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BA Thread Pitch Chart
BA threads are a precision fastening system with a rich history in British engineering. This BA Thread Chart serves as an essential reference guide for engineers, machinists, electronics technicians, model engineers, restorers and workshop professionals who work with small precision fasteners. Whether you're maintaining vintage equipment, assembling delicate mechanisms or working on specialist applications, understanding BA thread specifications is crucial for selecting the correct taps, dies and drill bits.
| Diameter | Pitch (mm) | Drill Size (mm) |
|---|---|---|
| 0 BA | 1.00 | 5.10 mm |
| 1 BA | 0.90 | 4.50 mm |
| 2 BA | 0.81 | 4.00 mm |
| 3 BA | 0.73 | 3.5 mm |
| 4 BA | 0.66 | 3.0 mm |
| 5 BA | 0.59 | 2.7 mm |
| 6 BA | 0.53 | 2.30 mm |
| 7 BA | 0.48 | 2.0 mm |
| 8 BA | 0.43 | 1.8 mm |
| 9 BA | 0.39 | 1.5 mm |
| 10 BA | 0.35 | 1.3 mm |
| 11 BA | 0.31 | 1.2 mm |
| 12 BA | 0.28 | 1.0 mm |
What is a BA Thread?
BA stands for British Association, a thread standard developed for small precision fasteners. Despite being part of British engineering standards, BA threads are based on a metric thread system, making them unique in the fastening world. BA threads use a distinctive 47.5° thread angle, which differs from both metric and imperial thread angles.
One of the defining characteristics of BA threads is their numbering system. Rather than being measured in inches or millimetres like conventional threads, BA thread sizes are numbered — with smaller numbers representing larger thread diameters and higher numbers indicating smaller, more precise threads. This inverse relationship can take some getting used to, but it becomes intuitive with practice.
BA threads are specifically designed for applications requiring fine threads and compact fasteners. Although largely replaced by modern metric threads in many industries, BA threads remain essential in specialist applications including model engineering, electronics, scientific instruments and vintage equipment restoration.
Common BA Thread Sizes
The BA thread chart covers a comprehensive range of sizes, from 0BA down to 12BA and beyond. The most commonly encountered BA thread sizes include:
- 0BA
- 1BA
- 2BA
- 3BA
- 4BA
- 5BA
- 6BA
- 8BA
- 10BA
- 12BA
As you move from 0BA to higher numbers, the thread pitch decreases and the major diameter becomes smaller. This makes higher BA numbers ideal for miniature assemblies and delicate work, while lower BA numbers suit larger precision components.
How to Read the BA Thread Chart
The BA thread dimensions chart above displays four essential columns to help you select the correct tooling:
- Diameter: The BA thread size designation (0BA, 1BA, 2BA, etc.)
- Pitch (mm): The distance between successive thread crests, measured in millimetres. This determines how fine or coarse the thread is.
- Drill Size (mm): The tap drill size required to prepare a hole before tapping. Using the correct drill size ensures proper thread depth and strength.
When selecting BA taps, BA dies or tap drill bits, cross-reference your thread size against this chart to ensure compatibility and accuracy. The tap drill size is particularly important — using an incorrect drill diameter can result in stripped threads or incomplete engagement.
Common Applications
BA threads have earned their place in precision engineering across numerous industries. You'll find British Association threads in:
- Model engineering and scale model construction
- Electrical equipment and switchgear
- Electronics and circuit board assemblies
- Scientific instruments and laboratory equipment
- Vintage equipment restoration and conservation
- Clocks, watches and horological mechanisms
- Precision machinery and fine mechanical assemblies
- Specialist aerospace and defence applications
The precision and compact nature of BA threads make them ideal wherever space is limited and reliability is paramount.
BA vs Metric Threads
A critical point for any engineer or machinist: BA threads are not interchangeable with metric threads. The differences are fundamental:
- BA threads use a 47.5° thread angle, while metric threads use a 60° angle
- BA pitch values differ from metric pitch specifications
- BA thread major diameters do not align with metric equivalents
- Metric fasteners should never be substituted for BA fasteners without verifying compatibility
Attempting to use metric taps or dies on BA threads, or vice versa, will result in poor engagement, stripped threads and potential equipment damage. Always verify your thread standard before selecting tooling.
Frequently Asked Questions
What does BA stand for in threading?
BA stands for British Association, a thread standard developed for precision fasteners in British engineering.
What angle is a BA thread?
BA threads use a 47.5° thread angle, which is unique to this standard and differs from metric (60°) and imperial (55°) threads.
Are BA threads still used today?
Yes. Although less common than metric threads, BA threads remain in active use for model engineering, electronics, scientific instruments, vintage restoration and specialist precision applications.
Are BA threads metric?
BA threads are based on a metric system despite being part of British engineering standards. However, they are not directly compatible with modern metric threads.
What is the difference between BA and metric threads?
BA and metric threads differ in thread angle (47.5° vs 60°), pitch specifications, major diameters and numbering systems. They are not interchangeable and require different taps, dies and tooling.