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Metric & American Standard Bolts' Designations and Grade Meanings

Fasteners are commonly used standard components in various industries of mechanical manufacturing, with many types and different standards in each country, including metric, American, and British systems. And fasteners have different mechanical performance levels. Under different standard systems, the performance levels and prefix markings of bolts are different. In order to better and reasonably use fasteners, we will classify and introduce metric and American bolts here.

Metric bolts (GB/ISO/DIN standard)

Metric bolts follow standards such as GB/T 3098.1 (equivalent to ISO 898-1) and use head digit codes to distinguish strength grades. They are the mainstream system used domestically and globally and are compatible with M-series metric threads.

🔹 Carbon steel/alloy steel metric bolts

✅ Performance level and scenario

Grade 4.6/4.8: Low carbon steel, tensile strength 400MPa; ordinary furniture, brackets, lightweight equipment.

Grade 5.6/5.8: medium carbon steel, tensile strength of 500MPa; ordinary machinery, agricultural machinery, medium load.

8.8 grade: medium carbon steel (quenched and tempered), tensile strength 800MPa; Automotive chassis, transmission components, heavy-duty frame.

Grade 9.8: Medium carbon steel (quenched and tempered), tensile strength of 900MPa; Construction machinery, motor base, high load structure.

Grade 10.9: Alloy steel (quenched and tempered), tensile strength 1000MPa; Key connections for heavy equipment, engines, and steel structures.

12.9 grade: high-strength alloy steel, tensile strength 1200MPa; Mold, aerospace, extreme load scenarios.

✅ How to read the prefix annotation?

▪️ Annotation format: X.Y (such as 8.8, 10.9)+abbreviation of the manufacturer's identification letter

▪️ Code definition:

Number before decimal point=Tensile strength 1/100 (MPa)

Number after decimal point=ratio of yield strength to tensile strength.

Example: 8.8 grade=tensile strength of 800MPa, yield strength of 640MPa (800 × 0.8);

Grade 10.9=tensile strength of 1000MPa, yield strength of 900MPa (1000 × 0.9).

🔹 Stainless steel bolt identification (A2/A4 series)

▪️ Annotation format: Material code - Strength grade

Stainless steel does not rely on heat treatment to enhance strength, but on cold work hardening to increase strength;

◾ Code definition: A2=304 stainless steel, A4=316 stainless steel; Last digit ÷ 10=tensile strength (MPa)

A2-70: Tensile strength ≥ 700MPa, the most widely circulated in the market, suitable for general outdoor and anti-corrosion scenarios

A2-80: Tensile strength ≥ 800MPa, deep cold work hardening, high tensile and heavy-duty working conditions, few in stock, many customized

A4-70: Tensile strength ≥ 700MPa, preferred for marine environment, chemical equipment, and strong corrosion conditions

A4-80: Tensile strength ≥ 800MPa, dual harsh working conditions of heavy load and strong corrosion, mostly customized goods

A2-50/A4-50: The standard exists (soft state, tensile strength ≥ 500MPa), but it is almost not produced domestically, has no stock, and is rarely used; Most of them are thin pieces, small-sized machine screws, or specially customized, which are rarely encountered in daily business.

American made bolts (SAE/ASTM standard)

American made bolts follow SAE J429 and ASTM standards, with strength grades distinguished by radial markings on the head. They are compatible with UNC/UNF American made threads and do not have metric 10.9/12.9 grade specifications.

🔹 SAE Standard Performance Level (for high-frequency orders)

Grade 2 (G2/SAE2): Head without engraved lines, low carbon steel, not heat-treated, tensile strength ≥ 414MPa, benchmarking metric grade 4.8; Suitable for ordinary engineering, wooden structures, and low load non critical fastening.

Grade 5 (G5/SAE5): 3 radial markings on the head, quenched and tempered medium carbon steel, tensile strength ≥ 621MPa, benchmarking metric grade 8.8; Suitable for automotive, motorcycle, and general high-strength mechanical assembly.

Grade 8 (G8/SAE8): 6 radial markings on the head, quenched and tempered medium carbon alloy steel, tensile strength ≥ 827MPa, benchmarking metric grade 10.9; Suitable for heavy machinery, critical load-bearing structures, and high-strength working conditions.

✅ Initial annotation rules

Grade 2 usually has no special line markings;

Grade 5 has three clear radial lines on the head;

Grade 8 has 6 radial lines on the head.

🔹 ASTM Steel Structure Special Bolts (Supplementary)

ASTM A307: Low carbon steel material, divided into A/B grades, used for general engineering and conventional connection of pipeline flanges;

ASTM A325: Quenched and tempered medium carbon steel, steel structure specific connection, strength benchmark SAE5 grade;

ASTM A490: Alloy steel material, high-strength steel structural connection, strength benchmark SAE8 level;

Grade 9 (G9/L9): Ultra high strength alloy steel, commonly used in aerospace and extreme heavy working conditions, rarely used in conventional foreign trade/industrial orders, only for understanding.

📊  metric system ↔ Precision Benchmarking Table for American Strength Grades

Metric grade

American SAE grade

Core Benchmarking Explanation

Level 4.8

SAE2 level

Ordinary low-carbon steel, suitable for light load, without heat treatment

Grade 8.8

SAE5 level

Medium carbon steel quenched and tempered, medium load, preferred for cost-effectiveness

Level 10.9

SAE Level 8

Alloy steel quenched and tempered, heavy-duty high strength, standard equivalent

Level 12.9

No corresponding American standard grade

There is no native 12.9 level in the US system, only non-standard customization is allowed

⚠️ Important reminder: This is only a strength numerical benchmark, and the material, heat treatment standards, and execution specifications may not be completely consistent. It cannot be used interchangeably across systems at will.

🎯 Summary of Core Identification

• Metric: Look at the top digits (4.8/8.8/10.9/12.9), with M thread

• American: Look at the head markings (none/3/6), with UNC/UNF threads