NEWS

A flat washer is specified by inner diameter (ID) = bolt diameter + ~0.4–1.0 mm clearance, outer diameter (OD), and thickness (t). An M8 flat washer (ISO 7089) has an 8.4 mm ID, 16 mm OD and 1.6 mm thickness. The washer's ID must always be slightly larger than the bolt — never smaller.

A metric hex nut is specified by the thread size (M) — an M8 nut fits an M8 bolt and takes a 13 mm wrench; M10 takes 16 mm; M12 takes 18 mm. Two dimensions matter for fit: the across-flats width (W), which equals the bolt's wrench size, and the nut height (m), which must be tall enough for the thread engagement.

Machine screws are sized by diameter × thread pitch × length (e.g. M4×0.7×20). Metric machine screws range from M1.6 to M10 (M3, M4 and M5 are the most common); inch machine screws are numbered #0–#12 (e.g. #8-32). "Machine screw" means the screw is designed to fasten into a tapped hole or with a nut — it does not cut its own thread.

For metric hex bolts (ISO 4014/4017), the two dimensions you need are across-flats width (W) — the wrench size — and head height (k). An M8 bolt has a 13 mm wrench size and 5.3 mm head height; an M10 has 16 mm and 6.4 mm; an M12 has 18 mm and 7.5 mm. Diameter is the thread diameter (M8 = 8 mm thread), and length is measured from under the head to the end of the thread, not including the head.

Choose stainless steel (304/316) when corrosion resistance is the priority — outdoor, marine, chemical, or humid environments — even though it costs 3–6 times more than galvanized. Choose galvanized (zinc-coated carbon steel) when you need high strength at low cost in moderate environments — indoor, dry, or mild outdoor use — because carbon steel grades (8.8/10.9) are far stronger than standard stainless (A2-70 ≈ 5.8 class). If the application is outdoor industrial or near the coast and strength matters, consider hot-dip galvanized high-strength bolts as the middle ground.