Machinery's Handbook, 31st Edition
Diameter of Tap Drill 2173 may wish to choose different tapping drill sizes to overcome this problem or for special purposes, and reference can be made to the pages mentioned above to obtain the minor diameter limits for internal pitch series threads. Reference should be made to this standard BS 1157:1975 (2004) for recommended tap ping hole sizes for other types of British Standard screw threads and pipe threads. Table 8. British Standard Metric Bolt and Screw Clearance Holes BS 4186:1967
Clearance Hole Sizes
Clearance Hole Sizes
Nominal Thread Diameter
Close Fit Series
Medium Fit Series
Free Fit Series
Nominal Thread Diameter
Close Fit Series 54.0 58.0 62.0
Medium Fit Series
Free Fit Series 62.0 66.0 70.0
1.6 2.0 2.5 3.0 4.0 5.0 6.0 7.0
1.7 2.2 2.7 3.2 4.3 5.3 6.4 7.4
1.8 2.4 2.9 3.4 4.5 5.5 6.6 7.6
2.0 2.6 3.1 3.6 4.8 5.8 7.0 8.0
52.0 56.0 60.0 64.0 68.0 72.0 76.0 80.0
56.0 62.0 66.0 70.0 74.0 78.0 82.0 86.0
66.0 70.0 74.0 78.0 82.0 87.0 93.0 98.0
74.0 78.0 82.0 86.0 91.0 96.0 101.0 107.0 112.0 117.0 122.0 127.0 132.0 137.0 144.0 155.0 165.0
8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 27.0 30.0 33.0 36.0 39.0 42.0 45.0 48.0
8.4 10.5 13.0 15.0 17.0 19.0 21.0 23.0 25.0 28.0 31.0 34.0 37.0 40.0 43.0 46.0 50.0
9.0 11.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 30.0 33.0 36.0 39.0 42.0 45.0 48.0 52.0
10.0 12.0 15.0 17.0 19.0 21.0 24.0 26.0 28.0 32.0 35.0 38.0 42.0 45.0 48.0 52.0 56.0
85.0 90.0 95.0 100.0 105.0 110.0 115.0 120.0 125.0 130.0 140.0 150.0
91.0 96.0 101.0 107.0 112.0 117.0 122.0 127.0 132.0 137.0 147.0 158.0
104.0 109.0 114.0 119.0 124.0 129.0 134.0 144.0 155.0
… … … … … … … … … … … … … … … … … … … …
All dimensions are given in millimeters. British Standard Clearance Holes for Metric Bolts and Screws.— The dimensions of the clearance holes specified in this British Standard BS 4186:1967 have been chosen in such a way as to require the use of the minimum number of drills. The recommendations cover three series of clearance holes, namely close fit (H 12), medium fit (H 13), and free fit (H 14) and are suitable for use with bolts and screws specified in the following metric British Standards: BS 3692, ISO metric precision hexagon bolts, screws, and nuts; BS 4168, Hexagon socket screws and wrench keys; BS 4183, Machine screws and machine screw nuts; and BS 4190, ISO metric black hexagon bolts, screws, and nuts. The sizes are in accordance with those given in ISO Recommendation R273, and the range has been extended up to 150 millimeters diameter in accordance with an addendum to that recom mendation. The selection of clearance holes sizes to suit particular design requirements can of course be dependent upon many variable factors. It is however felt that the medium fit series should suit the majority of general purpose applications. In the Standard, limit - ing dimensions are given in a table which is included for reference purposes only, for use in instances where it may be desirable to specify tolerances. To avoid any risk of interference with the radius under the head of bolts and screws, it is necessary to countersink slightly all recommended clearance holes in the close and
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