1. Code interpretation: What do numbers and letters represent?

Very simple:
Numbers (1, 2, 3)→ RepresentativesTolerance level。 The higher the number, the higher the accuracy and the tighter the fit.
Letter (B)→ RepresentativesFemale thread(i.e., threaded holes or nuts). If it is a bolt (male thread), then the letters are usedA(e.g. 1A, 2A, 3A).

So, 1B, 2B, and 3B are essentially three precision grades for internal threads.

2. The core difference between the three levels

The fundamental difference between the three isManufacturing tolerancesandApplication scenarios, you can see it with a table:

1. Level 1B: Loose and economical

• peculiarity: The tolerance is the largest, and the fit is the loosest. Even if the threads are burrs, minor damage, or dirt, they can be easily assembled with high fault tolerance.

• apply：
  ✅ Occasions where precision is not required
  ✅ Rough work that requires quick assembly
  ✅Hot-dip galvanized parts: It must be matched with 1B/1A to reserve space for the coating!

2. Level 2B: General Standard Type (Preferred!) ）

• peculiarity：The most commonly used and standardlevel. The perfect balance between precision, cost and assembly is achieved.

• apply：
  ✅The vast majority of general purpose machinery and equipment
  ✅ Automobiles, electronics, tools, furniture
  ✅If you don't know which one to choose, choose 2B with your eyes closed!

3. Grade 3B: precision and high configuration

• peculiarity: Minimum tolerance and highest accuracy. The mating has virtually no gaps and creates a very strong, loose-resistant connection.

• apply：
  ✅ Critical connections that require the highest levels of security and reliability
  ✅ aerospace and military products
  ✅ High-precision measuring instruments

3. How to choose and cooperate correctly?

When selecting internal threaded holes, be sure to consider the matching bolt grade! Follow these principles:

• Hole 1B→ Match1Aor2ABolts (loose fit)

• 2B hole→ Match2ABolts (most versatile, free fit)

• 3B hole→Must be equipped with 3ABolts (tight fit)

⚠️ Pay special attention：
Never put high precision3B holeMates with low-precision 1A or 2A bolts, otherwise it is easy to damage the threads and make assembly difficult!

4. Summary and memorization of mantras

Remember these three sentences and never get confused again:

• 1B→"Loose": For rough, easy-to-install or hot-dip galvanized environments.

• 2B→"Standard": Meets 95% of daily needs,Brainless is the first choice。

• 3B→"Precision": Used in high-precision fields, high cost.

5. How to use other thread standards?

Although "1B, 2B, 3B" is a code name unique to the United States, butAll thread standards have their own way of defining tolerances and mating, its core ideas are the same.

• Metric threads（Metric Thread）：
  Metric threads are usedTolerance belt(Tolerance Zone) to define precision.

• aboutFemale thread, indicated by capital letters (e.gG, H), followed by a number indicates the tolerance level (the smaller the number, the higher the accuracy).For specific tolerances, please refer to the internal and external thread deviation table.

• The most commonly used female tolerance belt is 6H, its status is equivalent to the 2B of the American system.

• Therefore, a metric thread would be marked as:M8x1.25-6H, instead of M8x1.25-2B.

• InchWyethWhitworth Thread：
  This is the traditional British standard and is now less used. It also has its own fit level, but the labeling is different. （Old BS 84[Close/Class 1 Fit, Medium/Class 2 Fit, Free/Class 3 Fit] →New BS 93【Precision Mechanical Engineering is a close match、General Engineering Intermediate Cooperation,High-Duty Fasteners are free to cooperate】）

• pipe threads（NPT/NPTF, G/R... ）：
  Pipe threads are another set of systems that usually fit throughTaper and seal formcomeConical density formImplement, not grade with numbers.

Summary and comparison

Conclusion:
yesOnly American threaded holes use the specific labeling specifications of "1B, 2B, 3B"。 This is a characteristic of the American uniform thread standard. When you see this label, you can immediately tell that it refers to American threads.

In practice, the easiest way to identify whether a thread is metric or US (imperial) is:

1. Look at the unit: in inch size (e.g. 1/4, #10) is made in the United States; Millimeters (e.g. M8, M12) are metric.

2. seetooth-shaped angle: Both the US and metric systems are 60°, while the Imperial Wyeth is 55° (requires professional tool measurement).

Next time you see the threaded hole specifications on the drawings, I believe you will be able to make the most professional and economical choice!