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Anyone can unscrew it? Selecting and installing security anti-tamper fasteners

2026-10-10

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why ordinary fasteners lose to a hardware-store screwdriver - threat model and security tiers

Outdoor cabinets, EV chargers, self-service kiosks and street furniture are opened every week with a driver bit bought at any hardware store, and the inspection team finds nothing but chewed-up sockets. The root cause is rarely strength; it is that the drive is public. Standard hexagon sockets to GB/T 70.1 (ISO 4762) and cross recesses to GB/T 818 can be backed out in about ten seconds, so an intruder needs no special tool at all. Before choosing a security fastener, split the problem into three questions. Who is the opponent: a curious passer-by, an opportunistic thief, or a professional with a kit? How much time do they have before the next inspection, 24 hours, 7 days or 30 days? And must the joint be serviceable afterwards, or is a one-time break acceptable? The answers set the tier. Casual tampering is stopped by an unusual recess; theft calls for a keyed drive that raises the working time; a determined professional is only stopped by shielding, welded nuts and alarm integration. Note that no global standard grades tamper resistance. In practice, engineers grade by how hard the matching bit is to obtain, verify drive geometry against ISO 10664 so the bit seats correctly, and verify destruction torque against ISO 898-7. Write the requirement on the drawing in a testable form, for example: exposed fasteners shall not be removable with non-dedicated tools within five minutes.

Four security tiers compared: special recesses, keyed drives, shear bolts and shielding

Security anti-tamper fasteners fall into four tiers by protection and cost. Tier one is special recesses: pin-in-torx sized to ISO 10664 and usually specified as T20, T25 or T30, pentalobe, triangular, dual-hole (snake-eye) and one-way clutch heads. They cost roughly 1.5 to 3 times an equivalent GB/T 70.1 screw and stretch removal time from ten seconds to one to three minutes, but the matching bits are openly sold online, so they only stop casual tampering. Tier two is patented keyed drives, non-standard lobed or five-lobe geometries, with bits issued under a numbered tool register: three to ten minutes of resistance at three to six times the cost, and the right choice for EV chargers and kiosks that need scheduled service. Tier three is shear bolts, where the head breaks off along a shear groove at the set torque and leaves a smooth cone with no grip. Removal means drilling out, so this suits guardrails, transformer fences and joints that are never serviced; budget four to eight times. Tier four combines shielding with chemical locking, welded nuts or covers plus anaerobic adhesive (minus 55 to plus 150 degrees Celsius, fixture in 20 minutes, full cure in 24 hours), tamper seals and electronic seals. For material, 8.8 zinc-plated carbon steel with 96 hours of neutral salt spray per ISO 9227 is enough indoors; outdoors and coastal sites need 316 (A4-80) stainless or Dacromet-coated 10.9 carbon steel at 480 to 1000 hours without red rust. Keep threads to 6g/6H per GB/T 197 (ISO 965-1) and drive hardness at or above 32 HRC per GB/T 230.1 so the recess resists drilling.

Installation torque, inspection and serviceability - making security practical

Two failures dominate: joints that are never properly tightened, and joints that cannot be opened again. Set installation torque from T equals K times d times F, with K around 0.14 for zinc-plated dry friction and 0.12 if lubricated, and preload F close to 0.7 times Rp0.2 times As. For an M8 property class 10.9 screw, As is 36.6 square millimetres and Rp0.2 is 940 MPa, giving F of about 24.1 kN and T of about 27 Nm at K of 0.14. Shear bolts add one check: the groove should break between 1.1 and 1.3 times the installation torque so the head shears only after the joint is seated. Calibrate torque tools to ISO 6789 every twelve months or 5000 cycles. On incoming lots, sample destruction torque to ISO 898-7 and require at least 1.5 times the installation torque, gauge threads to GB/T 3934 (ISO 1502), and inspect dimensions and finish to GB/T 2828.1 at AQL 2.5. For outdoor parts, re-verify neutral salt spray to ISO 9227: 480 hours for Dacromet, 1000 hours for 316 stainless, with no red rust. On the service side, keep a numbered register for dedicated bits, record recess type and torque in the maintenance manual, and hold at least two spare bits per recess so a site visit is never blocked. Security is not about making the joint impossible to open; it is about matching the tier to the inspection interval and the service cycle. For selection support, sample testing, or custom non-standard security anti-tamper fasteners built to your drawing, contact JIALI Industrial (164.hk) with the service environment, inspection interval and maintenance requirements.

Anyone can unscrew it? Selecting and installing security anti-tamper fasteners

Why ordinary fasteners lose to a hardware-store screwdriver - threat model and security tiers

Four security tiers compared: special recesses, keyed drives, shear bolts and shielding

Installation torque, inspection and serviceability - making security practical

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