British Association screw threads, or BA screw threads, are a largely obsolete set of small screw threads. The largest being 0BA at 6 mm diameter. They were used for miniature instruments and modeling.
They are unusual, in that they were probably the most "scientific" design of screw, starting with 0BA at 6.0-mm diameter and 1.0-mm pitch and progressing in a geometric sequence, where each larger number was 0.9 times the pitch of the last size.
They spoiled this by rounding to 2 significant figures in metric. Then converting to inches and rounding to the thousandth of an inch. This anticipated worldwide metrication by about a century. The design was first proposed by the British Association in 1884, with a thread angle and depth based on the Swiss Thury thread. It was adopted by the Association in 1903.
TPI stands for Threads Per Inch, which means the total number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.
The thread pitch is the distance between threads expressed in millimeters. In general, smaller fasteners have finer threads, so they have a lower thread pitch.
| Screw No. | Diameter of Screw Over Thread (D) | Threads per inch (TPI) | Pitch | Diameter at Thread Bottom | Trapping Drill Size |
|---|---|---|---|---|---|
| 16 BA | 0.0310" | 133.3 tpi | 0.0075" | 0.0220" | 73 Number drill | 0.6 mm | 0.0236" |
| 15 BA | 0.035" | 120.5 tpi | 0.0083" | 0.0250" | 70 Number drill | 0.7 mm | 0.0276" |
| 14 BA | 0.039" | 109.9 tpi | 0.0091" | 0.0280" | - | 0.8 mm | 0.0315" |
| 13 BA | 0.047" | 102 tpi | 0.0098" | 0.0352" | 62 Number drill | 0.98 mm | 0.0386" |
| 12 BA | 0.051" | 90.9 tpi | 0.0110" | 0.0378" | 60 Number drill | 1.05 mm | 0.0413" |
| 11 BA | 0.059" | 81.9 tpi | 0.0122" | 0.0443" | 56 Number drill | 1.2 mm | 0.0472" |
| 10 BA | 0.067" | 72.6 tpi | 0.0138" | 0.0504" | 54 Number drill | 1.4 mm | 0.0551" |
| 9 BA | 0.075" | 65.1 tpi | 0.0154" | 0.0565" | - | 1.55 mm | 0.061" |
| 8 BA | 0.086" | 59.1 tpi | 0.0169" | 0.0657" | 50 Number drill | 1.8 mm | 0.0709" |
| 7 BA | 0.0982" | 52.9 tpi | 0.0189" | 0.0753" | 46 Number drill | 2.05 mm | 0.0807" |
| 6 BA | 0.1100 " | 47.9 tpi | 0.0209" | 0.0849" | - | 2.3 mm | 0.0906" |
| 5 BA | 0.126" | 43 tpi | 0.0232" | 0.0981" | 37 Number drill | 2.65 mm | 0.1043" |
| 4 BA | 0.1420" | 38.5 tpi | 0.0260" | 0.1108" | 31 Number drill | 3 mm | 0.1181" |
| 3 BA | 0.1610" | 34.8 tpi | 0.0287" | 0.1266" | 29 Number drill | 3.4 mm | 0.1339" |
| 2 BA | 0.1850" | 31.4 tpi | 0.0319" | 0.1467" | 22 Number drill | 4 mm | 0.1575" |
| 1 BA | 0.2090" | 28.2 tpi | 0.0354" | 0.1665" | 16 Number drill | 4.5 mm | 0.1772" |
| 0 BA | 0.2360" | 25.4 tpi | 0.0394" | 0.1887" | 9 Number drill | 5.1 mm | 0.2008" |
