Metric thread, tapping drill and clearance hole sizes

Three holes matter around a metric bolt and only one of them is the bolt's own diameter. To cut a thread you drill smaller than the bolt: M8 wants 6.8 mm, not 8. To let the same bolt pass through a part you drill larger: 9 mm for an ordinary fit, or 8.4 if the two parts have to stay exactly where you put them. Get the two the wrong way round and you either snap a tap or end up with a joint that rattles. This page is those numbers transcribed out of three ISO standards rather than remembered: the diameter and pitch grid from ISO 261, the drill to use before tapping from ISO 2306, and the clearance hole from ISO 273. They are three separate documents for a reason — nobody has ever published them as one table without quietly rounding something.

How it is calculated

tapping drill ≈ nominal diameter − pitch · clearance hole = from ISO 273, fine, medium or coarse

ISO 2306 states the rule in its own scope: the drill diameter is "approximately equal to the nominal diameter of the thread minus the pitch". For M6 that is 6 − 1 = 5.00 mm and the standard agrees exactly. For M8 the rule gives 6.75 mm and the standard says 6.80, because the table is drawn from stocked drill sizes rather than from arithmetic; the same happens at M4.5, M9 and M12. That is why this page shows the published figure and tells you when the rule of thumb would have missed. The clearance hole is not arithmetic at all: ISO 273 says its values "result from bearing area calculations in connection with ISO bolt and nut product standards", and offers three series with tolerance fields H12, H13 and H14.

Source: ISO 2306:1972(E), Drills for use prior to tapping screw threads — Clause 1 — "This International Standard specifies the sizes of drills to be used prior to tapping parallel screw threads of normal length of engagement, the drill diameter being approximately equal to the nominal diameter of the thread minus the pitch"

Questions people ask

Why is the tapping drill smaller than the bolt?

Because the tap has to have material left to cut the thread out of. The hole you drill is roughly the minor diameter of the finished thread — the bottom of the valleys — and the tap raises the crests from what is left. Drill it at the nominal size and the tap has nothing to bite; drill it much undersize and the tap has too much to remove and snaps, which in a blind hole in an awkward place is an afternoon gone. The published drill sizes sit where the trade has settled: enough thread to hold, little enough load to tap by hand.

Which clearance series should I pick?

Medium unless you have a reason. Fine gives the least slop and so the most control over where the two parts end up, at the price of needing your two holes to line up properly. Coarse buys you room to shove things into position, which is what you want across a long run of fixings where small errors accumulate — a fence, a frame, anything set out with a tape. ISO 273 attaches tolerance fields to the three: H12 for fine, H13 for medium, H14 for coarse, "given for information only, for use where it is desirable to specify tolerances".

Does the drill size change with the material?

The published table does not, but what comes out of the hole does. ISO 2306 warns about it directly: "When tapping relatively soft material, there is a tendency for the material to be squeezed down towards the root and in such cases the minor diameter of the tapped hole may become smaller than the diameter of the drill used." Aluminium and mild steel close in on the tap; cast iron and hard steel do not. The standard also notes the practical trade: "The larger the drilled hole, within the relevant minor diameter tolerance, the more economical tapping becomes and the risk of tap breakage is reduced."

What does "coarse" mean, and is fine better?

Neither is better, and ISO 261 says so in as many words: "The words 'coarse' and 'fine' are given in order to conform to usage. No concept of quality shall, however, be associated with these words. It shall be understood that the 'coarse' pitches are the largest metric pitches used in current practice." Coarse is what you get when a bolt is called M8 with no pitch after it. The fine pitches listed here are equally standard, and equally certain to be the wrong thing to put in your toolbox next to the coarse ones, because M8 × 1 and M8 × 1.25 look identical to the eye and will start into each other's nuts before they bind.

Why are some sizes marked second choice?

ISO 261 ranks the diameters in three columns and tells you what to do with them: "Choose, for preference, diameters in column 1 of table 2 and, if necessary, in column 2 and then in column 3." First choice is M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30, M36 — the ones a hardware shop actually stocks. Second choice covers M3.5, M7, M14, M18, M22, M27, M33 and friends, which exist, are perfectly standard, and will have you ordering online. The standard has a couple of stranger notes too: the 1.25 mm pitch at M14 is reserved for spark plugs, and 35 mm exists only for bearing lock nuts.

Do I need to chamfer the clearance hole?

Often, yes, and for a reason that is easy to miss. A bolt is not square where the shank meets the head; there is a radius there. If the hole edge is sharp and the fit is close, the head sits on that radius instead of on the surface, and the joint is neither tight nor square. ISO 273 puts it plainly: "In cases where it is necessary to avoid interference between the edge of the hole and the underhead fillet of the bolt, a chamfer is recommanded" — the typo is the standard's, not ours.

Where do the tables come from?

Three ISO standards, all linked below, and each supplies exactly one column. ISO 261:1998 gives the nominal diameter and pitch grid and the first, second and third choice ranking. ISO 2306:1972 gives the drill to use before tapping the coarse series. ISO 273:1979 gives the clearance holes in the fine, medium and coarse series with their tolerance fields. ISO sells these standards, so the copies read for this page are the free preview PDFs published by iTeh Standards; the links go to those, and nothing here is blended from a second-hand chart.

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