How we inspect — tensile, torsion and gauge tests
The three tests we run on every lot: what they cover, the criteria and the procedure. What "we only state what we measure" means in practice.
"We only state what we measure" on our quality page is not a slogan but a procedure. Here are the three tests we run on each lot under the inspection plan for each product, with their scope and criteria, separating the reference standards from our in-house rules. The actual tests depend on material, shape, property class, applicable standard and customer requirements.
1. Tensile test — pulled until it breaks
The finished fastener is pulled to fracture to confirm the maximum tensile load and tensile strength. A load–displacement curve is recorded, but displacement is taken from the machine travel rather than an extensometer. Because it includes deformation of the machine and fixtures, we do not use it to determine the 0.2% proof stress to the standard; the curve serves as reference only.
| Item | Standard condition | Our practice |
|---|---|---|
| Basic scope | Stainless (ISO 3506-1 9.1.1): d 3–39 mm, length l ≥ 2.5d, thread length b ≥ 2d | Machine screws and bolts (including socket head cap screws) of length 2.5d or more. Below M3, outside the standard's tensile test range, tested to our in-house rule |
| Countersunk and other heads with reduced loadability | ISO 3506-1: tensile test applies when thread length b ≥ 3d, with separate acceptance conditions | In-house rule: tested at length 3.5d or more |
| Parts too short to tensile test | Steel: head soundness test (ISO 898-1), torsional test for heat-treated parts (ISO 898-7) Stainless: minimum breaking torque (ISO 3506-1 Table 8) |
Steel: head impact test (breaking torque for heat-treated parts); stainless: breaking torque |
These alternative tests do not measure the same thing as a tensile test. The head impact (head soundness) test strikes the head to check for cracks or failure at the head-to-shank junction; the breaking torque test checks torsional strength. A tensile strength value comes only from a tensile test.
Fixtures are made to suit each head type, and since August 2021 the results are registered and kept in our MES — the data behind every test certificate.
2. Torsional strength test — twisted until it breaks
We apply it to heat-treated steel screws and to stainless tapping and machine screws of nominal diameter 3.0 mm and above (in-house rule). The reference standard and test conditions depend on the product group.
| Product | Reference standard |
|---|---|
| Stainless metric bolts and machine screws | ISO 3506-1:2020 |
| Heat-treated steel metric bolts and machine screws | ISO 898-1, ISO 898-7 |
| Stainless tapping screws | ISO 3506-4:2025 |
| Heat-treated steel tapping screws | ISO 2702:2022 |
The equipment is a set of five TOHNICHI dial torque wrenches (DB1.5N4, DB6N4, DB12N4, DB25N, DB50N — 1.5, 6, 12, 25 and 50 N·m full scale). We choose the capacity to suit the expected breaking torque, record the maximum torque at fracture, and apply each wrench's correction formula (clockwise and anticlockwise) obtained from calibration.
How the part is held affects the value. ISO 3506-1 (9.5.2) clamps at least 1d of thread with at least two full threads projecting above the clamping device. Our in-house method clamps with a gap of one nominal diameter below the head (at least two threads on short screws). Results are judged against the applicable standard or the agreed inspection criteria.
3. Thread and cross-recess gauging — does it go in, is it too loose?
| Gauge | Criterion | What a failure means |
|---|---|---|
| Go gauge | Must pass the full thread without undue force | Possible interference on assembly from thread size or form |
| No-Go gauge | Must stop within two turns without undue force (metric threads — machine screws and bolts, ISO 1502) | Possible pitch diameter outside tolerance |
| Cross recess penetration gauge | Meets the gauge criterion of the applicable standard (ISO 4757) | Possible poor fit between bit and recess — too shallow and the bit slips (cam-out), too deep and the head neck may be weakened |
A No-Go failure means the thread size is out of tolerance; it does not by itself mean the joint will loosen under vibration, which depends on preload, joint stiffness, slip and locking design.
- Sampling inspection on every production lot under each product's inspection plan
- Regular calibration of Go/No-Go gauges and torque wrenches
- Results are tied to the lot number and managed within our traceability system
If you need the inspection data for a delivered lot, we confirm it with a test certificate. Certificates are issued only for lots that have actually shipped.
FAQ
What is a Go/No-Go test?
A check with limit thread gauges that the thread is within tolerance (ISO 1502). The Go gauge must pass all the way without undue force (if it binds, assembly may interfere); the No-Go gauge must stop within two turns (if it runs further, the pitch diameter may be out of tolerance).
How are short bolts tested?
When a part is too short to tensile test, steel parts get a head impact (head soundness) test or, if heat treated, a breaking torque test, and stainless parts a breaking torque test. These do not substitute for tensile strength; they check the soundness of the head junction or torsional strength. We tensile test from 2.5d, and from 3.5d for countersunk heads (in-house rule).
Why gauge the cross recess?
If the recess is too shallow the bit does not seat and slips, stripping the head (cam-out); if too deep, the section joining head and shank becomes thin and weaker. An ISO 4757 penetration gauge checks how far the bit enters.
What equipment is used for the torsion test?
Five TOHNICHI dial torque wrenches (1.5 to 50 N·m full scale). We choose the capacity to suit the expected breaking torque, record the maximum torque at fracture and apply each wrench's calibration correction formula.
References
- ISO 898-1:2013 Fasteners — Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — full-size tensile test (length over 2.5d), head soundness test for short screws
- ISO 898-7:1992 Torsional test and minimum torques for bolts and screws with nominal diameters 1 mm to 10 mm
- ISO 3506-1:2020 Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners — Part 1: Bolts, screws and studs with specified grades and property classes — breaking torque for short stainless screws
- ISO 2702:2022 Heat-treated tapping screws / ISO 3506-4:2025 (stainless tapping screws) — torsion test for tapping screws
- ISO 1502:1996 ISO general-purpose metric screw threads — Gauges and gauging — No-Go gauge two turns
- ISO 4757:1983 Cross recesses for screws — cross recess penetration gauge
Last revised 2026-10-12