What pilot hole for a tapping screw? — the hole sizes in KS are test conditions
When a tapping screw strips, the pilot hole is the first thing to check. What the hole diameters in KS B 0251 mean, what happens when the hole is too small or too large, and the caution for stainless tapping screws.
"The threads strip when we drive our tapping screws." We hear this often. When we inspect returned screws for heat-treatment hardness, thread form and dimensions, sometimes nothing is wrong. Then we also consider whether the pilot hole in the mating part was too small. A tapping screw forms its own mating thread as it goes in, so the pilot hole size changes the load on the screw considerably.
A tapping screw forms its own mating thread
An ordinary bolt goes into a pre-tapped thread. A tapping screw enters an untapped hole and forms the thread by displacing or cutting the base material. The load on the screw thread then depends on:
- Pilot hole diameter — the smaller it is, the more material must be displaced and the higher the load.
- Base material hardness — the harder, the higher the load.
- Base material thickness — the thicker, the longer the thread formed and the higher the driving torque.
For the differences between type 1, type 2 and trilobular screws, see Tapping screws type 1 and 2, and trilobular thread-forming screws.
The hole sizes in KS are test-plate conditions for checking the screw
KS B 0251 (steel and stainless steel tapping screws — mechanical properties) contains hole diameter tables. These are not recommended pilot holes for use, but test-plate conditions for the "drive test" that checks whether the screw was made correctly: an acceptance condition stating that, driven into a plate of this thickness and hardness with this hole, the screw must form a thread without its own thread deforming.
The tables and test conditions here follow KS B 0251:2009. The hole diameters, plate thicknesses, torsional torques and hardness values quoted are the same as in the 1999 edition. If a drawing or contract specifies another edition, check that edition.
Types 1–4 (KS B 0251 Annex A, Table A.5) — test plate: cold-rolled steel SPCC (70–85 HRB); driven without oil in the hole, without stopping, until the tapered point passes through
| Nominal diameter | Plate thickness | Hole (types 1, 2, 4) | Hole (type 3) |
|---|---|---|---|
| 2 | 1.17–1.30 | 1.8 | 1.8 |
| 2.5 | 2.1 | 2.2 | |
| 3 | 2.6 | 2.7 | |
| 3.5 | 1.85–2.06 | 3.0 | 3.1 |
| 4 | 3.4 | 3.6 | |
| 4.5 | 3.10–3.23 | 3.9 | 4.0 |
| 5 | 4.4 | 4.5 | |
| 6 | 5.2 | 5.4 |
mm. Hole diameter tolerance +0.05 / 0.
ST is the tapping screw thread designation of ISO 1478 (KS B 0248 main body); in ST3.5 the number is the thread major diameter in mm. It differs in pitch and thread dimensions from types 1–4 (nominal diameters 3, 4 mm etc., KS annex) common in Korea, so the two are not interchangeable.
ST tapping screws (KS B 0251 main body, Table 2; ISO 2702) — test plate: low-carbon steel, carbon 0.23% max, 130–170 HV; screws tested as received (with or without finish)
| Size | Plate thickness | Hole | Size | Plate thickness | Hole |
|---|---|---|---|---|---|
| ST2.2 | 1.17–1.30 | 1.905–1.955 | ST4.2 | 1.85–2.06 | 3.43–3.48 |
| ST2.6 | 1.17–1.30 | 2.185–2.235 | ST4.8 | 3.10–3.23 | 4.015–4.065 |
| ST2.9 | 1.17–1.30 | 2.415–2.465 | ST5.5 | 3.10–3.23 | 4.735–4.785 |
| ST3.3 | 1.17–1.30 | 2.68–2.73 | ST6.3 | 4.67–5.05 | 5.475–5.525 |
| ST3.5 | 1.85–2.06 | 2.92–2.97 | ST8 | 4.67–5.05 | 6.885–6.935 |
| ST3.9 | 1.85–2.06 | 3.24–3.29 |
Because the test uses a specified plate and thickness, these hole sizes may not suit aluminium, castings, plastics, or thicker or harder steel as they stand. Neither the product standard KS B 1032 nor the thread standard KS B 0248 gives pilot holes for use either, so the actual pilot hole must be set for the base material.
When the pilot hole is too small or too large
| Hole too small | Hole too large | |
|---|---|---|
| What happens | More material to displace, so driving torque rises | The formed thread is shallow and engagement is insufficient |
| Symptoms | Deformed (stripped) screw thread, damaged recess or bit cam-out, torsional fracture of the screw | Spins when tightening, pulls out under light load, cannot reach tightening torque |
| When the screw is inspected | Hardness and dimensions may be normal | Often nothing wrong with the screw |
The pilot hole is not the only cause of stripping. A base material harder or thicker than expected, or excessive driving speed, can give the same symptoms, so the product and assembly conditions must be checked together.
Driving torque and the torsional strength of the screw
With tapping screws the three torques must be in the right order:
Driving torque (forming the thread) < tightening torque < torque that strips the mating thread
Driving and tightening torque must also stay well below the torsional strength of the screw itself. A small pilot hole raises driving torque and narrows that margin. KS B 0251 sets the minimum torsional torque the screw must withstand. Having the order right does not by itself make the joint sound: screw fracture, recess damage, part deformation and tool torque scatter must also be considered.
| Nominal diameter | 2 | 2.5 | 3 | 3.5 | 4 | 4.5 | 5 | 6 | 8 |
|---|---|---|---|---|---|---|---|---|---|
| Steel types 2, 4 | 0.42 | 0.88 | 1.57 | 2.55 | 3.53 | 5.20 | 7.06 | 12.75 | 29.91 |
| Stainless types 2, 4 | 0.27 | 0.59 | 0.98 | 1.67 | 2.35 | 3.43 | 4.70 | 8.53 | 19.61 |
Minimum torsional torque, N·m. KS B 0251 Annex A Table A.3 (steel), Annex B Table B.2 (stainless). See the standard for types 1 and 3.
These are minimum requirements of a test that twists the screw to failure in a specified fixture, not recommended tightening torques for real parts.
Comparing types 2 and 4 of the same size, the stainless minimum torsional torque requirement is about 60–70% of steel. This is not a strength ratio of actual products, but because the minimum requirement is lower, check the margin between actual driving torque and damage torque separately for stainless tapping screws.
For stainless tapping screws (Annex B), KS B 0251 has no drive test. The tests are hardness, torsional strength and head soundness only, and the surface hardness requirement of 320 HV min is lower than for steel tapping screws (450 HV min). In other words, KS does not guarantee that a stainless tapping screw can form a thread in steel sheet. Before using stainless tapping screws in steel sheet, confirm drivability with the actual parts.
"The threads strip" — what to check
When we hear "the threads strip", we inspect the returned screws for heat-treatment hardness (surface and core), thread form and dimensions. If the screws are fine, the cause is more likely on the assembly side. The mating part is with you, so please check the following on site:
- Pilot hole diameter — measure the actual hole with a pin gauge, not the design value; worn drills or punched holes can be smaller than designed.
- Base material — whether material, hardness and thickness are as designed
- Screw material — whether stainless tapping screws are being used in steel sheet
- Assembly conditions — tool and speed
Sending the mating part or its drawing helps us narrow down the cause a great deal.
Set the actual pilot hole on the actual part
The KS test hole sizes can be a starting point for the same plate and thickness. Otherwise, drive into the actual base material while stepping the pilot hole diameter, and measure the maximum driving torque and the torque at which damage first appears (stripping of the mating thread, screw fracture or recess damage, whichever comes first). Choosing a pilot hole and tightening condition with enough margin between them, allowing for tool scatter, and checking the required clamp force and pull-out load, is the most reliable approach. Plastic base materials add other factors such as creep and cracking, so always confirm with the actual parts.
That is why we do not publish a pilot-hole table by base material. Base materials vary so much in material, hardness, thickness and hole-making method that a test on the actual part is more accurate than any number set by the screw maker.
We make and supply type 1 and type 2 tapping screws (KS B 1032 annex) in steel and stainless. We prepare samples so you can test on the actual parts when setting the pilot hole. Specification enquiry
FAQ
What pilot hole should I drill for a tapping screw?
It depends on the base material, its hardness and thickness. KS B 0251 gives hole diameters by size, but they are test conditions for checking the screw in steel plate of a specified thickness and hardness. Under the same conditions they can be a starting point; otherwise, measure the maximum driving torque and the torque at first damage on the actual part and choose with a margin that allows for tool scatter.
My tapping screw threads are stripping. Is the screw defective?
Both the product and the assembly conditions need checking. If returned screws show normal hardness, thread form and dimensions, we consider whether a small pilot hole, or a base material harder or thicker than expected, raised the driving torque. It is best to measure the actual hole rather than rely on the design value.
What do the hole diameter tables in KS B 0251 mean?
They are test-plate conditions for the drive test: an acceptance condition that the screw, driven into a plate of specified thickness and hardness with that hole, must form a thread without deforming. They are not recommended pilot holes for every base material. Types 1–4 in Annex A are driven into SPCC plate without oil in the hole; ST sizes in the main body are driven, as received, into low-carbon steel of carbon 0.23% max and 130–170 HV.
Can stainless tapping screws be used in steel sheet?
KS B 0251 has no drive test for stainless tapping screws, and their surface hardness requirement is lower than for steel. Since the standard does not guarantee drivability into steel sheet, confirm with the actual parts first. The minimum torsional torque requirement is also about 60–70% of steel for the same size (a comparison of minimum requirements, not of actual product strength).
References
- KS B 0251:2009 Steel and stainless steel tapping screws — Mechanical properties — Table 2, test plate thickness and hole diameter (ST2.2–ST8)
- KS B 0251:2009 Annex A — steel tapping screws types 1–4: Table A.3 torsional torque, Table A.5 test plate (SPCC) thickness and hole diameter
- KS B 0251:2009 Annex B — stainless steel tapping screws types 1–4: Table B.1 hardness (surface 320 HV min), Table B.2 torsional torque, Table B.3 tests (no driving test)
- KS B 0248 Tapping screw threads, KS B 1032 Cross-recessed tapping screws — no application pilot-hole table
Last revised 2026-10-09