GB/T193-2003 General Purpose Threads (General Purpose Metric Threads) – Diameter and Pitch Series

What is GB/T 193-2003?

In the global fastener industry, precision is the language of safety. GB/T 193-2003 is the Chinese national standard that aligns with ISO 261, defining the general-purpose metric screw threads used across billions of components worldwide. It establishes a rigorous framework for Diameter and Pitch Series, ensuring that a bolt manufactured in one facility perfectly mates with a nut produced in another.

At Ever Power, we recognize that selecting the correct thread combination is critical for mechanical integrity, especially when utilizing high-performance materials like 304 or 316 stainless steel.

The Three-Tier Diameter Hierarchy

One of the most vital rules in mechanical design is the priority of selection. GB/T 193-2003 categorizes diameters into three series to streamline manufacturing and inventory.

1. The First Series: The Primary Choice

Designers should always prioritize the First Series. These are the standard sizes you find in most commercial applications. Common diameters include M1, M1.2, M1.6, M2, M2.5, M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30, M36, M42, M48, M56, and M64. As diameters increase, the First Series continues with sizes like M72, M80, M90, and M100, reaching up to M300.

2. The Second Series: Secondary Preference

If the First Series cannot meet specific design requirements, the Second Series is the next alternative. This group includes M1.1, M1.4, M1.8, M2.2, M3.5, M4.5, M7, M9, M11, M14, M18, M22, M27, M33, M39, M45, M52, and M60. These sizes are frequently seen in specialized automotive or aerospace assemblies.

3. The Third Series: Restricted Application

The Third Series is intended for very specific technical needs and should be avoided whenever possible. Examples include M5.5, M15, M17, M25, M26, M28, M32, M35, M38, M40, M50, M55, M58, M62, M65, M68, M70, M75, M76, M78, and M82. Use of these sizes often increases lead times and procurement costs for fasteners.

Deciphering Pitch: Coarse vs. Fine Threads

Pitch determines how many threads are packed into a specific length and significantly influences a fastener’s resistance to loosening and its tensile strength.

Coarse Threads

Coarse threads are the “default” for most fasteners. They offer faster assembly and better resistance to stripping in softer materials. Under the GB/T 193 standard, coarse pitches follow a specific scale: for M1 it is 0.25mm; for M6 it is 1mm; for M12 it is 1.75mm; and for heavy-duty sizes like M64, the coarse pitch reaches 6mm.

Fine Threads

Fine threads provide higher adjustment precision and are less likely to vibrate loose. The standard defines multiple fine pitch options for larger diameters. For instance, an M20 diameter might utilize fine pitches of 1.5mm, 1mm, or even 2mm depending on the application. The available fine pitch spectrum in the standard includes values of 8mm, 6mm, 4mm, 3mm, 2mm, 1.5mm, 1.25mm, 1mm, 0.75mm, 0.5mm, 0.35mm, 0.25mm, and 0.2mm.

Specialized Design Considerations

When implementing GB/T 193-2003, engineers must adhere to several critical caveats:

• M14 × 1.25: This specific diameter/pitch combination is exclusively reserved for internal combustion engine spark plugs.
• M35 × 1.5: This is a highly specialized size primarily used for bearing lock nuts.
• Rule of Preference: Always select from the first series first, then the second, and only use the third series as a final resort. This ensures better availability of tooling and gauges.
• Pitch Constraints: Avoid selecting pitches listed in parentheses in the original standard documentation whenever possible, as these are non-preferred technical legacy sizes.

Ever Power’s Expert Recommendation: Material Matters

Standardizing the thread is only half the battle. At Ever Power, we specialize in the intersection of thread geometry and material science. When choosing a fine pitch for a stainless steel fastener (A2-70 or A4-80), remember that fine threads have a larger stress area but are more susceptible to galling. For high-vibration environments, we recommend a fine pitch combined with specialized anti-seize coatings to leverage the benefits of GB/T 193-2003 while mitigating material risks.