Gregory Bender

Drill bit size for metric taps

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models



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The formula for calculating the correct size of tap to use for a metric tap is very simple (no need for a huge table of values):

Major diameter - Thread pitch = Tap drill size

As an example, I needed to tap new threads in a fuel tank (for the petcocks). The size of the tap is 12 mm × 1 mm. The 12 mm is the major diameter of the tap. The 1 mm is the thread pitch of the tap. Plugging these values into the formula:

12 mm - 1 mm = 11 mm

And a 11 mm drill bit is needed. It is as easy as that. This works for fine and coarse thread pitches.