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Tapping Drill Chart and Preload Torque: The Two Numbers That Get Specified Wrong
A drawing gives you a thread callout: M8×1.25. A purchase order gives you a torque: T = K × F × d. Between them, these two numbers describe almost every threaded joint in the assembly.
Both are also routinely wrong in ways that do not show up until the joint fails.
The first error is a drill that is 0.2 mm too large, which costs you 25% of the thread height. The second is a torque figure quoted with no K-factor assumption, which means the same wrench setting produces preload errors of 30% or more depending on whether the threads are dry, oily, or new.
This article covers both, with the underlying formulas and the reference tables.
Part 1: The tapping drill
Why the hole diameter is a calculated dimension, not a preference
When a tap cuts an internal thread, it displaces material. The hole you drill before tapping determines how much material remains to form the thread profile.
Two quantities describe the geometry of an ISO metric thread:
| Symbol | Meaning | Unit |
|---|---|---|
| D | Nominal thread diameter | mm |
| P | Pitch — distance between adjacent thread crests | mm |
| H | Basic thread height | mm |
For ISO 68-1 metric threads, the basic thread height is:
H = 0.866025 × P
The drill diameter is chosen so that the tap forms full thread height without excessive cutting force. The two limits that bracket it:
- Too small → the tap must displace a much larger volume of material. Cutting force climbs, the tap breaks, and the thread is only partially formed.
- Too large → less material is available, so the formed thread is short and the root is weak.
The governing formula
The tap drill diameter that yields approximately 75% thread height:
D₀ ≈ D − P
Full thread height (100%) would require drilling to the minor diameter, which is impractical for cutting taps — the tap has no cutting edge left. 75% is the standard compromise, and it is what the drill tables below are built around.
Worked example, M12 coarse (P = 1.75):
D₀ = 12 − 1.75 = 10.25 → round to 10.2 mm
On thread height, the point worth making is this: the drilled hole becomes the minor diameter of the internal thread. The tap cuts outward from there and cannot cut below it, so the hole diameter is what limits the thread.
With D₀ = D − P, the resulting thread reaches approximately 75% of the basic thread height H = 0.866 × P. Drilling to the full minor diameter would give 100%, but leaves the tap no cutting edge — that is why it is not done.
This 75% figure is not an approximation to be tolerated; it is the assumption built into every thread strength table and every tapping torque data set you will look up. A thread produced at 75% height is what those tables describe, and the joint ratings you calculate from them are correct.
The practical consequence: D₀ ≈ D − P and the resulting thread is rated at approximately 75% of full height, which is what all thread strength tables and tapping torque data assume. That is the number that matters, and it is what the chart below gives you.
Tapping drill chart — metric coarse (ISO 261 / DIN 13)
| Thread | Pitch P (mm) | Tap drill Ø (mm) | Tolerance | Recommended drill |
|---|---|---|---|---|
| M3 | 0.5 | 2.5 | +0.1 / 0 | Ø2.5 |
| M4 | 0.7 | 3.3 | +0.1 / 0 | Ø3.3 |
| M5 | 0.8 | 4.2 | +0.1 / 0 | Ø4.2 |
| M6 | 1.0 | 5.0 | +0.1 / 0 | Ø5.0 |
| M8 | 1.25 | 6.8 | +0.1 / 0 | Ø6.8 |
| M10 | 1.5 | 8.5 | +0.1 / 0 | Ø8.5 |
| M12 | 1.75 | 10.2 | +0.15 / 0 | Ø10.2 |
| M14 | 2.0 | 12.0 | +0.15 / 0 | Ø12.0 |
| M16 | 2.0 | 14.0 | +0.15 / 0 | Ø14.0 |
| M18 | 2.5 | 15.5 | +0.15 / 0 | Ø15.5 |
| M20 | 2.5 | 17.5 | +0.2 / 0 | Ø17.5 |
| M22 | 2.5 | 19.5 | +0.2 / 0 | Ø19.5 |
| M24 | 3.0 | 21.0 | +0.2 / 0 | Ø21.0 |
Tolerances are one-directional. The tap drill is specified with a plus tolerance only — the hole may be marginally larger, never smaller. A hole drilled 0.05 mm undersize increases cutting force enough to break a tap, and the failure mode (a broken tap in the workpiece) costs far more than the hole.
Fine thread — a different answer, not a rounding
For fine threads the tap drill is larger, because the pitch is smaller and the same absolute thread height consumes proportionally less of the nominal diameter.
| Comparison | Coarse (M) | Fine (M×1.5, M×1.25) |
|---|---|---|
| Thread height | Larger | Smaller |
| Tap drill | Smaller | Larger (M12 fine ≈ Ø10.5, M16 fine ≈ Ø14.5) |
| Engagement length | Longer, ~6d | Can shorten to ~5d |
| Self-locking | Moderate | Good — resists loosening under vibration |
| Pull-out resistance | Higher | Slightly lower |
| Tapping difficulty | Easier | Tap rigidity is critical, breakage more likely |
Fine threads are specified where vibration is present and self-locking matters more than maximum pull-out capacity. The cost is in tapping: the smaller pitch means a longer, more slender tap with less cross-section, and it breaks more easily when the hole is on the tight side.
Depth and geometry requirements on the drawing
| Item | Recommended value |
|---|---|
| Effective thread depth | As designed; minimum 1.5d for thin-walled parts |
| Depth allowance below effective thread | +0.5P to +1.0P, plus 2–3 mm |
| Hole mouth chamfer | C0.5 to C1, spot at 60°–90° |
| Hole bottom | Flat or small cone; avoid a deep cone that eats into effective depth |
The chamfer is not cosmetic. A tapped hole without a chamfer leaves the thread crest at the surface, where the first thread engagement is incomplete and any burr sits directly in the load path.
Material changes the behaviour
| Workpiece material | Tapping behaviour | Process adjustment |
|---|---|---|
| Free-machining steel / brass | Cuts well | Standard tap, standard speed |
| Mild steel | Normal | Standard |
| Stainless steel (A2, A4) | Work-hardening, galling | Sharp tap, reduced speed, greater lubrication, larger drill by 0.1–0.2 mm |
| Aluminium | Soft, prone to sticking | Slightly larger drill, spiral point or form tap |
| Cast iron | Abrasive on the tap edge | Replace tap more often |
| Hardened steel / tool steel | Not tapable by cutting tap | Use a thread-forming (form) tap, drill to nominal |
The stainless case is the one that catches people. Work hardening in the hole wall makes the tap grab, which raises torque and breaks the tap. Drilling 0.1–0.2 mm larger reduces the material the tap must displace and reduces the torque accordingly.
Part 2: The preload torque
Why "torque" is not a force
Torque is a proxy. The number a torque wrench shows is only meaningful once you know the relationship between torque and axial force in that specific joint.
T = K × F × d
| Symbol | Meaning | Unit |
|---|---|---|
| T | Applied torque | N·m |
| F | Axial preload in the bolt | N |
| d | Nominal bolt diameter | mm |
| K | Torque coefficient (nut factor) | — |
K is the variable nobody specifies, and it is the one that decides whether your preload is right.
Where K comes from
K captures everything about the interface that turns torque into axial load: thread pitch angle, thread flank friction, washer face friction, and the surface condition of both surfaces.
| Condition | K value | Notes |
|---|---|---|
| Dry, as-received zinc plating | 0.20 | Typical for new zinc-plated fasteners |
| Slightly oiled | 0.15 | Common for pre-lubricated fasteners |
| Well lubricated | 0.10 | Oil, grease, or a controlled coating |
| Black oxide (as-produced) | 0.18–0.22 | Wide variation, test it |
| Hot-dip galvanized | 0.15–0.20 | Asphaltic coating, variable |
| Zinc-nickel (electroplated) | 0.12–0.18 | Note: this is a zinc coating, not zinc flake |
| Stainless, dry | 0.20–0.25 | Galling risk, consider anti-seize |
| Cadmium-plated | 0.18–0.25 | Legacy, still specified in aerospace |
The same torque, two different preloads
Take an M12 grade 8.8 bolt, required preload at 70% of yield:
Yield strength for 8.8 is 640 N/mm². Stress area at M12 is 84.3 mm².
F = 0.70 × 640 × 84.3 = 37,766 N
| K | Torque T = K × F × d |
|---|---|
| 0.10 (lubricated) | 45.3 N·m |
| 0.15 (lightly oiled) | 68.0 N·m |
| 0.20 (dry) | 90.6 N·m |
The same bolt, the same required preload, and a torque that varies by a factor of two.
This is why quoting a torque value without stating the K assumption is not a specification. It is a guess that may be off by 100%.
Working back from the torque
The more common direction is the reverse: a procedure says "tighten to 145 N·m", and the engineer needs to know what preload that actually produces.
F = T / (K × d)
With T = 145 N·m, d = 12 mm:
| K | Resulting preload | Against 37,766 N required | Verdict |
|---|---|---|---|
| 0.10 | 120,833 N | 320% | Bolt will fail on tightening |
| 0.15 | 80,556 N | 213% | Bolt will fail on tightening |
| 0.20 | 60,417 N | 160% | Bolt will fail on tightening |
| 0.25 | 48,333 N | 128% | Over-tensioned, thread stripping risk |
| 0.30 | 40,278 N | 107% | Over-tensioned |
| 0.35 | 34,524 N | 91% | Close to target, tolerance still matters |
| 0.40 | 30,208 N | 80% | Under-tensioned |
Read that table again. At K = 0.10, tightening to 145 N·m applies 120 kN to a bolt whose yield load is roughly 54 kN. The bolt fails during tightening, and the K-factor assumption is the only thing that determines whether it happens.
This is not a theoretical concern. It is why an assembly manual that specifies a torque without a lubrication condition is not a complete document.
Lubrication changes the answer, so specify it
If the assembly procedure applies oil, and the torque table was built from dry-condition data, the preload is systematically high. There are only two ways to fix this, and both have to be explicit on the drawing or in the assembly instruction:
- Specify the torque and the lubrication condition together.
- Specify the preload directly, and let the assembly house determine the torque.
Option 2 is better whenever the joint is safety-relevant, because it removes the variable entirely.
Torque values for common metric fasteners
Based on 70% of yield for grade 8.8, with the given K factor. Multiply the preload by 1.41 for grade 10.9 and 1.69 for grade 12.9 if the same percentage of yield is acceptable, or see the note below.
| Size | As (mm²) | Preload F (N) | K = 0.10 | K = 0.15 | K = 0.20 |
|---|---|---|---|---|---|
| M6 | 20.1 | 9,005 | 5.4 N·m | 8.1 N·m | 10.8 N·m |
| M8 | 36.6 | 16,397 | 13.1 N·m | 19.7 N·m | 26.2 N·m |
| M10 | 58.0 | 25,984 | 26.0 N·m | 39.0 N·m | 52.0 N·m |
| M12 | 84.3 | 37,766 | 45.3 N·m | 68.0 N·m | 90.6 N·m |
| M16 | 157 | 70,336 | 112.5 N·m | 168.8 N·m | 225.1 N·m |
| M20 | 245 | 109,760 | 219.5 N·m | 329.3 N·m | 439.0 N·m |
Torque values are not directly transferable between property classes. The relationship:
T ≈ 0.2 × d × F, where F is preload in newtons and d in millimetres, gives N·m directly for K = 0.2.
Higher grade does not mean higher torque for the same preload. If the design calls for the same preload in a grade 10.9 bolt as in a grade 8.8 bolt, the torque is the same. If the design calls for a higher preload in the stronger bolt, the torque rises accordingly. What must not happen is assuming that a stronger bolt can take more torque without re-deriving the preload.
Putting the two together
The tapping drill and the preload torque look unrelated, and they sit in different parts of the process. They are connected by the same idea: both are outputs of a design calculation, and both get corrupted when they are carried as a single number.
The drill diameter carries a tolerance because it is a manufacturing dimension. The torque carries a K-factor because it is a calculation with an assumption embedded in it. What neither can carry is the context that made it correct.
Three things to write on the drawing:
- The tap drill and its tolerance. Not "M8" — the M8 callout does not tell the machinist which drill to use.
- The thread engagement depth and the drill depth below it. Otherwise the tap runs out of room.
- For any joint where preload matters: the required preload, and the lubrication condition. Torque alone is not a specification.
About the process data behind this article
The drill diameters, thread heights, K-factor ranges and torque values above come from production practice rather than a textbook: tap breakage rates and their correlation with hole size, K-factor variation measured across batches as received, and preload verified by ultrasonic tension measurement.
Standard and non-standard fasteners, special-shaped parts, riveting fasteners, self-tapping screws and lean castings are produced from two plants in northern China — Handan (Yongnian) for standard fastener volume production, and Datong for high-strength, special and custom parts. Over 1,000 product types, with management systems certified to ISO 9001, ISO 14001 and ISO 45001, and in-house dimensional, mechanical, coating and torque inspection with batch-level material traceability.
This is stated as process context for the technical content, not as a sales pitch. Where a figure above does not match your drawing or your service condition, the drawing wins.
NXHD Manufacturing (contact@nxhdmfg.com) is a precision fastener and custom metal parts manufacturer with dual production bases in Handan and Datong, China. We supply standard and non-standard fasteners to OEM customers across Europe, the Americas and Asia.
Frequently asked questions
How do I calculate the correct tapping drill diameter?
For ISO metric threads, the common formula is drill diameter = major diameter − pitch, which gives approximately 75% thread engagement. For materials that tap cleanly this is the standard recommendation; for ductile materials a slightly larger hole in the region of 85–90% of full thread is common, at the cost of reduced thread strength. The drawing should state the nominal thread, the pitch and the hole diameter explicitly — a thread callout alone does not define the drilled hole.
What tolerance should the tapped hole be held to?
The hole must be checked with a thread plug gauge rather than measured directly with a micrometer, because a thread gauge verifies the fit of the internal thread rather than the hole wall. Holes that are tight will strip the tap or produce a high torque and a malformed thread; holes that are loose will reduce thread engagement and stripping resistance. Specify the gauge requirement on the drawing.
How is preload torque calculated?
T = K × F × d, where F is the required preload, d is the nominal diameter and K is the torque coefficient. Preload is normally targeted at about 70% of the yield strength of the property class for the standard assembly method. The K value must reflect the actual surface condition of the production lot — dry, oiled, lubricated or coated.
Why does a lubricated bolt break at a torque value that worked when dry?
Because lubrication drops K from about 0.20 to 0.10–0.12. With an unchanged torque setting the delivered preload falls by roughly half while the joint still appears tight. The correct practice is to establish a measured K on the actual production lot through a torque-preload (TTI or tension) test and specify against that value.
In what order should a multi-bolt flanged joint be tightened?
Work from the centre outward in a cross pattern, in three passes — approximately 30%, then 60%, then 100% of the final torque. Sequential tightening from one side loads the last bolt with the accumulated error, which is a common cause of bolt breakage during assembly.
Does NXHD supply to drawing with tapping and torque requirements?
Yes. Non-standard parts are produced from a customer drawing or sample with first articles submitted for approval before the production run, and standard items ship within 48 hours while custom parts typically take 3–7 days to first article. Both plants run in-house dimensional, mechanical, coating and torque inspection.
