An M10 bolt of property class 8.8 needs to mount a plate. What clearance hole, wrench size and tightening torque belong to it?
Three independent choices per thread size
The clearance hole (ISO 273) sets how roomy the hole in the part being fastened is — fine, medium or coarse, depending on the positioning accuracy needed. The wrench size (ISO 4014/4017 for hex, ISO 4762/DIN 912 for socket) sets which tool fits. Both follow directly from the thread size, but they're independent tables — one doesn't predict the other.
Worked example: M10, class 8.8
For M10, the clearance table gives 10.5 mm (fine), 11.0 mm (medium) or 12.0 mm (coarse); the wrench size is 16 mm (hex) or 8 mm (socket). The stress area A_s is 58.0 mm². Class 8.8 has a yield strength Rp0.2 = 800 × 0.8 = 640 N/mm². At 75% utilisation, the preload is 0.75 × 640 × 58.0 ≈ 27,840 N. With a nut factor K = 0.2, the tightening torque follows as T = 0.2 × 27,840 × 10 / 1000 ≈ 55.7 N·m.
The nut factor is an assumption, not a constant
K ≈ 0.2 is the common rule of thumb for unlubricated, black- or phosphate-finished steel fasteners. With certain lubricants or coatings, K can decrease — a lower K at the same torque means a higher actual preload, which can overload a bolt if you keep using the table K. Use validated friction data for the specific fastener, coating and lubricant. Stainless steel alone does not imply a lower K; also assess galling risk.
Assumptions and applicability limits
The clearance, wrench-size and stress-area tables are standard values (ISO 273, ISO 4014/4017, ISO 4762/DIN 912, ISO 898-1). The tightening torque is a rule-of-thumb calculation — not a standard requirement — that doesn't account for friction variation between bolts, reuse, dynamic loading or a specific tightening procedure (angle-controlled, torque-limited). For critical joints, use a qualified assembly procedure, validated friction data and preload verification where required. A torque wrench controls applied torque, not achieved preload.