Torx Socket Head Cap Screws (Hexalobular Socket Head Cap Screws)
On production lines and in field service, many “fastener failures” start as drive failures: the bit slips, the recess rounds, torque ramps unpredictably, and the joint never reaches the intended preload. When that happens, the symptoms show up later as vibration loosening, gasket leakage, fretting at interfaces, or repeated rework during maintenance.
Torx (hexalobular) socket head cap screws are specified to reduce that variability. The internal hexalobular drive is standardized by ISO 10664 (shape, basic dimensions, and gauging method), so the interface can be controlled across bits and screws. For the screw itself, ISO 14579 defines the product characteristics for hexalobular socket head cap screws from M2 to M20 (product grade A), making it easier for engineers and distributors to align drawings, inspection plans, and sourcing.
- Reduce recess stripping risk
- Transfer torque via flanks
- Stabilize assembly torque scatter
- Standardize drive per ISO 10664
- Source per ISO 14579 sizes
- Support automation-friendly tightening
Technical Specifications
Product Name
Torx Socket Head Cap Screws / Hexalobular Socket Head Cap Screws
Standards
ISO 14579 (hexalobular socket head cap screws, product grade A), drive interface per ISO 10664
Material
Alloy steel (e.g., SCM435 / 42CrMo4 equivalents), Stainless steel A2 / A4, 17-4PH (on request)
Diameter Range
M2–M20 per ISO 14579
Grades
10.9 / 12.9 (ISO 898-1), A2-70 / A4-80 (ISO 3506-1)
Surface Finish
Black oxide, zinc plated, zinc-nickel, phosphate/oil, inorganic zinc flake; stainless passivation
Certifications
ISO 9001; EN 10204 3.1 material cert; RoHS/REACH declarations on request; PPAP support (program-based)
1: “Tight” torque reading, low real preload
What happens: The bit/recess interface deforms or slips early; torque spikes before clamp load is achieved. In vibration environments, that becomes preload loss and loosening.
Engineering response: Specify a standardized hexalobular drive (ISO 10664) to control engagement geometry and gauging. For critical joints, define target preload and validate with torque-tension testing on the real joint stack-up.
2: Tool wear + rework in high-cycle production
What happens: Worn bits and inconsistent operator angle lead to rounding, scrap, and time loss (especially on hardened 12.9 screws).
Engineering response: Implement bit life control (scheduled replacement), depth engagement checks, and consistent installation condition (dry vs lubricated). ISO-defined drives support more repeatable inspection and supplier alignment.
3: Joint relaxation from embedding (paint, aluminum, soft interfaces)
What happens: Under-head bearing surface seats into the mating material after tightening or thermal cycling, reducing preload.
Engineering response: Use hardened washers where appropriate and control seating flatness. If the joint is aluminum + high-strength alloy steel, pay attention to galvanic corrosion protection and surface treatments.
4: Strength requirement is real, not marketing
What matters: For alloy-steel property classes, minimum tensile strengths are 1040 MPa (10.9) and 1220 MPa (12.9) per commonly referenced ISO 898-1 tables.
Engineering response: Select grade based on required clamp load and safety factor, then control friction so preload matches intent.
Below is a standard dimension example aligned to ISO 14579 “dimensions” search intent. ISO 14579 defines head geometry (including head height k) and specifies drive size by “Socket No.” (Torx/TX size).
Note: Thread length b depends on nominal length L and ordering style (fully threaded vs partially threaded). The table below uses a fully-threaded example (b = L) commonly used for short lengths; confirm b/L on your drawing or PO.
| d (Thread) | P (Pitch, mm) | s (Torx size, TX “Socket No.”) | k (Head height, mm, max) | b (Thread length, example) |
|---|---|---|---|---|
| M3 | 0.5 | TX10 | 3.0 | 12 |
| M4 | 0.7 | TX20 | 4.0 | 20 |
| M5 | 0.8 | TX25 | 5.0 | 20 |
| M6 | 1.0 | TX30 | 6.0 | 25 |
| M8 | 1.25 | TX45 | 8.0 | 30 |
| M10 | 1.5 | TX50 | 10.0 | 40 |
| M12 | 1.75 | TX55 | 12.0 | 45 |
(Drive sizes TX10/TX20/TX25/TX30/TX45/TX50/TX55 and k-max values shown above follow ISO 14579 dimension listings. )
Torque & Preload (control the variable that actually moves):
Torque is an indirect method; preload depends heavily on friction. Use the practical relationship T = K × F × d (K varies with lubrication and coating).
For critical joints: define target preload as a fraction of proof load, then validate with torque-tension testing. Mechanical property requirements for 10.9/12.9 are commonly referenced from ISO 898-1 tables.
Lubrication (write it into the process, not a guess):
If you switch from dry to oiled/anti-seize, the same torque can produce materially different preload.
Specify the tightening condition on the work instruction and keep it consistent across suppliers and sites.
Washers (when they’re not optional):
Use hardened washers when seating on aluminum, coated surfaces, or soft interfaces to reduce embedding and preload relaxation.
For stainless (A2/A4), washers can also reduce surface damage and help manage galling risk when combined with anti-seize.
Hole Clearance (ISO 273):
Clearance holes should follow ISO 273 series (close/normal/loose) for predictable assembly. ISO 273 defines clearance holes for general-purpose applications and notes special cases should follow design requirements.
Misfit risk: tight clearance + positional stack-up = side-loading during tightening → torque scatter and damaged drive.
Tooling control (Torx needs discipline too):
Use the correct TX bit size and replace bits proactively. Worn bits increase recess damage and increase assembly variability—especially on hardened 12.9 screws.
Related Products
FAQ
What standard covers Torx socket head cap screws?
ISO 14579 covers hexalobular (Torx/TX) socket head cap screws from M2 to M20 (product grade A).
The drive interface geometry itself is standardized by ISO 10664.
What is the difference between ISO 14579 and ISO 4762 (DIN 912)?
ISO 14579 uses a hexalobular (Torx/TX) internal drive, while ISO 4762/DIN 912 uses an internal hex.
Choose based on your torque transfer needs, tool control, and recess damage risk in the real assembly process.
What Torx (TX) size matches M6 socket head cap screws?
For ISO 14579, M6 commonly corresponds to TX30 as the listed “Socket No.”
Always verify TX size on the drawing or supplier inspection report.
Should I lubricate Torx socket head cap screws when tightening?
Only if your torque specification is defined for that lubrication condition.
Lubrication changes friction and therefore clamp load at the same torque; lock the condition in the work instruction and validate torque-to-preload on the actual joint.
What clearance hole should I use for metric socket head cap screws?
Use ISO 273 clearance hole series (close/normal/loose) for general-purpose bolt and screw assemblies.
Pick the fit based on alignment tolerance and assembly needs to avoid side-loading and torque scatter.