Screw Shear Strength

Material Shear Strength (Steel) = 0.345 kN/mm2 (25 tons/in2) from http://www.efunda.com/processes/metal_processing/stamping.cfm
Screw diameters From http://www.engineersedge.com/screw_threads_chart.htm
Screw Size       Min Min Dia Area Shear Steel Shear Strength
Basic       In Pounds/in^2
0-80 0.0442 0.006138 343.70 56000
0-80 0.0447 0.006277 351.52
1-64. 0.0532 0.008891 497.92
1-64. 0.0538 0.009093 509.22
1-72. 0.0554 0.009642 539.95
1-72. 0.056 0.009852 551.71
2-56. 0.0635 0.012668 709.39
2-56. 0.0641 0.012908 722.86
2-64. 0.0662 0.013768 771.00
2-64. 0.0668 0.014019 785.04
3-48. 0.0727 0.016604 929.84
3-48. 0.0734 0.016926 947.83
3-56. 0.0764 0.018337 1,026.89
3-56. 0.0771 0.018675 1,045.80
4-40. 0.0805 0.020358 1,140.07
4-40. 0.0813 0.020765 1,162.84
4-48. 0.0857 0.023073 1,292.11
4-48. 0.0864 0.023452 1,313.30
5-40. 0.0935 0.027465 1,538.02
5-40. 0.0943 0.027937 1,564.45
5-44. 0.0964 0.029195 1,634.90
5-44. 0.0971 0.02962 1,658.73
6-32. 0.0989 0.030729 1,720.80
6-32. 0.0997 0.031228 1,748.75
6-40. 0.1065 0.035633 1,995.43
6-40. 0.1073 0.03617 2,025.52
8-32. 0.1248 0.04893 2,740.10
8-32. 0.1257 0.049639 2,779.77
8-36. 0.1291 0.05236 2,932.18
8-36. 0.1299 0.053011 2,968.63
10-24. 0.1379 0.059742 3,345.54
10-24. 0.1389 0.060611 3,394.24
10-32. 0.1508 0.071442 4,000.74
10-32. 0.1517 0.072297 4,048.64
1/4-20. 0.1876 0.110564 6,191.61
1/4-20. 0.1887 0.111865 6,264.43
1/4-28. 0.2052 0.132283 7,407.86
1/4-28. 0.2062 0.133576 7,480.23
5/16-18. 0.2431 0.185661 10,396.99
5/16-18. 0.2443 0.187498 10,499.89
5/16-24. 0.2603 0.212862 11,920.27
5/16-24. 0.2614 0.214665 12,021.23
3/8-16. 0.297 0.277117 15,518.54
3/8-16. 0.2983 0.279548 15,654.69
3/8-24. 0.3228 0.327353 18,331.79
3/8-24. 0.3239 0.329588 18,456.94
7/16-14. 0.3485 0.381553 21,367.00
7/16-14. 0.3499 0.384625 21,539.01
7/16-20. 0.3749 0.441551 24,726.85
7/16-20. 0.3762 0.444618 24,898.63
1/2-13. 0.4041 0.513012 28,728.68
1/2-13. 0.4056 0.516828 28,942.35
1/2-20. 0.4374 0.601046 33,658.55
1/2-20. 0.4387 0.604624 33,858.93
5/8-11. 0.5119 0.823228 46,100.77
5/8-11. 0.5135 0.828382 46,389.40
5/8-18. 0.5554 0.969084 54,268.73
5/8-18. 0.5568 0.973976 54,542.66
3/4-10. 0.6255 1.229149 68,832.34
3/4-10. 0.6273 1.236233 69,229.07
3/4-16. 0.6718 1.417849 79,399.52
3/4-16. 0.6733 1.424187 79,754.49
7/8 -9. 0.7368 1.70549 95,507.42
7/8 -9. 0.7387 1.714297 96,000.63
7/8-14. 0.7858 1.939876 108,633.04
7/8-14. 0.7874 1.947784 109,075.88
1 -8. 0.8446 2.241052 125,498.94
1 -8. 0.8466 2.251679 126,094.00
1 -12. 0.896 2.522121 141,238.77
1 -12. 0.8978 2.532265 141,806.81
1-1/8-7. 0.9475 2.820384 157,941.53
1-1/8-12. 1.021 3.274925 183,395.80
1-1/4-7 1.0725 3.613637 202,363.65
1-1/4-12 1.146 4.125904 231,050.62
1-3/8-6 1.1681 4.28657 240,047.92
1-3/8-12 1.2709 5.074259 284,158.50
1-1/2-6 1.2931 5.253081 294,172.54
1-1/2-12 1.3959 6.121509 342,804.50
1-3/4-5 1.5019 7.086502 396,844.11
2 - 4-1/2 1.7245 9.342783 523,195.86