Submit an R.F.Q. — Cap Nuts
Technical sales team responds within 24 hours. EPC & project pricing across all cap nut types, grades and sizes.
R.F.Q. Submitted Successfully
Our technical team will contact you within 24 hours.
Reference: sales@rrhydraulics.com
Cap Nuts
A comprehensive engineering reference for EPC contractors, mechanical engineers, procurement heads and TPI agencies — covering cap nut (acorn nut) types, thread protection and sealing principles, dimensional standards, material grades, surface treatments and full project documentation requirements.
Cap Nut Types, Engineering Function
& Thread Protection Principles
Standard · High · Dome · Low · Knurled · Nylon · Acorn Nut
Definition and Engineering Function
A cap nut (also called an acorn nut, dome nut or blind nut) is an internally threaded fastener with a closed domed crown that encloses the projecting end of a bolt or stud. The cap nut serves three simultaneous engineering functions: thread protection — shielding the exposed bolt thread end from corrosion, mechanical damage, contamination and dirt ingress; operator safety — eliminating a sharp projecting thread end that creates laceration hazard in accessible machinery, equipment and structural installations; and aesthetic finishing — providing a visually clean, polished dome termination on exposed bolt ends in architectural metalwork, handrails, signage, consumer goods and decorative fixtures.
The protective function relies on the closed crown geometry: the dome completely encloses the bolt thread end, blocking atmospheric exposure to moisture, acids and chlorides. This is especially important on carbon steel bolt assemblies in outdoor environments, where the projecting thread end — thinner in cross-section than the bolt body — is the first point at which corrosion initiates and propagates back through the joint. A correctly specified cap nut with the bolt end engaging 50–80% of the available internal thread depth provides complete enclosure without the dome interior bottoming on the bolt end before the hex section achieves its full clamping torque.
The most common cap nut installation error is cap-out: the bolt end contacts the inside of the dome before the hex section is fully torqued. The joint appears loaded (the wrench feels resistance) but is actually carrying only the compressive reaction of the dome against the bolt end — not the intended thread preload. Cap-out occurs when the bolt thread projection above the seating surface exceeds the cap depth minus minimum thread engagement. Always measure actual bolt projection and compare against cap depth before specifying DIN 1587 (standard) or DIN 917 (high). If projection is too great for DIN 1587, specify DIN 917 or trim the bolt to the correct length.
Cap Nut Anatomy — The Four Key Dimensional Features
Cap Nut Types — Complete Engineering Descriptions
| Type | Standard | Crown | Cap Depth | Primary Function | Application |
|---|---|---|---|---|---|
| Standard Cap Nut | DIN 1587 | Compact dome | Moderate | Protection + aesthetics | General industrial, structural, automotive |
| High Cap Nut | DIN 917 | Cylinder + dome | Deep | Protection + full engagement | Long-stud assemblies, high-load applications |
| Dome Cap Nut | Custom / arch. | Full hemisphere | Shallow | Aesthetic finish | Handrails, balustrades, signage, facades |
| Low Cap Nut | Custom / reduced | Shallow dome | Reduced | Protection in low clearance | Panel assemblies, electronics |
| Knurled Cap Nut | Custom | Standard dome | Moderate | Hand-adjustable + protection | Tooling, fixtures, instrumentation |
| Nylon Cap Nut | Custom PA66 | Moulded dome | Moderate | Isolation + light protection | Food, pharma, electrical, decorative |
Dimensional Data, Governing Standards
& Thread Engagement Reference
Crown Height · Cap Depth · Width Across Flats · Overall Height
DIN 1587 — Standard Cap Nut Dimensions
| Thread | Pitch (mm) | W/F s (mm) | W/C e (mm) | Overall Height H (mm) | Hex Height h (mm) | Crown Height c (mm) | Cap Depth d (mm) |
|---|---|---|---|---|---|---|---|
| M3 | 0.50 | 5.5 | 6.35 | 8.0 | 3.0 | 5.0 | 4.5 |
| M4 | 0.70 | 7.0 | 8.08 | 10.0 | 3.8 | 6.2 | 5.5 |
| M5 | 0.80 | 8.0 | 9.24 | 12.5 | 4.5 | 8.0 | 7.0 |
| M6 | 1.00 | 10.0 | 11.55 | 15.0 | 5.5 | 9.5 | 8.5 |
| M8 | 1.25 | 13.0 | 15.01 | 19.0 | 6.8 | 12.2 | 11.0 |
| M10 | 1.50 | 16.0 | 18.48 | 23.0 | 8.4 | 14.6 | 13.5 |
| M12 | 1.75 | 18.0 | 20.78 | 27.5 | 10.8 | 16.7 | 15.5 |
| M16 | 2.00 | 24.0 | 27.71 | 35.0 | 14.8 | 20.2 | 19.0 |
| M20 | 2.50 | 30.0 | 34.64 | 44.0 | 18.0 | 26.0 | 24.5 |
| M24 | 3.00 | 36.0 | 41.57 | 52.0 | 21.5 | 30.5 | 29.0 |
DIN 1587:2000. W/F = width across flats; W/C = width across corners. Cap depth = maximum usable internal thread depth from open face to dome inner surface. Thread tolerance class 6H. All dimensions in mm.
| Thread | Pitch (mm) | W/F s (mm) | Overall Height H (mm) | Hex Height h (mm) | Cylinder Section (mm) | Cap Depth d (mm) | Max Bolt Projection (mm) |
|---|---|---|---|---|---|---|---|
| M5 | 0.80 | 8.0 | 16.0 | 4.5 | 4.0 | 11.0 | 6.0 |
| M6 | 1.00 | 10.0 | 20.0 | 5.5 | 5.0 | 14.0 | 8.0 |
| M8 | 1.25 | 13.0 | 25.0 | 6.8 | 7.0 | 18.0 | 10.5 |
| M10 | 1.50 | 16.0 | 30.0 | 8.4 | 8.5 | 21.5 | 13.0 |
| M12 | 1.75 | 18.0 | 36.0 | 10.8 | 10.0 | 25.5 | 15.5 |
| M16 | 2.00 | 24.0 | 46.0 | 14.8 | 12.0 | 31.5 | 19.5 |
| M20 | 2.50 | 30.0 | 56.0 | 18.0 | 14.0 | 38.0 | 23.5 |
DIN 917:2000. The cylinder section between hex and dome provides significantly greater cap depth vs DIN 1587 for the same thread size. Max bolt projection = cap depth − minimum thread engagement (1.0×d). Always verify actual bolt projection before ordering.
| Thread Size | TPI (UNC) | W/F (in) | Overall Height H (in) | Cap Depth d (in) | Max Bolt Projection (in) |
|---|---|---|---|---|---|
| 1/4" | 20 | 7/16 | 25/64 | 15/64 | 9/64 |
| 5/16" | 18 | 1/2 | 31/64 | 19/64 | 11/64 |
| 3/8" | 16 | 9/16 | 37/64 | 23/64 | 13/64 |
| 1/2" | 13 | 3/4 | 49/64 | 15/32 | 17/64 |
| 5/8" | 11 | 15/16 | 61/64 | 37/64 | 5/16 |
| 3/4" | 10 | 1-1/8 | 1-5/64 | 43/64 | 23/64 |
| 1" | 8 | 1-1/2 | 1-27/64 | 55/64 | 7/16 |
ASME B18.2.2-2010. Thread tolerance class 2B per ASME B1.1. Max bolt projection = cap depth − (1.0×d minimum thread engagement). Verify actual bolt projection for each application.
Governing Standards
DIN 1587 is the primary European standard for standard hex cap nuts (acorn nuts), covering M3 to M24 in steel and stainless steel. It defines width across flats, overall height, crown height and thread tolerance class 6H. DIN 917 covers high cap nuts with the extended cylindrical section, M5 to M20, providing greater internal thread depth for long-projection bolt assemblies. Both standards are widely referenced in European EPC, machinery and architectural specifications. ASME B18.2.2 covers inch-series hex cap nuts from 1/4" to 1" UNC.
L_eng_min = 1.0 × d // Minimum thread engagement = 1× nominal diameter for ISO 6H/6g
// WORKED EXAMPLE: M10 DIN 1587 (d_cap = 13.5 mm) vs M10 DIN 917 (d_cap = 21.5 mm)
// DIN 1587: L_proj_max = 13.5 − 10.0 = 3.5 mm // bolt projection must not exceed 3.5 mm
// DIN 917: L_proj_max = 21.5 − 10.0 = 11.5 mm // DIN 917 accommodates longer stud projections
// If measured bolt projection > DIN 1587 limit → SPECIFY DIN 917 HIGH CAP NUT
| Thread | DIN 1587 Cap Depth (mm) | DIN 1587 Max Proj. (mm) | DIN 917 Cap Depth (mm) | DIN 917 Max Proj. (mm) | Use DIN 917 When Projection > |
|---|---|---|---|---|---|
| M6 | 8.5 | 2.5 | 14.0 | 8.0 | 2.5 mm |
| M8 | 11.0 | 3.0 | 18.0 | 10.5 | 3.0 mm |
| M10 | 13.5 | 3.5 | 21.5 | 13.0 | 3.5 mm |
| M12 | 15.5 | 3.5 | 25.5 | 15.5 | 3.5 mm |
| M16 | 19.0 | 3.0 | 31.5 | 19.5 | 3.0 mm |
| M20 | 24.5 | 4.5 | 38.0 | 23.5 | 4.5 mm |
"Max Proj." = cap depth − (1.0×d minimum thread engagement). Specify DIN 917 when the measured bolt thread projection above the seating surface exceeds the DIN 1587 max projection for the same thread size.
Material Grades, Mechanical Properties
& Surface Treatments
Zinc Plate · HDG · Decorative Chrome · Passivation · Electropolish
| Grade | Standard | Proof Load (MPa) | Hardness | Temp (°C) | Corrosion | Key Application |
|---|---|---|---|---|---|---|
| CS Class 6 | ISO 898-2 / DIN 1587 | 600 | ≤302 HV | −50 to +300 | Low | General industrial, structural, indoor/coated outdoor |
| SS 304 (A2-70) | ISO 3506-2 | 700 | ≤320 HV | −196 to +650 | High | Outdoor, marine-adjacent, food equipment, handrails |
| SS 316L (A4-70) | ISO 3506-2 | 700 | ≤320 HV | −196 to +650 | Very High | Offshore, chemical plant, chloride, food-contact |
| Duplex 2205 | ASTM A182 F51 | — | ≤310 HB | −50 to +300 | Very High | Offshore structural, sour service, subsea covers |
| Super Duplex | ASTM A182 F53 | — | ≤310 HB | −50 to +280 | Extreme | High-chloride subsea, severe sour service |
| Brass (CuZn37) | BS 2872 / ASTM B16 | — | ~120 HV | −50 to +180 | Good | Plumbing, marine, architectural decorative |
| Zinc Die-Cast | ASTM B86 / EN 1774 | — | ~90 HB | −20 to +120 | Moderate | Light-duty aesthetic, automotive trim, covers |
| Inconel 625 | ASTM B564 | — | ≤241 HB | −200 to +980 | Extreme | High-temp, chemical process, turbine stud covers |
| PA66 GF30 Nylon | ISO 527 / custom | — | ~100 Shore D | −40 to +120 | Excellent | Food, pharma, electrical isolation, decorative |
| PVDF (Kynar) | ASTM D3222 | — | ~80 Shore D | −40 to +150 | Extreme | High-purity chemical, semiconductor, ultraclean |
Material Selection and Application Guidance
Carbon steel cap nuts in DIN 1587 / DIN 917 with zinc electroplate (ASTM B633 SC3, minimum 13 µm) or hot-dip galvanise (ASTM A153 Class C, 45–86 µm) are the standard for outdoor structural, industrial and civil engineering applications. For decorative architectural applications requiring a chrome appearance, bright decorative chrome (ASTM B456, Cu+Ni+Cr) over zinc electroplate is the standard finish. Critical note for HDG cap nuts: hot-dip galvanising deposits zinc inside the thread bore and dome interior; the internal thread must be re-tapped after galvanising per DIN 267-10 to restore the 6H thread tolerance. Specify "HDG + re-tapped bore" on the purchase order.
Stainless steel 316L cap nuts with passivation per ASTM A380 are the standard for offshore, coastal, chemical plant and food-adjacent applications. For food-contact and pharmaceutical environments (3-A Sanitary Standards, EHEDG), electropolish per ASTM A967 is applied to achieve Ra ≤ 0.8 µm — eliminating micro-crevices on the dome surface where bacteria could harbour. The sealed dome of an electropolished SS cap nut presents a smooth, cleanable, crevice-free surface that cannot trap process fluid, cleaning chemicals or sanitising agents.
Cap nuts are among the most visually prominent fasteners in architectural metalwork — handrails, balustrades, glass facades, signage and furniture. For polished architectural SS cap nuts the standard sequence is: SS 304/316 blank → tumble deburr → electro-barrel polish to Ra ≤ 0.4 µm → passivation per ASTM A380. For brass cap nuts in plumbing and marine architectural fittings, chrome-plating per ASTM B456 Service Condition SC2 provides a bright durable finish. Specify all units from the same manufacturer batch — minor dimensional and finish variations between batches are visible in polished assemblies at close inspection range.
| Finish | Standard | Thickness (µm) | Salt Spray (hrs) | Max Temp (°C) | Notes & Application |
|---|---|---|---|---|---|
| Plain / Bare | — | — | <24 | — | Indoor dry storage only; VCI for export |
| Black oxide + oil | MIL-DTL-13924 | 0.5–2 | 24–48 | 150 | Indoor machinery; dark aesthetic finish |
| Zinc electroplate SC3 | ASTM B633 SC3 | 13 min | 200 | 150 | Standard outdoor industrial carbon steel |
| Yellow chromate + ZP | ASTM B633 + ISO 4520 | 13–20 | 240 | 150 | Improved salt spray vs. clear zinc; industrial outdoor |
| Decorative chrome | ASTM B456 SC2 | Ni 10 + Cr 0.3 | 144 | 200 | Architectural, plumbing fittings, automotive trim |
| Hot-dip galvanise | ASTM A153 Class C | 45–86 | 500+ | 200 | Outdoor structural; re-tap internal thread bore after HDG |
| Zinc-nickel alloy | ASTM B841 | 8–15 | 500–1000 | 200 | Offshore-adjacent, coastal outdoor |
| Dacromet / Geomet | ISO 10683 | 8–12 | 720+ | 300 | High-temp, high-strength; no H₂ embrittlement risk |
| SS 316L passivation | ASTM A380 | N/A (passive) | 1000+ | 870 | Offshore, food, pharma, architectural SS |
| Electropolish + passivation | ASTM A967 + A380 | −5 to −25 µm | 1000+ | 870 | Food-contact, pharmaceutical, cleanroom, 3-A EHEDG |
| Lacquer (clear/colour) | OEM spec | 15–40 | 200–400 | 80 | Brass decorative, colour-coding, retail display |
Hot-dip galvanised cap nuts: internal thread bore must be re-tapped post-galvanising to restore 6H tolerance. Specify "HDG + re-tapped bore" explicitly in purchase order.
Inspection, QC Protocols, Applications
& Export Documentation
Structural · Architectural · Offshore · Food · Plumbing
Inspection and Quality Control
Three dimensions are critical for cap nut QC and must be verified on every production batch: (1) Overall height — confirms whether the unit is DIN 1587 (standard) or DIN 917 (high), establishing the cap depth available; (2) Cap depth — measured from the open face to the inside of the dome using a calibrated depth micrometer or pin gauge; this is the parameter directly determining cap-out risk; (3) Thread gauge per Go/No-Go ISO 1502 or ASME B1.2 — confirming the internal thread is within tolerance class 6H and free of damage, debris or zinc blockage (in the case of HDG cap nuts post-re-tap). Cap depth must be stated as a mandatory measured characteristic in the dimensional inspection report for all cap-out-critical applications.
The dome crown must be inspected for cracks, porosity, incomplete closure and seam defects at the crown apex — the most common manufacturing defect in cast, die-cast and stamped cap nuts. A cracked or porous crown does not provide the intended thread end protection and will allow moisture ingress through the defect. For stainless steel cap nuts in offshore or pressure-related applications, liquid penetrant inspection (LPI/PT) per ASTM E165 is specified on a sample coupon per batch. For forged carbon steel cap nuts in structural applications, visual inspection plus magnetic particle inspection (MPI/MT) per ASTM E709 of the crown and transition radius is required.
Type 3.1: Minimum for all EPC, OEM and structural project supply. Must state material standard, grade, heat/lot number, chemical analysis, mechanical test results and heat treatment condition. Cap depth and crown height must be explicitly stated as measured characteristics in the MTC where cap-out risk is critical. Type 3.2: Co-validated by purchaser-nominated TPI body (Bureau Veritas, DNV, Lloyds, SGS, TÜV). Mandatory for offshore, NACE, nuclear-adjacent and pressure vessel applications.
Positive Material Identification by XRF or OES spectrometry is mandatory for all stainless steel, duplex and super duplex and exotic alloy cap nuts to confirm alloy composition matches the MTC. For polished architectural SS cap nuts, PMI also verifies that SS 316L (not SS 304) has been supplied — SS 304 and SS 316L are visually identical after polishing, and the distinction is critical for coastal and chloride-exposed installations where SS 304 would corrode within months.
Applications by Industry
Carbon steel cap nuts, zinc-plated or hot-dip galvanised, are used on exposed structural bolt ends in steel framing, column base plates, road bridges, pedestrian bridges and precast concrete panel connections. They prevent thread end corrosion that initiates at an unprotected projecting bolt end and wicks back through the joint over time. In pedestrian-accessible areas, cap nuts provide mandatory injury prevention by eliminating sharp thread ends at waist and head height. For coastal and marine-adjacent structural applications, SS 316L cap nuts with passivation are specified.
Polished SS 316L or chrome-plated brass cap nuts are standard on stainless handrails, glass balustrade systems, architectural louvres, cable facade tensioner end-fittings, signage mounting systems and curtain wall fixing systems. The smooth dome profile and high-polish finish produce a visually clean, premium appearance consistent with the surrounding architectural metalwork. Electropolished SS cap nuts on exposed handrail posts satisfy ISO 9227 Salt Spray Class C5-M corrosion requirements for coastal and industrial atmospheric exposure, providing a 25+ year service life without visible corrosion.
Cap nuts on offshore and process plant instrumentation stud terminals, cable gland threaded protrusions, pipe support stud ends and equipment drain plug extensions protect thread ends from sour atmosphere, chloride spray and mechanical damage during maintenance. Duplex 2205 or super duplex cap nuts with EN 10204 3.2 MTC, PMI and TPI witness are standard for offshore sour service zones. NACE MR0175 / ISO 15156 hardness compliance (≤ 22 HRC / 237 HB) applies to carbon and low-alloy steel cap nuts in H₂S service zones.
Electropolished SS 316L cap nuts on process equipment, mixing vessel bolt ends, filter housing bolting and conveyor structure exposed fasteners satisfy 3-A Sanitary Standards and EHEDG cleanliness requirements for food-contact fastener design. The sealed dome prevents food product, cleaning chemicals or CIP solutions from lodging in the thread recess. NSF/ANSI 51 certification of the cap nut material is required for direct food-contact applications. Ra ≤ 0.8 µm surface roughness (3-A minimum) or Ra ≤ 0.4 µm (EHEDG recommended) is mandatory for hygienic-grade cap nuts.
Brass or zinc-plated carbon steel cap nuts on threaded pipe stud ends, valve packing gland studs, heat exchanger cover bolting and HVAC duct hanger threaded rod provide corrosion protection and a clean installation finish. In potable water systems per EN 1717 and BS 6700, the cap nut material must be approved for water contact — brass per BS 2872 CW614N or SS 316L are the approved materials. Zinc alloy die-cast cap nuts are not approved for potable water contact applications.
Export Packaging and Preservation
- Cap nuts packed in heat-sealed polypropylene bags, batch-labelled with PO number, type (DIN 1587 / DIN 917 / ASME B18.2.2), grade, thread size, heat/lot number and quantity
- VCI (Volatile Corrosion Inhibitor) poly liner for plain and black-oxide carbon steel cap nuts intended for sea freight or long-term site storage
- Polished SS and architectural finish cap nuts individually wrapped in tissue or separated by foam to prevent dome surface contact marking during transit
- Bags packed in double-wall corrugated cartons with foam or kraft paper void fill; open face (thread bore) oriented downward to prevent moisture ingress into dome interior during transit
- Cartons secured on ISPM-15 heat-treated timber pallets with stretch wrap and minimum two steel strapping bands per pallet layer
- MTC, cap depth inspection certificate, crown integrity inspection record, thread gauge certificate and all project documents in waterproof sealed envelope attached to exterior of each crate
- Outer crate labels: PO number, item tag, material grade, type (DIN 1587 / DIN 917), standard, heat number, quantity, gross weight, country of origin, handling instructions
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 for offshore / NACE / pressure vessel applications |
| 02 | Dimensional Inspection Report | DIN 1587 / DIN 917 / B18.2.2 | AQL 1.0 per ISO 2859; cap depth as mandatory measured characteristic |
| 03 | Thread Gauge Certificate | ISO 1502 / ASME B1.2 | Go/No-Go per heat lot; post-re-tap re-check for HDG cap nuts |
| 04 | Cap Depth Gauge Report | DIN 1587 / project spec | Depth micrometer / pin gauge — mandatory for cap-out-critical applications |
| 05 | Crown Integrity Inspection | ASTM E165 (LPI) / E709 (MPI) | LPI for SS offshore; MPI for CS forged structural / offshore |
| 06 | Proof Load Test Report | ISO 898-2 / ASTM A194 | Per heat/lot — mandatory for structural grade cap nuts |
| 07 | Hardness Test Report | ISO 6507 / ISO 6506 | Cross-section mapping for NACE sour service cap nuts |
| 08 | PMI Report (XRF / OES) | Project specification | 100% of SS, duplex, alloy and exotic grade cap nuts |
| 09 | Surface Coating Certificate | ASTM A153 / B633 / A380 / A967 | Required for all finishes — Ra value stated for food-grade electropolish |
| 10 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include cap nut manufacture |
| 11 | Packing List / Traceability Record | Project MRB format | Heat-number-level traceability to PO line item |
| 12 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports cap nuts (acorn nuts) in all types — DIN 1587 (standard), DIN 917 (high), dome, low-profile, knurled and nylon — in carbon steel, stainless steel 304/316L, duplex & super duplex, brass and Inconel. Full EN 10204 3.1/3.2 MTC documentation, ISO 9001:2015 QMS, TPI witness by BV/DNV/Lloyds/SGS/TÜV accommodated. M3–M24 metric and 1/4"–1" inch. Electropolished SS 316L cap nuts with Ra ≤ 0.4 µm for food/pharma applications. NACE MR0175-compliant grades available with full cross-section hardness certification. 48-hour express dispatch on in-stock sizes.