MIL-DTL-32258 — Self-Locking Nut, Nickel-Copper Alloy

MIL-DTL-32258 covers 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 Issued June 28, 2007, it superseded MIL-N-25027/1A.

Type: Self-Locking Heavy Hex Nut Material: Nickel-Copper Alloy (Monel) Issued: 28 Jun 2007 Supersedes: MIL-N-25027/1A FSC Class: 5310

At a Glance

Full Title

Nut, Self-Locking, Ring-Type Insert, Heavy Hex, Ni-Cu Alloy

Diameter Range

1/4" – 2½"

Thread Series

Coarse Pitch & 8-Pitch (UNJ)

Thread Class

3B, Right-Hand

Temp Ratings

250°F (NICU) / 450°F (NICUV)

Locking Element

Ring-Type Non-Metallic Insert

Optional Coating

Dry Film Lubricant, SAE AS5272 Type I

Application

Shipboard, Level I Piping Systems

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 nuts are engineered to resist fatigue cracking and loosening caused by vibration.

Both coarse-pitch and eight-pitch thread series are covered, in nominal diameters between 1/4" and 2½". The specification superseded MIL-N-25027/1A on June 28, 2007, and the resulting nuts are dimensionally and functionally interchangeable with the equivalent MIL-N-25027/1 parts.

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.
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).

Part Number / Temperature Cross-Reference

MIL-DTL-32258 nuts are interchangeable with MIL-N-25027/1 by nominal diameter and temperature rating.

Nominal Diameter

Temperature Rating

A 0.500" nut at 250°F cross-references as M32258-UCA08 under MIL-DTL-32258, equivalent to legacy M25027/1-08C.

Key Requirements

Requirement	Detail
Thread Class	3B, right-hand, UNJ profile with controlled root radius
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)

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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 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.

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.

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.

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