Understanding tightening torque: tightening to Nm correctly, extensions, property class

Workshop guideUpdated 2026-09-13ELDI / Racing Planet editorial team

Every torque table quotes figures in newton metres. What the figure actually means, why an oiled thread changes it and what happens when you hang a hook spanner on the torque wrench as an extension are things many mechanics only half know. The result is sheared studs and stripped aluminium threads.

This guide explains the principle behind tightening torque, the types of torque wrench, the influence of friction and lubrication, the formula for extensions and the property classes 8.8 and 10.9. By the end you will know what to watch out for on aluminium threads and with the wheel hub spanner.

What a newton metre is

Torque is force times lever arm. One newton metre is produced when you pull with a force of one newton on a lever one metre long. One kilogram of mass exerts a weight force of about 9.81 newtons on earth. So if you hang 1 kg on a horizontal lever one metre long, you get around 10 Nm. Hold a spanner 20 cm long and pull with 50 N, that is about 5 kg, and the result is 10 Nm.

When tightening a bolt, torque is only an auxiliary measure. What you actually want is preload: the bolt is stretched like a spring and presses the parts together. That force cannot be measured in the workshop, but the torque can. The table figures are chosen so that the bolt is preloaded to about 70 to 90 per cent of its yield point.

Only about a tenth of the torque goes into stretching the bolt. The rest is turned into friction under the head and in the thread. That is why lubrication plays such a large part.

Torque wrenches: break-back, click, beam

Type	Function	For the workshop
Break-back wrench	The head noticeably breaks over when the figure is reached	Easy to read, robust, takes practice to reset
Click wrench (releasing)	Releases with an audible click, a spring sets the figure	The workshop standard, wind it back to the lowest figure after use
Beam wrench	A pointer on a scale shows the bending of the beam	Simple, no adjusting mechanism, hard to read in tight spaces
Electronic wrench	Strain gauges, display with an audible signal	Accurate, also measures angle of rotation, depends on batteries

A wrench is only as accurate as its calibration. A click wrench left in the cupboard under tension loses spring force. Wind it back to the lowest figure after every use. Pull slowly until it releases, then stop.

For a moped engine you usually need two ranges: a small wrench for M5 and M6 in aluminium and a medium one for axle, flywheel and clutch nuts. In the bottom fifth of its range a wrench works inaccurately.

Friction, oil and the table figure

The figures in a torque table apply to a particular friction condition. The standard tables for metric bolts assume a lightly oiled or black tempered thread. A dry, rusty thread has more friction: at the same torque, less preload is produced. A thread lubricated with grease or copper paste has less friction: at the same torque, considerably more preload is produced, and in extreme cases the bolt or the thread fails.

Rule of thumb: stick to the condition the manufacturer specifies. If the workshop manual for your machine says nothing about it, tighten clean and lightly oiled. With copper paste or thread lock you reduce the torque, and many manufacturers quote 10 to 20 per cent. For spark plugs the part-turn rule is safer, as described in changing the spark plug and reading the plug.

The thread pitch plays a part too. Metric coarse thread: M6 with 1.0 mm, M8 with 1.25 mm, M10 with 1.5 mm, M12 with 1.75 mm. A fine thread produces more preload at the same torque and therefore has its own figures in the table. You will find all the pitches in the thread table, and you can identify unknown threads with the thread finder.

Reading property class 8.8 and 10.9

The property class is marked on the head of every standard steel bolt from M5 upwards, for example 8.8 or 10.9. The two numbers give away the material figures:

• The first number times 100 is the tensile strength in N/mm². So 800 N/mm² for 8.8, and 1000 N/mm² for 10.9.
• The second number times the first times 10 is the yield point in N/mm². So 8 x 8 x 10 = 640 N/mm² for 8.8, and 10 x 9 x 10 = 900 N/mm² for 10.9.

The yield point is the stress above which the bolt stretches permanently. A 10.9 bolt therefore takes around 40 per cent more preload than an 8.8 of the same size and is given a correspondingly higher torque in the table. The reverse also applies: tighten an 8.8 bolt to the 10.9 figure and you go past the yield point. The bolt stretches permanently, loses preload and fails the next time it is tightened.

Treat bolts with no marking as 8.8 or below. Nuts carry a single number: nut 8 with bolt 8.8, nut 10 with bolt 10.9. The sizes across flats to ISO 272 are in the across flats table: M6 AF10, M8 AF13, M10 AF16 or AF17 old, M12 AF18 or AF19 old. On older machines you will come across the old sizes across flats.

Extension on the torque wrench: the formula

Some nuts can only be reached with a hook spanner or a crowfoot spanner, which changes the length of the torque wrench. One example is the wheel hub spanner, 17 mm for Sachs hubs, which grips the ring nut as a hook spanner and is driven by a socket on the torque wrench.

The torque wrench measures the torque at its head. If an attachment extends the lever arm away from the handle, a greater torque is produced at the bolt than the wrench indicates. You therefore have to set the wrench to a smaller figure:

Worked example: wrench with an effective length of 400 mm, crowfoot spanner 80 mm, target torque 60 Nm. Then M_set = 60 x 400 / (400 + 80) = 50 Nm. You set 50 Nm and get 60 Nm at the nut.

Special cases: if the attachment stands at right angles to the wrench, you set the target figure. If it points back towards the handle, E becomes negative and you set a higher figure. A socket extension at right angles between the square drive and the socket does not change the torque.

The torque calculator works out the figure for your attachment once you enter L, E and the target torque.

Aluminium threads: less is more

Crankcases, cylinder heads and covers on moped engines are cast aluminium. The thread in the aluminium is considerably weaker than the steel bolt in it. The table figure for 8.8 applies to a steel mating thread and is too high in aluminium. The tables for aluminium therefore quote lower figures.

For studs in aluminium the rule is: tighten the nut to the table figure, but screw the stud itself in hand tight only. Do not retighten anything while it is hot: the aluminium has expanded, and the preload is higher than it is cold.

Tightening correctly: step by step

1. Look the figure up

   Take the figure from the workshop manual for your machine. If there is none, take the figure for the thread size and property class from the tightening torque table, and for aluminium the reduced figure.

2. Check the thread

   Thread clean, undamaged and dry or lightly oiled as the manufacturer specifies. With thread lock or paste, reduce the torque.

3. Set the wrench

   Set the target figure. With an attachment in line with the wrench, convert it using the formula or the torque calculator.

4. Keep to the sequence

   Where several bolts hold one component, such as a cylinder head or the case halves, tighten crosswise in at least two stages: first all of them to about half the figure, then all of them to the full figure.

5. Pull slowly up to the signal

   Pull evenly, without jerking, with your hand in the middle of the handle. Stop after the click. No second click.

6. Retighten after settling

   Joints with gaskets settle after the first warm-up. Check them again cold at the target figure.

The tool finder shows which tools you need for your model. The spark plug is tightened with the spark plug spanner, 21 mm to the part-turn rule. For axle nuts, head bolts and ring nuts the rule is: torque wrench, not feel.