My old Machinery's Handbook doesn't seem to provide a tap drill size for M14-1.25 hand tap (modern spark plug size). Maybe I'm losing my marbles, but I can't find it. Is there a simple formula for determining tap drill size, given the metric size? TIA, Dave

The general formula for finding metric tap drill sizes is: Nominal diameter - pitch

In the stated case for a M14-1.25 thread that would come out to a metric drill size of 12.75 mm.

12.75mm is 0.502 in

I assume the general formula gives the tap drill size for nominal 0.75% thread which is considered practical for a strong thread in average machineable material.

To test this I converted 14mm and 1.25mm to their inch equivalents and calculated the inch size of drill for 0.75% thread. It came out to 0.503 in.

No! M14 * 1.25 is a fine pitch. So you would be way off for standard pitch. Your (and other rules of thump using a simple factor) are simply useless. It is always NominalSize - Pitch. No matter wether fine or standard.

Sorry Nick. I guess we have a problem with translation here. Pitch is distance between the crests of a thread. Pitch is also the reciprocal of the # of threads per unit measure; whatever that unit may be; inches, millimeters, etc. AFAIK, there is no specific definition of (std) and (fine). For the terms to be meaningful in any calclulation they must have a specific identity re. to the quantity to be calculated. That seems to be the case in the 7 examples you cited.

There is no "your formula". I merely changed mm to inches in order to use an "inch" formula I am familiar with and to illustrate the fact it comes out with only a tiny difference compared to the "metric' formula.

I believe the "metric" formula for tap drill size is: Basic major diameter minus thread pitch

The "inch" formula I used is: Tap drill diameter = nominal diameter - [ (1.299 x %) / pitch ]

where: Pitch is # of threads per inch and % is expressed as a decimal

Which gives the drill size as diam - pitch (for those subtractively challenged).

Except for these: Thread Table Calc M4.5 x .75 3.70 3.75 M8 x 1.25 6.80 6.75 M9 x 1.25 7.80 7.85 M12 x 1.75 10.20 10.25

M4 x .35 3.60 3.65 M10 x 1.25 8.8 8.75 M14 x 1.25 12.8 12.75

I suppose that the calculated value is not a readily available drill size, hence it is rounded (some up, some down).

These must be typo's M8 x .5 7.00 7.50 M8 x 1 7.50 7.00 M16 x 1.5 15.0 14.5 M36 x 3 36.0 33.0 !! Pity the poor guy who blindly follows the table and uses a 36mm drill for a M36 thread.

Bob (who wonders why he doesn't go to bed instead of verifying tables that he'll *never* use)

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