British Association Screw Threads | BA Thread Chart

British Association Screw Threads | BA Thread Chart

  British Association Screw Threads | BA Thread

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, and to some extent still are, used for miniature instruments and modelling.

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 then spoiled this by rounding to 2 significant figures in metric and 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.

  Thread Count (TPI)

TPI stands for Threads Per Inch, 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.

  Thread Pitch

Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters. In general, smaller fasteners have finer thread so they have lower thread pitch.

  BA Thread Charts

BA Thread Chart

  British Association Screw Threads Charts | BA Thread Chart

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"

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