Product Introduction

A hex bolt is about as simple as a fastener gets — six flat sides, a shank, a thread. The simplicity is the point. Wrench it down and the torque turns into clamping force that holds two parts together for years. What separates a good hex bolt from a cheap one is everything you can't see: the alloy, the cold-heading process, the thread tolerances, and the paperwork behind the material.

Here's the short version of the 304/316 question. Both are austenitic stainless steels — chromium forms a self-healing oxide film that does the corrosion fighting. The difference is molybdenum. 316 carries 2.0–2.5% of it, and that small addition is what stops chloride pitting and crevice corrosion cold. That's why the ISO 3506 system calls 304 steel group A2 and 316 group A4. Same bolt shape, different chemistry, different environment.

Full thread (ISO 4017) means the thread runs from under the head to the tip — the right call for tension loads and long engagement. Partial thread (ISO 4014) leaves a plain shank with a bigger shear section, the usual pick for structural joints that take sideways loads. We also ship complete assemblies: bolt, nut, washer, all one alloy, all one grade. No mixing, no galvanic surprises at the jobsite.

One honest warning before you order. DIN 933 and DIN 931 are retired standards — ISO 4017 and ISO 4014 replaced them decades ago. Most of the market still asks for the DIN numbers, and that's fine, but across-flats (WAF) differs on M10, M12, M14 and M22. A 17 mm wrench fits a DIN M10; an ISO M10 needs 16 mm. We confirm the standard with you before shipping, so your crews don't end up with the wrong sockets on site.

Full Specification

1 Chemical Composition (ISO 3506-1:2020, mass %, max unless shown as range)

2 Mechanical Properties (ISO 3506-1:2020, minimum values)

Elongation A measured over the actual bolt length after fracture; d = nominal thread diameter. Class 80 stays limited to cold-worked small sizes in the 2020 edition — for bigger diameters in high strength, talk to us about class 50 or duplex (FA) grades.

3 Reference Tightening Torque (K ≈ 0.20, oiled thread, target ≈ 0.7 × Rp0.2)

Reference values for new, oil-lubricated threads. Real joints differ — friction coefficient, joint stiffness, sealing needs. Use a torque wrench, tighten in 2–3 steps, and re-torque once. Preload column is for A2-70; A4-80 allows roughly 1.33× the preload.

Size Chart

1 Full Thread Hex Bolts — ISO 4017 (product grade A)

Grade A applies up to d ≤ 24 mm with L ≤ 10d or L ≤ 150 mm; beyond that, grade B. ISO 4017 range M1.6–M64; DIN 933 stops at M52.

2 WAF Difference — DIN vs ISO (the wrench trap)

Most-confused parameter in the trade. An M10 ISO bolt under a 17 mm DIN wrench will slip and round the corners. Confirm the standard before tooling up.

3 Partial Thread — ISO 4014 Thread Length b (L ≤ 200 mm)

4 Assembly Kit Quick Reference (Bolt + Nut + Washer)

Every kit is one alloy, one grade: A2 bolt + A2 nut + A2 washer, or the same in A4. Spring washers (DIN 127) are available on request, though for vibration duty we usually steer you to nylon-insert lock nuts (ISO 7040) or threadlocker instead.

Applications

South America Market Notes

If you're buying for South America, the environment usually makes the decision for you. Chilean and Peruvian mining sites sit in saline dust with high humidity — 316 takes it, 304 doesn't. Coastal infrastructure from Cartagena to Viña del Mar lives in salt spray year-round. Brazilian food plants run chlorinated CIP washes that will pit 304 over time. Wineries, fruit packers, aquaculture cages — they all settle on A4 for anything that touches seawater or washdown chemicals.

For indoor, dry, or general outdoor duty, A2-70 is the honest cost-effective pick, and we'll tell you straight when you're overspending. A quick rule we hand every buyer: chloride exposure of any kind → A4; dry and sheltered → A2; anything structural in shear → partial thread; anything in pure tension → full thread.

• Copper mining & processing — Chile, Peru (saline dust, A4-80)
• Desalination & solar plants — Atacama coast, northern Chile (A4)
• Salmon aquaculture — southern Chile (seawater, A4 kits)
• Agribusiness & food processing — Brazil, Argentina (CIP chemicals, A2/A4)
• Wineries & fruit packing — Chile, Argentina (washdown, A2/A4)
• Coastal infrastructure & ports — Brazil, Colombia, Peru (A4)

Why Fasto

We've been making fasteners since before a lot of our customers' engineers started in the trade. Twenty-plus years of export, our own factory floor — not a trading desk with a catalog. ISO 9001 and IATF 16949 sit on the wall; CE, SGS and ROHS documentation sits on the file shelf. When we say a bolt is A4-80, it's because the melt certificate says so, and you get a copy.

• Own factory with 20+ years of fastener export experience
• IATF 16949 & ISO 9001 quality systems; CE / SGS / ROHS product documentation
• ISO / DIN / ANSI fluency — WAF and standard confirmed before every shipment
• Full paperwork pack: EN 10204 3.1 certs, mechanical reports, salt spray reports
• M6–M24 assemblies stocked; mixed-load containers; OEM marking & pre-assembly options
• Engineers on the line for torque, galling, and material selection questions

The part that matters most on a product page is trust, and trust in fasteners is built on paperwork and consistency. That's the part we don't outsource.

Pain Points & Solutions 

FAQ 

Q1. What's the difference between 304 and 316 stainless steel?

ES: ¿Cuál es la diferencia entre acero inoxidable 304 y 316?   PT: Qual a diferença entre inox 304 e 316?

Molybdenum. That's the whole story, really. 316 carries 2.0–2.5% of it, 304 carries none. Mo is what stops chloride ions from punching through the passive film and starting pits. In the ISO 3506 system, 304 is steel group A2 and 316 is A4. The PREN numbers say it plainly: 18–19 for 304, 23–25 for 316.

Q2. Will 304 stainless bolts rust in coastal areas?

ES: ¿El inox 304 se oxida en la playa?   PT: Inox 304 enferruja no mar?

Inland and dry, 304 will behave for decades. Put it in salt spray — a coastal deck, a pool area, a road that gets de-icing salt — and you'll start seeing rust streaks and pits, usually within a couple of years. Chlorides attack the passive film and 304 doesn't have the molybdenum to fight back. For anything near seawater, spec A4/316 and don't look back.

Q3. What do A2-70 and A4-80 mean?

ES: ¿Qué significa A2-70 y A4-80?   PT: O que significam A2-70 e A4-80?

The letter is the steel group, the number is strength. A2 = 304, A4 = 316. 70 means a minimum tensile strength of 700 MPa, 80 means 800 MPa. So A2-70 is a cold-worked 304 bolt and A4-80 is a cold-worked 316 bolt. That's the language used on the head marking and the mill certs, so it pays to read it.

Q4. Are DIN 933 and ISO 4017 the same?

ES: ¿ISO 4017 y DIN 933 son lo mismo?   PT: ISO 4017 e DIN 933 são a mesma coisa?

Mostly yes, with a trap. The old DIN 933 became ISO 4017 and the geometry is the same — except across-flats. On M10, M12, M14 and M22 the wrench size changed. A 17 mm socket fits a DIN M10; the ISO M10 needs 16 mm. We confirm the standard on every order so your tooling doesn't surprise you.

Q5. Why do stainless bolts seize during installation?

ES: ¿Por qué se traban los tornillos de acero inoxidable?   PT: Por que parafusos de inox travam?

Stainless is tough and it work-hardens fast. Under heat and pressure the threads cold-weld together — that's galling, and once it starts you're cutting the bolt out. Prevention: anti-seize on the threads, torque in two or three steps, keep speed under 200 rpm, and pair A2 bolts with A4 nuts to lower the risk.

Q6. Full thread or partial thread — which one do I need?

ES: ¿Rosca completa o parcial, cuál usar?   PT: Rosca inteira ou parcial, qual usar?

Full thread (ISO 4017) — thread runs head to tip, for tension loads and deep engagement. Partial thread (ISO 4014) — plain shank under the head, bigger cross-section for shear, the usual choice in structural steel joints. If in doubt, tell us the load case and we'll point you right.

Q7. What torque should I apply to an M12 stainless bolt?

ES: ¿Qué torque para un perno M12 inoxidable?   PT: Qual torque para um parafuso M12 inox?

As a starting point with oiled threads (K ≈ 0.20): M12 A2-70 runs 75–92 N·m, A4-80 runs 100–122 N·m. Those numbers assume a clean, lubricated thread and a target preload around 70% of proof strength. Real joints vary — friction changes everything — so use a torque wrench and verify. The full M6–M24 table ships with the spec sheet.

Q8. Is 316 stainless magnetic?

ES: ¿El inox 316 es magnético?   PT: Inox 316 é magnético?

Austenitic stainless is non-magnetic in theory. Cold working — cold heading, thread rolling — converts some austenite to martensite, and that shows up as weak magnetism. For magnetic-sensitive jobs, ask for annealed product and we'll control the process for you.

Q9. What certificates come with an order?

ES: ¿Qué certificados incluye el pedido?   PT: Quais certificados acompanham o pedido?

Every order ships with EN 10204 3.1 material certificates plus mechanical test reports, and salt spray reports where required. For customs you get the full pack: commercial invoice, packing list, certificate of origin, mill certs. Brazilian and Colombian buyers tell us this alone saves them days at the border.

Q10. Which grade should I use at a mining site?

ES: ¿Qué grado usar en minería?   PT: Qual classe usar em mineração?

A4-80, no contest. Mining sites in Chile and Peru sit in saline dust with high humidity — that's a chloride attack on every surface. 304 will pit, and in stressed threads pitting turns into stress corrosion cracking. A4 has the molybdenum headroom to live there. Use A4 nuts and washers too; mixing grades invites galvanic trouble.