You need a torque wrench if you’re serious about engine building or the safety of key components like brake caliper mounting bolts or spindle nuts
Inside Patrick Williamson’s den of ancient and modern – and plenty of junk too
How tight is tight enough?there are those blessed with the touch of a concert pianist who have such inbuilt self-calibration they never strip a thread or leave a fastener too loose. For everyone else there is the torque wrench.
Not that you need one for every job, although if you are particularly unpractised or ham-fisted a torque wrench dispenses with guess-work.after time you will find yourself less reliant on one as your feel improves.
Torque wrenches are indispensable for some tasks especially where even tightening or safety are critical; cylinder heads and caliper bolts are obvious candidates here. When given torque values are at
astronomical levels, for example crankshaft gear and clutch centre nuts, it’s hard to gauge if you’ve gone tight enough and a decent wrench is helpful there.at the other end of the spectrum, those small fasteners sometimes seem to go from loose to stripping the thread with the merest fraction of a turn.
Accurate torquing relies on a properly calibrated quality wrench. Rather than a one-size fits all tool, you ideally want one for small torque figures, one for medium and one for large.that’s because typically each of these will be more accurate within their given ranges than one that attempts to span them all.accuracy also depends on clean threads, or indeed greased or oiled threads if that’s what the manual specifies.
And what exactly is torque?
Torque is twisting force around a shaft. In the context of fasteners, the bolt, screw or stud is the shaft.when the amount of force applied to the lever and its distance of its end from the centre of the fastener are multiplied, there’s your torque; distance x force = torque.
Torque wrench types
The most basic type of torque wrench and indeed the cheapest is the deflection beam type where the handle bends as pressure is applied and a pointer aligns with a scale on the handle.then there are the ones where you preset the torque required using a micrometer type barrel. Once the desired torque is reached the wrench clicks and the ratchet momentarily lets go.there are also electronic ones that beep when you hit optimum tightness.
Always release the spring pressure on a wrench where you preset the torque. Don’t use your torque wrench as a socket drive. For critical application accuracy, some people have their torque wrenches properly calibrated. For our purposes this is needless expense. Most are only out by a few percent at the extreme ends of their ranges. Note that some torque wrenches have a marker on the handle where you should try to centre the pressure you place upon it.
How much torque do I use?
This is dictated by the fastener size, its material, what it’s holding together or screwing into or onto and whether it’s compressing a gasket or not.where a torque figure is given, that’s the one you want to stick to.where there is a range of acceptable torque, go with the number in the middle.
Before getting the torque wrench out, fit the fasteners and get them handtight. Don’t
tighten one fully while leaving the others loose where you have more than one fastener securing a component, let’s say a cylinder head for example.
Tighten evenly and work on diagonal opposites from the centre out. Once a component is handtight, break out the torque wrench, again sticking to the rule on diagonal opposites.
A barrel preset type torque wrench clicks or ‘lets go’ when the correct torque is reached.there are two schools of thought on whether you should go around the fasteners with the torque wrench again after it has clicked. Some say that to do so is to risk overtightening, others say its a decent insurance policy. In truth you will rarely find a fastener that takes further tightening with the torque wrench after it’s been done once.
Unless otherwise stated, torque figures given are for clean and dry threads in good condition.an oily bolt will be too tight or even snap if you use the dry torque figure. Conversely a rusty or dirty fastener or one with damaged threads will be too loose despite the torque wrench telling you it’s tight because of the increase in friction.
Extending your reach
Sometimes you’ll need to use a crows foot or dogbone extension to get onto a fastener. In these cases the figure set on the wrench will no longer be the same as that applied to the fastener.you have to do a little sum.
Measure the normal effective length of the wrench and the extended effective length. Divide the first by the second (this will be less than one) then multiply it by the desired torque to get the new figure you should set the wrench to.
For example:you want 12Nm on a bolt. Your wrench’s normal effective length is 300mm.the dogbone adds an effective 80mm. 300 divided by 380 is 0.79. 12 multiplied by 0.79 is 9.48. So something around 9.5Nm is what the wrench should be set to.
Windswept numpty in sleeveless jerkin is here to help
Not a tricky tool to use. But a very necessary one
A torque wrench is really no more than a big, accurate spanner