GB/T196-2003 Standard General Purpose Threads – Basic Dimensions

1. Scope and Applicability

This standard specifies the basic dimensions for general-purpose metric screw threads based on GB/T 192 (Basic Profile) and GB/T 193 (Diameter/Pitch Series). It applies to standard mechanical fastening.

Critical Note: While these threads offer precision fits, the thread profile itself does not provide a seal. For applications requiring gas or liquid tightness (such as pipe fittings), tapered threads (like NPT or BSPT) or additional sealing agents (O-rings, washers) must be used.

2. Basic Dimensions Table (Unit: mm)

The table below covers the standard range from Micro-screws (M1) to Heavy Engineering bolts (M300). Standard Coarse Threads are highlighted for quick reference, as they are the default choice for most industrial applications.

3. Expert Selection Advice from Ever Power

Selecting the right thread specification is not just about checking a table; it’s about application suitability. Here are three critical factors to consider:

A. Coarse vs. Fine Pitch: Which to Choose?

• Coarse Thread (Standard): Prioritize this for most applications. Coarse threads are more durable, less prone to cross-threading during assembly, and offer better fatigue resistance. They disassemble faster and are more tolerant of plating thickness.
• Fine Thread: Choose fine pitch for thin-walled components, fine adjustments, or applications requiring higher self-locking capabilities against vibration. Warning: Fine threads are more susceptible to “galling” (cold welding), especially in stainless steel applications.

B. The Critical Role of Pitch Diameter (D₂)

The Pitch Diameter is the most important dimension for thread fit. It is the theoretical diameter where the width of the thread tooth equals the width of the thread groove. In manufacturing and quality control (gauging), the pitch diameter tolerance determines whether a nut and bolt will assemble correctly or be too loose/tight.

C. Tapping Drill Size (Minor Diameter D₁)

For internal threads (nuts or tapped holes), the Minor Diameter (D₁) dictates the drill bit size.

Rule of Thumb: Drill Size ≈ Nominal Diameter – Pitch.

Example for M10x1.5: Drill size ≈ 10 – 1.5 = 8.5mm.

Drilling too large reduces thread engagement height (stripping risk), while drilling too small increases tapping torque and risks breaking the tap.