MIL-DTL-32258 — Self-Locking Nut, Nickel-Copper Alloy
MIL-DTL-32258 is the U.S. defense specification covering ring-type, self-locking, heavy hex nuts machined from nickel-copper alloy (Monel), with a controlled root radius UNJ thread profile, for general shipboard applications including Level I piping systems. Issued June 28, 2007, it superseded MIL-N-25027/1A and remains the current standard for Monel self-locking nuts across U.S. Navy and allied shipbuilding programs.
At a Glance
Overview
MIL-DTL-32258 covers ring-type, self-locking, heavy hex nuts machined from nickel-copper alloy (Monel), with screw threads cut to a controlled root radius UNJ thread profile, for use in general shipboard applications including Level I piping systems and associated components. The ring-type non-metallic insert is compressed into the top of the nut bore, gripping the mating bolt or stud threads to resist loosening from vibration and thermal cycling without relying on torque alone.
Both coarse-pitch and eight-pitch thread series are covered, in nominal diameters between 1/4" and 2½", to Class 3B, right-hand fit per ASME B1.15. The specification superseded MIL-N-25027/1A on June 28, 2007, and the resulting MIL-DTL-32258 nuts are dimensionally and functionally interchangeable with the equivalent legacy MIL-N-25027/1 parts, which simplifies re-procurement on older hull and piping drawings that still reference the earlier designator.
Nickel-copper alloy (commercially known as Monel 400/K-500) was selected as the base material for its resistance to seawater corrosion, its low magnetic permeability, and its retained strength at elevated temperature — all properties that matter in shipboard piping, seawater systems, and other marine environments where standard steel or stainless fasteners would corrode or gall prematurely.
- 250°F-rated nuts are permanently marked with material symbol "NICU"; 450°F-rated nuts are marked "NICUV" — both markings applied per MIL-STD-792.
- Nuts are marked on the wrenching surface with the manufacturer's identification symbol, material symbol, and lot number for full traceability.
- An optional graphite-free dry film lubricant coating (SAE AS5272 Type I) may be specified, with nickel or copper plate as an optional pretreatment.
- Tensile strength is verified per NASM1312-8 (Fastener Test Methods, Method 8 — Tensile Strength), and locking torque is checked over multiple installation-removal cycles to confirm the insert retains its self-locking function.
Part Number / Temperature Cross-Reference
MIL-DTL-32258 nuts are interchangeable with MIL-N-25027/1 by nominal diameter and temperature rating.
Key Requirements
| Requirement | Detail |
|---|---|
| Thread Class | 3B, right-hand, UNJ profile with controlled root radius, per ASME B1.15 |
| Marking | Manufacturer symbol, material symbol (NICU/NICUV), lot number — permanent per MIL-STD-792 |
| Tensile Test | NASM1312-8 (axial tension at room temperature) |
| Surface Discontinuities | ASTM F812 |
| Material Chemistry | ASTM B880 (Ni/Ni-alloy/Co-alloy chemical check analysis) |
| Optional Coating | SAE AS5272 Type I dry film lubricant, nickel or copper pretreatment |
Compliance & Sourcing References
Verify full specification text and procurement history through official government sources before finalizing drawings or purchase orders.
The full MIL-DTL-32258 specification text, including revisions and amendments, is publicly available through the Defense Logistics Agency's ASSIST QuickSearch portal. Procurement history, NSN cross-references, and cage code data for MIL-DTL-32258 hardware can also be checked via DLA DIBBS. Ananka Fasteners manufactures to the current, active revision on file and supplies full Mill Test Certification with every shipment.
MIL-DTL-32258 nuts are interchangeable with MIL-N-25027/1 nuts of the same diameter and temperature rating. It's also related to NASM25027, which covers self-locking nuts more broadly across multiple base materials and temperature classes.
MIL-DTL-32258 — Frequently Asked Questions
What does the "NICU" marking mean?
It's the material symbol permanently marked on MIL-DTL-32258 nuts rated to 250°F. Nuts rated to 450°F are marked "NICUV" instead — both identify the nickel-copper alloy composition and temperature class at a glance.
Is MIL-DTL-32258 interchangeable with MIL-N-25027/1?
Yes — the specification states that nuts covered by MIL-N-25027/1 are interchangeable with those covered by MIL-DTL-32258 of the equivalent nominal diameter and temperature rating, so existing drawings referencing the older number do not need to be redrawn.
What thread profile does MIL-DTL-32258 use?
A UNJ thread profile with a controlled root radius, Class 3B fit, right-hand, covering both coarse-pitch and eight-pitch thread series per ASME B1.15.
Why is nickel-copper alloy (Monel) used for these nuts?
Monel resists seawater and marine-atmosphere corrosion far better than standard steel or many stainless grades, holds up to elevated shipboard operating temperatures, and has low magnetic permeability — all reasons it's specified for Level I shipboard piping hardware.
Does Ananka Fasteners supply Mill Test Certification with MIL-DTL-32258 nuts?
Yes, every shipment includes full Mill Test Certification and material traceability documentation covering chemistry, tensile performance, and lot marking, in line with the specification's testing requirements.
Need MIL-DTL-32258 Monel Nuts?
Ananka Fasteners manufactures MIL-DTL-32258 self-locking nickel-copper alloy nuts with full material traceability and Mill Test Certification.
Prefer email? sales@anankafasteners.com
