A bolt is a thread with jobs attached: a head to turn, a nut to clamp, a washer to spread the load and a torque that turns tension into grip. This guide covers the numbers you need at the bench.
Head and nut sizes are not always the same
For most SAE bolts the hex head and the finished hex nut take the same wrench, but not always: a 7/16" cap screw head is 5/8" across flats while its nut is 11/16", and a 9/16" head is 13/16" against a 7/8" nut. Metric splits differently. ISO 4032 nuts for M10, M12, M14 and M22 are 16, 18, 21 and 34 mm, while older DIN 934 nuts are 17, 19, 22 and 32 mm.
Torque is a way of setting tension
The torque you apply is mostly lost to friction. Only a small part stretches the bolt. Our torque figures target 75% of the bolt’s proof load with a nut factor K of 0.20 for clean, dry, unplated steel. Oil, anti-seize or zinc plating cut friction, so the same torque produces more tension: reduce the figures by 20–25% when the threads are lubricated.
T = K × F × d F = 0.75 × proof stress × tensile stress area
Grades and property classes
SAE Grade 2 is low-carbon steel, Grade 5 is medium-carbon quenched and tempered (three radial lines on the head) and Grade 8 is alloy steel (six lines). Metric class 8.8 sits close to Grade 5 and 10.9 close to Grade 8. 18-8 stainless is corrosion-resistant but weaker than Grade 5 and prone to galling, where threads seize and tear under load.
Questions
Answered plainly.
01What size wrench fits a 1/4" bolt?+−
A standard 1/4"-20 hex cap screw and its finished hex nut both take a 7/16" wrench. Hex washer-head and flange bolts can differ, so check the head itself on the hex sizer above.
02Can I use a 13 mm wrench on a 1/2" nut?+−
It fits, but loosely. 13 mm is 0.512", so it is 0.3 mm oversize on a 1/2" (12.7 mm) hex and can round a tight nut. A 1/2" wrench will not go over a 13 mm nut at all. 19 mm and 3/4" are close enough to swap.
03How tight should a 1/2"-13 Grade 5 bolt be?+−
About 75 ft-lb dry with our model (K = 0.20, 75% of proof load), or roughly 57 ft-lb if the threads are oiled. Follow the equipment manufacturer’s figure when one is published: gaskets, soft materials and engineered joints often call for less.
04Why do stainless bolts seize?+−
18-8 stainless threads gall: under pressure the oxide layer breaks and the surfaces cold-weld. Use anti-seize, tighten slowly without power tools, and stay below the stainless torque figure, which is lower than Grade 5.
05SAE or USS washer?+−
SAE washers are narrow and fit closely around the bolt. USS washers have a larger outside diameter to spread the load on wood and soft materials. A 1/4" SAE washer is 0.625" across; the USS version is 0.734".
06What is a flange nut for?+−
A flange nut has a built-in washer face, usually serrated, which spreads the load and resists vibration. A 1/4" flange nut has a flange about 0.59" across. Use it where a loose washer could get lost or where you can only reach one side.
07How do I tell a Grade 5 from a Grade 8 bolt?+−
Look at the head. Three radial lines mark Grade 5, six mark Grade 8, and a plain head is usually Grade 2. Metric bolts are stamped with the class itself, such as 8.8 or 10.9.