> But if you're e.g. repairing existing equipment, then the customary and SI units are incompatible. Say, a 1/4 inch bolt head is too large for a 6 mm wrench, b

> But if you're e.g. repairing existing equipment, then the customary and SI units are incompatible. Say, a 1/4 inch bolt head is too large for a 6 mm wrench, but too small for a 8 mm (which is typically the next step up from 6mm). Where is the problem? Buy a 6.35mm wrench for your 6.35mm bolt head.