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The number on a bolt head is its strength class: 8.8 = 800 MPa tensile strength / 640 MPa yield (standard structural steel), 10.9 = 1000 MPa / 900 MPa (high-strength, automotive & heavy machinery), 12.9 = 1200 MPa / 1080 MPa (highest strength, critical precision applications, but brittle and hydrogen-sensitive). For most general use, 8.8 is correct; use 10.9 where loads are high; 12.9 only where its brittleness is acceptable.
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.
The number on a bolt head is its strength class: 8.8 = 800 MPa tensile strength / 640 MPa yield (standard structural steel), 10.9 = 1000 MPa / 900 MPa (high-strength, automotive & heavy machinery), 12.9 = 1200 MPa / 1080 MPa (highest strength, critical precision applications, but brittle and hydrogen-sensitive). For most general use, 8.8 is correct; use 10.9 where loads are high; 12.9 only where its brittleness is acceptable.
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.
