Posted by BONCOO FASTENERS on May 8th 2026
The Complete Guide to 304 (18-8) Stainless Steel Nuts: Grades, Standards, and Selection
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## Why 304 Stainless Steel Nuts Are the Industry Standard
If you're specifying fasteners for an outdoor structure, a food processing facility, or a coastal installation, **304 stainless steel nuts** are likely your first choice — and for good reason.
The **18-8 designation** (18% chromium, 8% nickel) refers to the base composition of AISI 304 austenitic stainless steel. This specific alloy delivers a combination of corrosion resistance, mechanical strength, and cost-effectiveness that makes it the most widely used stainless fastener material in North America.
But not all "304 nuts" are created equal. Understanding the **grade designations**, **DIN/ISO standards**, and **dimensional specifications** is critical to selecting the right nut for your application — and avoiding costly failures down the line.
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## 18-8 vs. 304 vs. A2: What's the Difference?
These three terms are often used interchangeably, but they refer to different classification systems:
| Term | System | Meaning |
|------|--------|---------|
| **18-8** | Composition-based | 18% Chromium, 8% Nickel minimum |
| **304** | AISI/SAE designation | Specific grade within the 18-8 family |
| **A2** | DIN/ISO material code | Metric designation for 304-equivalent stainless |
**Key takeaway:** When a supplier lists a nut as "18-8 (304) Stainless Steel" or "A2-70," they are referring to the same base material. The difference lies in the **strength grade**:
- **A2-70**: Minimum tensile strength of **700 MPa** (≈ 101,500 psi). This is the standard grade for 304 stainless hex nuts.
- **A2-80**: Cold-worked to achieve **800 MPa** (≈ 116,000 psi). Higher strength, but more limited availability.
At BOMCOO, all our 304 stainless nuts are rated **A2-70** and manufactured to recognized DIN standards — with **Material Test Certificates (MTC)** available upon request.
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## The 4 Most Common 304 Stainless Steel Nut Types
### 1. DIN 934 / ISO 4032 — Hex Full Nuts (Style 1)
**The workhorse of stainless fastening.**
- **Width across flats (AF)**: Standard hex dimension per DIN/ISO
- **Applications**: General-purpose structural bolting, machinery assembly, construction
- **Strength**: A2-70 grade standard
- **Availability**: M3 through M24 in stock
This is the nut you reach for when you need a reliable, corrosion-resistant hex nut for any standard bolt application. It's the most commonly stocked and most competitively priced stainless nut in the industry.
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### 2. DIN 934 / ISO 4033 — Hex Full Nuts (Style 2)
**The heavier-duty sibling.**
- **Key difference**: Thicker body than Style 1, providing greater thread engagement and higher proof load
- **Applications**: High-strength structural connections, heavy machinery, bridge and infrastructure bolting
- **When to choose Style 2 over Style 1**: When your bolt is Grade 8.8 or higher and you need a nut with matching load capacity
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### 3. DIN 985 / ISO 7040 — Nylon Insert Lock Nuts (Nyloc)
**Vibration resistance meets corrosion resistance.**
- **Key feature**: Embedded nylon ring creates a friction lock on the bolt thread
- **Applications**: Automotive, marine, HVAC, and any assembly subject to vibration or dynamic loads
- **Temperature limit**: Nylon ring effective up to approximately **120°C (250°F)**. Beyond this, consider all-metal lock nuts
- **Reusability**: Not recommended — the nylon ring compresses after initial installation
**Critical note for marine applications:** The nylon insert is not stainless steel. In prolonged saltwater exposure, the insert may degrade. For pure marine-grade locking, consider **316 (A4) stainless lock nuts** or **all-metal DIN 980 lock nuts**.
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### 4. DIN 6923 / ISO 4161 — Hex Flange Nuts
**Serrated grip, no washer needed.**
- **Key feature**: Integrated flange with serrated bearing surface distributes load and resists loosening
- **Applications**: Automotive, structural steel, heavy equipment — anywhere you'd normally use a nut + washer combination
- **Advantage**: Reduces parts count, saves assembly time, and provides built-in anti-loosening function
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## Metric vs. Inch: A Quick Reference for US Buyers
Many US procurement professionals are more familiar with inch-series nuts (SAE J995, ASME B18.2.2). Here's a practical cross-reference for the most requested metric sizes:
| Metric Size | Thread Pitch (mm) | Approx. Inch Equivalent | Wrench Size (mm) |
|-------------|-------------------|--------------------------|-------------------|
| M6 | 1.0 | ≈ 1/4" | 10mm |
| M8 | 1.25 | ≈ 5/16" | 13mm |
| M10 | 1.5 | ≈ 3/8" | 17mm |
| M12 | 1.75 | ≈ 1/2" | 19mm |
| M16 | 2.0 | ≈ 5/8" | 24mm |
| M20 | 2.5 | ≈ 3/4" | 30mm |
**Note:** These are approximate equivalents for reference only. Metric and inch threads are **not interchangeable**. Always verify thread specifications before procurement.
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## When 304 Is Not Enough: Know Your Upgrade Path
304 (A2) stainless steel handles most indoor and moderate outdoor environments with ease. But there are scenarios where you should upgrade to **316 (A4) stainless**:
| Environment | 304 (A2) Performance | 316 (A4) Performance |
|-------------|----------------------|----------------------|
| **Dry indoor** | Excellent | Overkill |
| **Outdoor, rain exposure** | Good — minor surface staining possible | Excellent |
| **Coastal / salt spray** | Moderate — pitting risk over time | Excellent |
| **Chemical / acid exposure** | Limited | Superior (2-3% molybdenum addition) |
| **Food processing / washdown** | Acceptable (with regular cleaning) | Preferred (CFDA compliance) |
| **High temperature (>400°C)** | Risk of carbide precipitation | Better stability |
**Bottom line:** If your installation is within 5 miles of a coastline, in a chemical plant, or in a food-grade washdown zone — specify **316 (A4-80)** nuts instead of 304.
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## Why Material Traceability Matters (The MTC Question)
In industrial procurement, a nut without a **Material Test Certificate (MTC)** is a nut without a pedigree.
An MTC (also called a "mill certificate" or "certificate of conformance") documents:
- The **chemical composition** of the steel batch (Cr, Ni, C, Mn content)
- The **mechanical properties** tested (tensile strength, yield strength, hardness)
- The **heat number** for full batch traceability
- The **standard compliance** (DIN 934, ISO 4032, etc.)
**Why this matters for your project:**
1. **QA documentation**: Engineers and inspectors require MTCs for structural, marine, and food-grade installations
2. **Liability protection**: If a fastener fails, the MTC proves you specified and procured the correct material
3. **Audit compliance**: ISO 9001, AS9100, and many industry-specific audits require material traceability
**At BOMCOO, MTCs are included with every order — not as a premium add-on, but as a standard part of our product.** Because in B2B fastener procurement, traceability isn't optional. It's the minimum.
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## Quick Selection Checklist
Before you place your next nut order, run through this 5-point verification:
- [ ] **Material confirmed**: 304 (A2-70) for general use, or 316 (A4-80) for harsh environments?
- [ ] **Standard specified**: DIN 934 for hex nuts, DIN 985 for nyloc, DIN 6923 for flange?
- [ ] **Thread pitch correct**: Standard pitch (M6×1.0, M8×1.25, M10×1.5) or fine pitch?
- [ ] **Wrench size verified**: Do your tools match the AF dimension?
- [ ] **MTC available**: Can your supplier provide material traceability documentation?
If any of these are unclear, **contact our technical team** — we'll help you specify the exact part before you order.
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## Ready to Specify?
BOMCOO Fasteners stocks a full range of **304 (18-8) stainless steel metric nuts** — from M3 to M24 — manufactured to DIN standards with MTC documentation included.
**For standard quantities:** [Shop BOMCOO Nuts on our website](/categories/nuts/)
**For bulk orders (1,000+ pcs) or custom specifications:** [Request a Quote](mailto:info@bomcoofasteners.com) or visit our [Amazon Flagship Store](https://www.amazon.com/stores/BOMCOOFasteners) for smaller quantities with Prime shipping.
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*BOMCOO Fasteners — Stainless Steel Specialists. DIN & ISO Compliant. MTC Included. USA Domestic Shipping.*