and low types and choose the finish
DIN 917 (11/2021) covers the low-pattern hexagon cap nut, M4 to M72, product grade A, in coarse and fine threads. For M8 it gives a 13 mm width across flats like DIN 1587, but a height of 12 mm and a thread depth of 9.21–9.79 mm, against 15 mm and 10.7–11.4 mm for the high type; it lists about 9.5 kg per 1,000 M8 nuts in steel. [2][1]
NASA's Fastener Design Manual notes that zinc plating, including hot-dip galvanizing, is sacrificial and keeps protecting where the coating is scratched, with a useful service limit of 250 °F (its corrosion-inhibiting qualities degrade above 140 °F); nickel plating tarnishes unless followed by chromium, must be baked after plating to prevent hydrogen embrittlement, and is good to 1,100 °F; phosphate coatings are mildly protective and are oiled, waxed or painted. [3]
Editorial calculation: swapping an M8 high-type cap nut for a low type saves 15 − 12 = 3 mm of height but cuts the minimum thread depth from 10.7 to 9.21 mm, so the bolt end must stop about 1.5 mm shorter. [1][2]
04
Name the stainless alloy group for inch-series nuts
DIN 1587 fixes the shape of a metric cap nut, not its stainless alloy. For inch-series stainless nuts, ASTM F594 sets the material requirements: its scope covers stainless steel nuts 0.25 to 1.50 in. inclusive in nominal diameter, in seven alloy groups (ten austenitic, two ferritic, four martensitic and one precipitation-hardening alloy), with chemical, mechanical and corrosion-resistance tests. [4]
F594 states that bolts, hex cap screws and studs used with these nuts are covered by ASTM F593 and, unless otherwise specified, must conform to F593 and be of the same alloy group, so a stainless acorn nut and its bolt are ordered as a pair. Its values in inch-pound units are the standard; SI values in parentheses are for information only. [4]
Editorial calculation: 0.25 to 1.50 in. is 6.35 to 38.1 mm (× 25.4), so the F594 range reaches beyond the M4–M24 span of DIN 1587 at the top end but does not cover the M4 and M5 sizes. [4][1]
Fill the specification fields with values for your joint
A cap nut quotation needs values for your joint. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
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Compiled and source-reviewed by the CNFX editorial team.
Published product pages show what a company offers, not tested performance. Confirm the legal seller, factory, standard and scope for your order. How we source and review
· Page last updated 13 October 2026
Buyer questions
What sizes does DIN 1587 cover?
High-type hexagon domed cap nuts from M4 to M24, product grades A and B (2014 edition). [1]
What is the difference between DIN 1587 and DIN 917 cap nuts?
DIN 1587 is the high type; DIN 917 is the low type — for M8 the low type is 12 mm high with 9.21–9.79 mm thread depth, against 15 mm and 10.7–11.4 mm. [1][2]
Which standard covers the material of stainless inch acorn nuts?
ASTM F594 covers stainless steel nuts 0.25 to 1.50 in. in seven alloy groups, such as Group 1 (304 family) and Group 2 (316, 316L), and pairs them with F593 bolts of the same alloy group. [4]
Sources and what still needs a direct check
Sources checked 2026-10-12. The dimension table reproduces DIN 1587 (2014) values as published by a fastener standards site, and ASTM's F594 scope page gives the stainless alloy groups for inch-series nuts; dimensions and material for your nuts must come from the supplier's drawing, certificate and inspection.
