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The tables accessible from this page cover both imperial and metric threads commonly available.
Rather than specifying a specific drill size for each thread, a range of sizes is indicated. Using a drill towards the minimum size will result in a stronger fit (fuller thread depth), while the maximum size produces a weaker fit (approximately 50% thread depth).
For general modelling purposes, any drill (imperial, number or metric) falling within the range will be satisfactory although it is better to be closer to the minimum than the maximum for a reasonable depth of thread. Avoid using the exact minimum except in an emergency as this may give rise to excessive strain on the tap.
Hint: A "rule of thumb" calculation worth remembering is:
Tapping Drill Size = Screw Diameter - Thread Pitch.
Revised and extended 09-Nov-2003
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