ISO 965-2 & IS 14962-2 Internal Threads – Minor Diameter & Plain Plug Gauge

In thread inspection, internal threads (nuts) are one of the most critical features to control. Among all parameters, the minor diameter (D1) plays a key role in ensuring proper thread strength and assembly.

As per ISO 965-2:1998 & IS 14962-2:2001, limits for internal threads are clearly defined, and based on these limits, we can design Plain Plug Gauges for accurate minor diameter checking.

Standard Pitch Size List

What ISO 965-2 & IS 14962-2 Says About Internal Threads

ISO 965-2 defines:

• Limits of Pitch Diameter (D2)
• Limits of Minor Diameter (D1)
• Applicable for:
  • Coarse thread series
  • Fine thread series
  • Medium tolerance class

Internal Thread Parameters

For internal threads (Nut), 2 important dimensions are:

1. Pitch Diameter (D2)
   • Controls fit between bolt & nut
   • Checked by Thread Plug Gauge (GO / NO-GO)
2. Minor Diameter (D1)
   • Controls strength of thread
   • Checked by Plain Plug Gauge
     • How to decide Plain Plug Gauge GO & NOGO size – You can check from above calculator

Internal Thread Limits

Example for M6 Internal Thread:

• Pitch Diameter (D2):
  • Max: 5.500 mm
  • Min: 5.350 mm
• Minor Diameter (D1):
  • Max: 5.153 mm
  • Min: 4.917 mm

👉 These values are the base for gauge design.

Design of Plain Plug Gauge (Minor Diameter)

Now the main question

👉 How to Make Plain Plug Gauge for Internal Thread?

Step 1: Take Minor Diameter Limits

From ISO table: which calculator given above – select the Size of Thread

Example: M6 thread

• D1 (Max) = 5.153 mm
• D1 (Min) = 4.917 mm

Step 2: Understand GO & NO-GO Concept

Go Size – Check Minimum Size

NO SIZE – Check Maximum Size

Step 3: Design Formula

GO Plug Gauge Size:

👉 GO = Minimum Minor Diameter

Note – For M6 Thread Minor dia will be GO = 4.917 mm

NO-GO Plug Gauge Size:

👉 NO-GO = Maximum Minor Diameter

Note – For M6 Thread Minor dia will be NOGO = 5.513 mm

Why Minor Diameter Checking is Important?

Minor diameter checking is extremely important in internal threads because it allows fast, reliable, and 100% inspection at the shop floor level.

In practical manufacturing conditions, it is not feasible for operators to check every part using a thread plug gauge, as it takes more time and requires careful handling. To solve this problem, a plain plug gauge (minor diameter gauge) is used.

✅ Key Reasons:

• 100% Inspection Possible
  • Operators can quickly check every part using a minor diameter plug gauge without slowing down production.
• Faster Than Thread Plug Gauge
  • Plain plug gauges are simple and easy to use, enabling quick decision-making on the shop floor.
• Prevents Customer Rejection
  • By ensuring minor diameter is within limits, defective threads are identified early, so no rejected parts reach the customer.
• Indirect Thread Quality Check
  • If the minor diameter is out of limit:
    • The thread is either too tight or too loose
    • This directly affects thread functionality

👉 Important Practical Rule:

If Minor Diameter Plug Gauge fails → Thread is also rejected

Standard Pitch Size

Conclusion

ISO 965-2 provides clear limits for internal thread dimensions, and using these limits:

✔ We can design accurate Plain Plug Gauges

✔ Ensure proper minor diameter control

✔ Improve thread strength and quality