Bolts: grades, wrench sizes and torque

November 3, 2026  ·  Indumor

Tighten it till it creaks. A good share of the bolts in the world were done up by that recipe, and it breaks more of them than any overload does. A bolt works stretched, like a spring: the tightening torque lengthens it by a few hundredths of a millimeter, that stretch is what clamps the parts, and as long as the spring stays tensioned the joint rides out the vibration. Push the tightening past the yield point and the spring stops being a spring: the bolt stretches without coming back, the preload collapses, and within hours the nut works loose by itself, the thread strips or the head ends up in the wrench. A 12.9 tightened by feel with a pipe on the handle snaps as easily as a 4.6; it just gives less warning.

So tightening torque is not an opinion. It is a number that follows from the bolt class, the diameter and the friction. This chapter brings the classes and grades with their head markings, the head and wrench dimensions of everyday hardware, metric torque charts computed with the VDI 2230 method and inch charts with the SAE method, the stress area of each thread, and what you need to know about thread engagement and the torque wrench itself.

Classes and grades: what the head says

A metric bolt carries its property class on the head: two numbers with a dot between them. The first is the tensile strength in hundreds of MPa, and the two multiplied together, times ten, give the yield strength. An 8.8 holds 800 MPa and starts yielding at 0.8 × 800 = 640; a 10.9 holds 1000 and yields at 900; a 12.9, 1200 and 1080. The classes are defined in ISO 898-1 with the minimums in the table. Class 4.6 is the plain commercial bolt, 8.8 the one for machines and structures, and 10.9 and 12.9 are quenched-and-tempered alloy steels that you do not weld or hot-dip galvanize without thinking twice, because the hydrogen from the bath embrittles them.

A nut carries a single number, one hundredth of the stress it can hold: a class 8 nut carries the proof load of an 8.8 bolt without stripping. The rule is a nut of the same number as the bolt class, or higher. Put a class 5 nut on a 10.9 bolt and the nut threads shear before the bolt reaches its preload, and the tightening goes nowhere. A nut stronger than the bolt does no harm; a weaker one does.

Inch bolts follow SAE J429, which stamps lines instead of numbers: none for grade 2, three for grade 5, six for grade 8. Structural bolts follow ASTM: A307 for common and anchor bolts, and A325 and A490 (now grouped under F3125) for pretensioned connections, which have their own guide on structural bolts. Stainless bolts under ISO 3506 are marked with the steel and the strength: A2-70 is a 304 at 700 MPa, A4-80 a 316 at 800.

1 ksi = 6.895 MPa: 55 ksi = 379 MPa, 74 = 510, 85 = 586, 105 = 724, 120 = 827, 150 = 1034. Proof load is the stress the bolt must carry without permanent set.

Head, wrench and nut dimensions

Two systems share the drawers in most shops. ISO wrench sizes (ISO 4014 and 4017 for bolts, 4032 for nuts) and the old DIN sizes (931/933 and 934) differ on four threads: M10 went from 17 to 16 mm, M12 from 19 to 18, M14 from 22 to 21 and M22 from 32 to 34. Because DIN hardware kept selling for decades after the change, one machine can carry M12 bolts with 18 mm heads and 19 mm heads side by side, and you need both wrenches. The table gives both sizes.

b = 2d + 6 for lengths up to 125 mm; 2d + 12 from 125 to 200; 2d + 25 above that. ISO 4017 is threaded to the head. Last column: the wrench size of the old DIN hardware, still common in Latin American shops.

A socket head is k = d high and sits in a counterbore about 2 mm larger than dk (DIN 974). DIN 913 has a flat point, 914 a cone point, 915 a dog point and 916 a cup point; all four take the same key.

A split lock washer is not a lock: at 8.8 and above the preload flattens it and it stops acting as a spring. It works on small 4.6 and 5.8 bolts; to keep a joint from loosening, use a prevailing-torque nut or the right preload.

Inch wrenches are measured across flats just like metric ones, and some match almost exactly: a 3/4 fits a 19 mm nut with five hundredths to spare, and a 7/16 fits an 11 mm. Others look alike and are not: a 1/2 (12.7 mm) does not go on a 13 mm M8 nut, and a 15/16 does not go on a 24. For a wrench to fit it must be larger than the nut, and to keep from rounding it, by less than 0.3 or 0.4 mm.

A wrench fits if it is larger than the nut; with more than 0.3 or 0.4 mm to spare it rounds the corners on the first hard pull.

Tightening torque: where the number comes from

Torque charts come out of a calculation. You decide how far to stretch the bolt (the preload F_M), take it to 90 % of its yield while counting the torsion that thread friction twists into it during tightening, and then work out the torque needed to beat the friction under the head, the friction between the threads and the slope of the thread itself. That is the VDI 2230 method, the German guideline for bolted joints, and it is where the charts in the catalogs come from.

The split is the one in the drawing, and it explains why torque depends so much on friction: close to half goes into the head rubbing on the washer, about 40 % into the threads, and only 10 to 15 % actually stretches the bolt. An oiled bolt, or one with assembly paste, has a μ of 0.10 instead of 0.14, and with the same wrench reading it ends up about 30 % tighter; on a 12.9 that is enough to take it past yield. The charts in this chapter are for μ = 0.14 (black or zinc-plated bolt, dry or lightly oiled); with lubricant you recalculate, and the calculator does it.

A guide only: the torque actually needed depends on friction (finish, plating, oil, paste), on the washer and on tightening speed. The current edition of VDI 2230-1 uses 3/2 instead of 2 in the torsion term and its chart runs 8 to 11 % higher (M12 8.8: 93 N·m); this chart stays on the conservative side.

The last column of the first table is the chart found in every catalog. It agrees up to M16; from M20 up the computed values run 3 to 4 % higher because ISO 898-1 sets 660 MPa yield for 8.8 bolts over 16 mm and the old charts used 640. Either one will do: a click-type torque wrench is allowed a 4 % error (ISO 6789), and the real friction of a thread varies a good deal more than that.

Inch torque: SAE grades 2, 5 and 8

American manuals use a shorter formula, T = K · F · d, where K is a nut factor that lumps all the friction together and F is the clamp load, taken as 75 % of the proof load of the grade. K is about 0.20 for a black bolt dry (some tables go up to 0.30), 0.15 to 0.20 zinc plated and 0.12 to 0.18 lubricated. The result comes out in lbf·in and is divided by 12 for lbf·ft (ft-lb on most wrench scales); multiply by 1.356 for N·m (chapter 1). The table uses K = 0.20 and matches the charts in American fastener catalogs. The stress areas of the inch threads are below; pitches and tap drills are in chapter 5.

A guide only. Zinc plated: K ≈ 0.15 to 0.20 (cut up to 25 %); oiled or with paste: K ≈ 0.12 to 0.18 (cut 10 to 40 %). The wrench size is for regular hex cap screws (ASME B18.2.1).

Thread engagement, studs and the wrench

The torque charts assume the female thread is as strong as the bolt. In a nut of the right class that is guaranteed; in a tapped hole it depends on the material. In quenched and tempered steel, a thread length equal to the diameter is enough, but in mild steel (S235, F-24) an 8.8 bolt asks for 1.6 to 1.8 d; cast iron needs 1.25 to 1.5 d and aluminum 2 d, because the soft thread strips before the bolt stretches. Studs come with that length built into the end that goes into the part: DIN 938 for steel, 939 for cast iron, 835 for aluminum and 940 for soft materials. Pitches and tap drills for the hole are in chapter 4.

Guide values for coarse thread and an 8.8 bolt; they change with the bolt class and the strength of the material.

That leaves the torque wrench, which is a measuring instrument, not a breaker bar. It is not for loosening, it never gets a pipe on the handle, you bring it up to torque in one continuous pull until the click and stop there: a second pull adds nothing. Click wrenches are stored at the bottom of the scale so the spring does not take a set, and they are calibrated every 5000 cycles or once a year, whichever comes first (the ISO 6789 default; company procedure overrides it). And one that fell off the bench onto the floor gets calibrated before it is used again, however nicely it still clicks.

Frequently asked questions

What does 8.8 on a bolt mean?

It holds 800 MPa in tension and starts yielding at 640 MPa (0.8 × 800). It is the everyday class for machines and structures and goes with a class 8 nut. A 10.9 holds 1000 and yields at 900; a 12.9, 1200 and 1080.

What torque for an M12 8.8 bolt?

About 85 N·m (8.7 kgf·m, 63 lbf·ft) dry or lightly oiled, μ = 0.14. A 10.9 takes 125 N·m and a 12.9, 146. If the thread carries assembly paste the same torque over-stretches it: cut 20 to 30 %.

What wrench size is an M10 bolt?

ISO 4014/4017 hex: 16 mm; old DIN 931/933 hex: 17 mm; DIN 912 socket head: 8 mm key; ISO 4032 nut: 16. M12 takes 18 (ISO) or 19 (DIN); M8 takes 13 in both systems.

Sources: ISO 898-1:2013 (mechanical properties of carbon and alloy steel bolts); ISO 898-2:2022 (nuts); SAE J429:2014 (inch grades); ASTM F3125 (A325 and A490) and ASTM A307; ISO 3506-1 (stainless fasteners); VDI 2230-1:2015 (systematic calculation of bolted joints: preload and assembly torque); ISO 4014, ISO 4017 and ISO 4032 (hex bolts and nuts); ISO 4762 / DIN 912 (socket head cap screws); DIN 7991 (countersunk); DIN 913 to 916 (set screws); ISO 7089 (flat washers); DIN 127 (split lock washers); ISO 273 (clearance holes); ASME B1.1 (unified threads, stress area); ASME B18.2.1 (inch hex heads); DIN 938, 939, 835 and 940 (studs); ISO 6789 (torque wrenches).