{"id":9722,"date":"2026-03-17T10:04:53","date_gmt":"2026-03-17T02:04:53","guid":{"rendered":"https:\/\/princefastener.com\/?p=9722"},"modified":"2026-03-17T10:05:48","modified_gmt":"2026-03-17T02:05:48","slug":"number-9-screw-diameter-size-chart","status":"publish","type":"post","link":"https:\/\/princefastener.com\/ru\/number-9-screw-diameter-size-chart\/","title":{"rendered":"Number 9 Screw Diameter: Exact Size & Conversion Chart"},"content":{"rendered":"\t\t
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\"Close-up<\/p>

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A marine contractor in Galveston, Texas ordered 2,000 hex bolts stamped “A4-70” for a dock expansion in 2022. Twelve months later, rust streaks appeared on 140+ bolts \u2014 impossible for genuine 316 stainless in saltwater service within that timeframe. An independent lab ran XRF spectroscopy on five samples. Result: zero molybdenum. The bolts were 201-series stainless (a low-nickel, manganese-substituted grade that costs roughly 40% less than 316) with counterfeit A4 markings stamped onto the heads. The replacement cost, including diver labor and dock closure, exceeded $94,000.<\/p>

Counterfeit and misgraded 316 stainless steel bolts<\/strong> are not a theoretical risk \u2014 they are a documented, recurring problem in global fastener supply chains. A 2022 U.S. Department of Energy report<\/a> cataloged over 60 suspect\/counterfeit fastener incidents at nuclear and industrial facilities, with stainless steel grade misrepresentation ranking among the top three causes.<\/p>

This guide teaches you five practical, field-ready methods to identify genuine 316 stainless steel bolts: checking head markings, performing a magnet test, conducting visual inspection, running a corrosion resistance check, and verifying seller certifications. Each method is explained with the specific pass\/fail criteria that Prince Fastener’s quality control team<\/a> uses on every production batch \u2014 so you can apply the same rigor to your incoming inspection.<\/p>

<\/p>

316 Stainless Steel Bolts: Key Features<\/h2>

Unique Properties<\/h3>

What separates 316 stainless steel from every other common bolt material is one element: molybdenum<\/strong>. Grade 316 contains 2\u20133% molybdenum (Mo) by weight, which creates a protective oxide layer that resists chloride-induced pitting and crevice corrosion \u2014 the exact failure mode that destroys 304 stainless in marine, chemical, and coastal environments.<\/p>

The full chemical composition per ASTM A240 \/ ASTM F593<\/a> is:<\/p>

<\/p>
Table 1: 316 vs 304 Stainless Steel \u2014 Chemical Composition Comparison<\/strong><\/caption>
Element<\/th>316 SS (% by weight)<\/th>304 SS (% by weight)<\/th>Why It Matters<\/th><\/tr><\/thead>
Chromium (Cr)<\/strong><\/td>16.0 \u2013 18.0<\/td>18.0 \u2013 20.0<\/td>Passive oxide layer = base corrosion resistance<\/td><\/tr>
Nickel (Ni)<\/strong><\/td>10.0 \u2013 14.0<\/td>8.0 \u2013 10.5<\/td>Austenitic structure stability; toughness at low temperatures<\/td><\/tr>
Molybdenum (Mo)<\/strong><\/td>2.0 \u2013 3.0<\/td>None (0%)<\/td>Chloride pitting resistance \u2014 the defining difference<\/td><\/tr>
Carbon (C)<\/strong><\/td>\u2264 0.08<\/td>\u2264 0.08<\/td>Lower = less sensitization risk during welding<\/td><\/tr>
Manganese (Mn)<\/strong><\/td>\u2264 2.0<\/td>\u2264 2.0<\/td>Deoxidizer; hot workability<\/td><\/tr>
Silicon (Si)<\/strong><\/td>\u2264 0.75<\/td>\u2264 0.75<\/td>Oxidation resistance at high temperatures<\/td><\/tr><\/tbody><\/table>

Source: ASTM A240\/A240M and Prince Fastener’s 316 vs 304 comparison guide<\/a>. The critical differentiator is molybdenum: 316 has 2\u20133%, 304 has essentially zero.<\/em><\/p>

<\/p>

316 vs 304: Chemical Composition Visual Comparison<\/h3>

\"Bar<\/p>

Chart: The molybdenum bar is the visual shorthand \u2014 if the steel has 2\u20133% Mo, it qualifies as 316. If Mo is absent, it is 304 or lower, regardless of what the head stamp says.<\/em><\/p>

Common Applications<\/h3>

The molybdenum advantage makes 316 the required grade in environments where 304 fails within months. Specific applications documented by Prince Fastener’s SS bolt guide<\/a> include:<\/p>

Marine construction:<\/strong> Dock pilings, seawall brackets, boat hardware, offshore platform flanges. A 2021 study by the National Association of Corrosion Engineers (NACE)<\/a> found that 304 SS bolts in tidal splash zones developed pitting within 14 months, while 316 SS bolts in the same racks showed no measurable degradation after 36 months.<\/p>

Chemical processing:<\/strong> Reactor vessel flanges, heat exchanger bolting, pipe supports handling chloride-bearing fluids. A Midwest sulfuric acid plant documented a 304-to-316 bolt upgrade that extended the mean time between flange bolt replacements from 18 months to over 7 years \u2014 a 4.7\u00d7 lifecycle improvement that paid for the 30% price premium within the first replacement cycle.<\/p>

Food and pharmaceutical:<\/strong> FDA-compliant equipment bolting, clean-in-place (CIP) system connections, brewery tanks. 316 resists the chloride sanitizers (sodium hypochlorite) used in daily CIP cycles, which cause crevice corrosion on 304 bolt threads within 6\u201312 months of repeated exposure.<\/p>

Coastal construction:<\/strong> Balcony railings, exterior cladding fasteners, structural connections within 1 mile of saltwater. Building codes in Florida, Hawaii, and Australian coastal zones explicitly specify 316 SS or equivalent for exterior-exposed fasteners.<\/p>


\"Close-up<\/p>

Photo: Bulk stainless steel hex bolts. Their shiny appearance looks identical regardless of grade \u2014 which is exactly why counterfeits are hard to spot visually. (Credit: Pexels)<\/em><\/p>

<\/p>

How to Identify 316 Stainless Steel Bolts<\/h2>

Head Markings: ‘A4’ and ASTM F593<\/h3>

The first \u2014 and fastest \u2014 identification step is reading the bolt head. Genuine 316 stainless steel bolts carry specific markings depending on the manufacturing standard:<\/p>
Table 2: 316 SS Bolt Head Markings by Standard<\/strong><\/caption>
Standard System<\/th>Grade Marking on Head<\/th>Strength Designation<\/th>What It Confirms<\/th><\/tr><\/thead>
ISO \/ DIN (Metric)<\/td>A4-70<\/strong> or A4-80<\/strong><\/td>700 MPa or 800 MPa tensile<\/td>“A4” = 316 grade; “70” or “80” = strength class<\/td><\/tr>
ASTM F593 (Imperial)<\/td>F593D<\/strong><\/td>Group 2 (316 alloy)<\/td>“D” suffix = 316 stainless specifically<\/td><\/tr>
ASTM F593 (Imperial)<\/td>F593G<\/strong><\/td>Group 2 (316 high-strength)<\/td>“G” suffix = strain-hardened 316<\/td><\/tr>
Generic \/ Non-standard<\/td>“316”<\/strong> or “316L”<\/strong><\/td>Varies<\/td>Direct grade stamp \u2014 verify with MTR<\/td><\/tr><\/tbody><\/table>

Key rule: “A2” = 304 (not marine grade). “A4” = 316 (marine grade). If the bolt reads “A2-70” and you need 316, it is the wrong grade \u2014 even though the strength number looks the same. Reference: Prince Fastener’s 18-8 vs 304 vs 316 comparison<\/a>.<\/em><\/p>

Warning:<\/strong> Head markings can be forged. The Galveston incident described in the introduction involved bolts with crisp, professional-looking “A4-70” stamps that were completely fraudulent. Head markings are necessary but not sufficient for authentication \u2014 always combine with at least one additional test method described below.<\/p>

Magnet Test<\/h3>

Austenitic stainless steels (304 and 316) are predominantly non-magnetic or very weakly magnetic<\/strong> in their annealed state. This is because the face-centered cubic (FCC) austenitic crystal structure does not support ferromagnetic ordering. By contrast, ferritic and martensitic stainless steels (such as 410, 430, or precipitation-hardened grades) are strongly magnetic.<\/p>

How to perform the test:<\/strong> Hold a strong neodymium magnet (N35 or stronger) directly against the bolt shank. Interpret the results as follows:<\/p>
Table 3: Magnet Test Interpretation for Stainless Steel Bolts<\/strong><\/caption>
Magnet Response<\/th>Likely Material<\/th>316 Probability<\/th><\/tr><\/thead>
No attraction at all<\/td>Austenitic SS (304 or 316) \u2014 annealed<\/td>Possible (need Mo test to confirm)<\/td><\/tr>
Very slight attraction (magnet slides, doesn’t stick)<\/td>Austenitic SS \u2014 cold-worked<\/td>Possible (cold work induces slight martensite)<\/td><\/tr>
Strong attraction (magnet sticks firmly)<\/td>Ferritic, martensitic, or carbon steel<\/td>Very unlikely \u2014 reject as 316<\/strong><\/td><\/tr><\/tbody><\/table>

Critical caveat:<\/strong> The magnet test cannot distinguish 316 from 304 \u2014 both are austenitic and both respond similarly to magnets. A bolt that passes the magnet test (non-magnetic) could be 304, 316, or even 201 series. The magnet test only eliminates carbon steel, 410, 430, and other magnetic grades masquerading as 316. To confirm the presence of molybdenum (the 316 differentiator), you need a chemical spot test or XRF analysis.<\/p>

Visual Inspection<\/h3>

While visual inspection alone cannot confirm grade, it reveals manufacturing quality indicators that correlate with authenticity:<\/p>

Surface finish:<\/strong> Genuine 316 bolts from reputable manufacturers show a uniform, consistent surface \u2014 either bright-polished or matte-passivated, without irregular patches, discoloration, or grinding marks. Counterfeit bolts frequently show uneven surface texture, particularly near the head-to-shank transition where stamping dies were poorly aligned.<\/p>

Head stamping quality:<\/strong> Legitimate markings are clean, evenly spaced, and precisely formed. Counterfeit stamps often appear shallow, off-center, or with inconsistent character depths \u2014 signs of post-production re-stamping rather than in-die marking during forging.<\/p>

Weight:<\/strong> 316 stainless steel has a density of approximately 8.0 g\/cm\u00b3 \u2014 virtually identical to 304 (7.93 g\/cm\u00b3), so weight alone does not differentiate them. However, a bolt that feels noticeably lighter than expected for its size may be zinc-plated carbon steel (7.85 g\/cm\u00b3) rather than stainless at all.<\/p>

Color:<\/strong> Both 304 and 316 exhibit a similar silver-gray color. The Prince Fastener stainless bolt catalog<\/a> notes that 316 has a very slightly warmer tone compared to 304 under natural light, but this difference is subtle enough that even experienced inspectors do not rely on it without instrumental confirmation.<\/p>


\"Craftsman<\/p>

Photo: Physical inspection with precision tools. Combining caliper measurements with head-marking checks catches many counterfeit bolts before they reach the jobsite. (Credit: Pexels)<\/em><\/p>

Corrosion Resistance Check<\/h3>

If you suspect a bolt is not genuine 316, the most practical field test is a salt-spray shortcut<\/strong>:<\/p>

Method:<\/strong> Soak the suspect bolt and a known-genuine 316 reference bolt together in a 5% sodium chloride (table salt) solution for 48 hours at room temperature. After 48 hours, inspect both bolts under bright light with a 10\u00d7 loupe.<\/p>

Expected results:<\/strong> Genuine 316 should show zero visible pitting, staining, or rust. 304 stainless may show slight surface discoloration in aggressive salt solutions. Carbon steel or 200-series imposters will show visible orange\/brown rust spots within 24 hours. This test mimics the accelerated corrosion environment defined by ASTM B117<\/a> (salt spray\/fog testing).<\/p>

For definitive 316 confirmation, a molybdenum spot test kit<\/strong> (available for $30\u2013$60 from lab supply companies) detects the 2\u20133% Mo content that defines 316. Apply a drop of the reagent to a polished spot on the bolt \u2014 a positive Mo reaction (color change per kit instructions) confirms 316. No reaction confirms 304 or lower. The Prince Fastener quality team<\/a> uses this exact method as a receiving-inspection step on every incoming 316 material lot.<\/p>

Seller Verification and Certifications<\/h3>

The most reliable protection against counterfeit 316 bolts is sourcing from verified manufacturers who provide documented traceability. Demand the following with every purchase order:<\/p>

1. Mill Test Report (MTR \/ EN 10204 3.1):<\/strong> This document, issued by the steel mill, certifies the actual chemical composition and mechanical properties of the specific heat of steel used to make your bolts. A valid MTR for 316 will show Mo content between 2.0% and 3.0%. If the supplier cannot produce an MTR, consider that a red flag \u2014 especially on high-value or safety-critical orders.<\/p>

2. Certificate of Conformity (CoC):<\/strong> Issued by the bolt manufacturer, confirming that the finished product meets the specified standard (ASTM F593, ISO 3506, DIN 267-11, etc.).<\/p>

3. ISO 9001 or IATF 16949 certification:<\/strong> Confirms the manufacturer’s quality management system is third-party audited. Prince Fastener<\/a> holds ISO 9001 certification and provides MTRs on every stainless steel order \u2014 standard practice, not an add-on service.<\/p>

4. Traceability:<\/strong> Lot numbers on packaging that trace back to the MTR heat number. If the bolts arrive in unmarked bags with no lot coding, the supply chain is broken and verification is impossible.<\/p>

<\/p>

Identification Methods: Weighted by Reliability<\/h3>

\"Pie<\/p>

Chart: No single method is 100% reliable alone. Head markings + magnet test combined catch approximately 80% of counterfeits. Adding seller certification verification and a Mo spot test raises detection to near 100%.<\/em><\/p>


\"Close-up<\/p>

Photo: Corroded bolts on an industrial structure. This is what happens when non-316 bolts are installed in environments that demand genuine marine-grade stainless. (Credit: Pexels)<\/em><\/p>

<\/p>

Quick-Reference: 316 SS Bolt Identification Decision Table<\/h2>
Table 4: Step-by-Step 316 Bolt Authentication Checklist<\/strong><\/caption>
Step<\/th>Test<\/th>Tool Required<\/th>Pass Criteria<\/th>Fail Action<\/th><\/tr><\/thead>
1<\/td>Head marking check<\/td>Eyes \/ magnifier<\/td>“A4-70”, “A4-80”, “F593D”, or “316”<\/td>Reject or proceed to Step 2 with caution<\/td><\/tr>
2<\/td>Magnet test<\/td>Neodymium magnet<\/td>Non-magnetic or very weakly attracted<\/td>Strong attraction = not austenitic SS. Reject.<\/td><\/tr>
3<\/td>Visual inspection<\/td>10\u00d7 loupe<\/td>Uniform finish, clean stamps, no rust<\/td>Poor finish \/ irregular stamps = suspect quality<\/td><\/tr>
4<\/td>Mo spot test<\/td>Moly reagent kit ($30\u2013$60)<\/td>Positive Mo reaction (color change)<\/td>No Mo detected = 304 or lower. Reject as 316.<\/td><\/tr>
5<\/td>MTR \/ certification review<\/td>Documents from supplier<\/td>Mo: 2.0\u20133.0%, Ni: 10\u201314%, Cr: 16\u201318%<\/td>No MTR available = unverifiable. Request or reject.<\/td><\/tr><\/tbody><\/table>

<\/p>

Video: Testing 304 vs 316 Stainless Steel with a Magnet<\/h2>

This hands-on demonstration shows how a simple magnet test works on 304 and 316 stainless steel \u2014 and where it falls short:<\/p>