Understanding Markings and Grades on Nuts and Bolts
Take a look at bolts in your toolbox, and you'll likely spot some markings on them. While they might seem like random scratches or manufacturer branding, these marks actually tell you everything you need to know about that bolt's strength and what you can use it for.
Getting the right bolt matters more than most people think. Pick one that's too weak for the job, and things can go badly wrong - especially in cars, buildings, or machinery where lives depend on bolts doing their job properly.
This guide will walk you through what all these bolt head markings mean. By the end, you'll know exactly what you're looking at and how to pick the right bolt every time.
The Background Behind Bolt Markings
The push for standardized bolt markings came from real problems in the industry. Early manufacturing only cared about basic size and thread measurements, but as machinery got more complex and manufacturing went global, this wasn't enough.
Organizations like ASTM and SAE created marking systems to standardize bolt grades and identify material strength (later in this guide, we'll break down both ASTM and SAE standards). These marks went beyond simple size stamps - they showed exactly what the bolt was made of and how strong it was.
Today's bolts get their marks through stamping or laser etching. The marks tell you the grade, who made it, and what it's made from. This information means manufacturers can check they're using the right bolt for each job.
When a single bolt failure can bring down machinery or structures, these markings matter. They're part of making sure every bolt meets safety standards across construction, automotive, aerospace, and other industries.
Common Markings on Nuts and Bolts Explained
Fasteners typically have a combination of letters, numbers, and symbols stamped onto them. These markings indicate:
Grade or Strength Level
These marks tell you how strong the bolt is and what it's made from. You'll spot them as raised lines or numbers on the bolt head. SAE Grade 5 has three lines spaced like a Mercedes logo (at 12, 4, and 8 o'clock), while Grade 8 has six lines evenly spaced around the head.
The number of lines doesn't match the grade—they're designed to make each grade instantly recognizable at a glance, even in poor lighting or awkward positions.
Manufacturer's Marks
Look for stamped letters, numbers, or symbols that show who made the bolt. These marks are registered and unique to each manufacturer. Major manufacturers like Fastenal use 'F' while Brighton-Best uses 'B.'
These marks are intended for traceability—if there's ever a recall or quality issue, manufacturers can track exactly which factory and even which production batch a bolt came from.
Material Properties and Finish
These markings tell you what the bolt is made from and how it's protected against corrosion. Common marks include '18-8' for stainless steel containing approximately 18% chromium and 8% nickel or '316' for marine-grade stainless, which includes molybdenum for advanced corrosion resistance.
Additionally, stainless steel bolts may have ASTM F593 markings, such as F593C, F593D, and F593G, indicating different types of stainless steel. F593C corresponds to 304 stainless steel, F593D covers 316 stainless steel, and F593G is a high-strength variant used in demanding applications.
For surface treatments, finish markings vary by manufacturer, but some general conventions exist. For example, zinc-plated bolts may be marked with 'Z,' while hot-dip galvanized bolts may have a 'G' marking. However, these are not universally standardized.
Thread Information
Thread markings show the bolt's size and how fine or coarse the threads are. In SAE systems, you'll see markings like '1/2-13’—meaning a half-inch bolt with 13 threads per inch. Metric equivalents use marks like 'M10-1.5', showing a 10mm bolt with 1.5mm between threads.
These markings often appear alongside other information. Fine threads (marked UNF or with more threads per inch) are common in precision machinery, while coarse threads (UNC) work better for general construction. Some specialized bolts also show their thread direction—'LH' means left-hand threads.
Heat Treatment and Testing Marks
Heat treatment marks show how the bolt was strengthened. Common industry terms include 'Q&T' for quenched and tempered and 'HT' for heat-treated, though these designations are not always physically stamped on bolts.
Instead, the material grade itself (such as '8.8' for metric bolts) often implies the heat treatment process used.
Some bolts carry proof test marks—a dot or dimple showing they've been tested to their proof load without failing, though again, this is not a universal standard.
Construction bolts often need extra testing. ASTM A325 and A490 bolts receive special markings indicating they have passed rigorous strength tests, with A325 typically marked 'A325' on the bolt head.
SAE vs. Metric Markings: What's the Difference?
SAE (Society of Automotive Engineers) and metric fasteners use completely different marking systems to show their strength. You'll probably encounter both regularly in different types of equipment.
SAE Grade Markings
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SAE bolts have a pattern of radial lines on their heads.
- No lines usually mean it's a Grade 2 bolt—basic strength for general use.
- Three lines spaced like a Mercedes logo mark a Grade 5 bolt, good for most automotive work.
- Six lines evenly spaced around the bolt head show it's Grade 8, meant for heavy-duty applications.
Essentially the stronger the bolt, the more lines you'll see on its head.
Metric Grade System
The metric system, standardized by ISO (International Standards Organization), keeps things simpler with a two-number system stamped on the head.
The first number shows tensile strength (divided by 100), while the second indicates how much it can stretch before breaking.
- An 8.8 bolt is medium-strength
- 10.9 is high-strength
- 12.9 is ultra-high-strength.
| SAE & Metric Bolt Markings Strength Comparison Table | ||||
|---|---|---|---|---|
| Bolt Head Marking | SAE Grade | Metric Grade | Tensile Strength (PSI) | Common Uses |
| No Marking | Grade 2 | 4.6 | 58,000 PSI | Light-duty applications, furniture, general use |
| 3 Radial Lines | Grade 5 | 8.8 | 120,000 PSI | Automotive, machinery, structural |
| 6 Radial Lines | Grade 8 | 10.9 | 150,000 PSI | Heavy-duty, aerospace, industrial |
| N/A | N/A | 12.9 | 177,000 PSI | High-performance machinery, extreme loads |
Tensile Strength, Yield Strength, and Proof Load Explained
Earlier, we mentioned how metric grades like 8.8 tell us about a bolt's strength, but there's more to these numbers than meets the eye.
Tensile Strength
When you stretch a bolt until it breaks, that breaking point is its tensile strength. For metric 8.8 bolts, that first '8' translates to 800 MPa (megapascals). SAE Grade 5 bolts break at 120,000 PSI, while Grade 8s can take 150,000 PSI before snapping.
Yield Strength
Before a bolt breaks, it reaches a point where it starts to stretch like a rubber band and won't bounce back. That's its yield strength. The second number in metric grades (the .8 in 8.8) shows where this happens—at 80% of its breaking strength.
Proof Load
Manufacturers test bolts by pulling them with heavy loads—that's the proof load test. If a bolt passes, it might get a small dot or dimple mark. These tests typically use about 85-95% of the yield strength. Construction bolts, like A325s, must pass these tests before they can be used in buildings.
Knowing these limits helps match the right bolt to the job. Your garden fence probably doesn't need Grade 8 bolts, but you definitely want them under your car's engine mounts.
Specialized Markings: Stainless Steel, Aluminum, and More
While we covered basic material markings earlier, some fasteners have unique marking systems for specific applications and materials.
Stainless Steel Grades
- 'A2' or '18-8': Standard stainless for everyday corrosion resistance
- 'A4' or '316': Marine-grade with added molybdenum for saltwater use
- '410': Magnetic stainless used in high-temperature applications
- '17-4': High-strength stainless found in aerospace parts
- 'F593C, F593D, and F593G': ASTM F593 stainless grades for industrial, marine, and structural applications.
Aluminum Fasteners
- '2024-T4': Aircraft-grade aluminum with specific heat treatment
- '6061-T6': Common structural aluminum, good for general use
- 'AL' or 'ALU': Basic marking for standard aluminum grades, though not standardized across all industries.
Military and Aerospace
- 'MS' followed by numbers: Military specification fasteners
- 'NAS': National Aerospace Standard bolts
- 'AN': Air Force-Navy spec hardware
Specialized Applications
- 'B7': High-strength alloy for pressure vessels
- 'L7': Low-temperature service bolts
- 'NACE': Certified for sour gas service
- 'MRO': Maintenance and repair operations grade
Putting Your Bolt Knowledge to Work
Bolt markings might seem like a maze of lines, numbers, and letters at first glance. But now you can see they create a clear system that takes the guesswork out of choosing the right fastener.
From a simple Grade 2 bolt holding up your mailbox to a high-strength 12.9 keeping an engine together, each marking tells you exactly what that fastener can handle.
Put your knowledge to work and browse our complete range of quality fasteners. Whether you need a single specialty bolt or a bulk order, we're your go-to fastener specialists, ready to help with any order size.
Frequently Asked Questions
Do all bolts need grade markings?
No. Some low-strength (Grade 2) bolts have no head markings. Stainless steel bolts often use different marking systems. But any bolt for structural or safety-critical use should have clear grade markings.
Can I convert between SAE and metric bolts?
Never substitute SAE bolts for metric or vice versa. Even though some grades seem similar (like SAE Grade 5 and metric 8.8), the threads are completely different. Mixing systems can lead to cross-threading, improper fit, and dangerous failures. If your equipment specifies metric bolts, use metric. If it needs SAE, use SAE.
How do I tell if a bolt is counterfeit or substandard?
If you always buy from reputable suppliers such as Fastenere, this won’t be an issue. Otherwise, watch for warning signs: inconsistent marking quality, missing manufacturer marks, unusual packaging, or prices that seem too good to be true. Counterfeit bolts may have fuzzy or irregular grade markings compared to the crisp lines on genuine ones.