Méthode serrage dynamométrique

Tutorial : how to correctly tighten with a torque wrench

With the increasing number of people working themselves directly on their cars and motorcycles I found interesting – and I hope useful for you – to write an article on how to correctly use and tighten with a torque wrench. This tool is well known by a lot of mechanics but it is sadly very regularly “badly” used leading to wrong tightenings, loosening or overtightened bolts with tons of chances to have damages on their related threads.

What you have to clearly remember when you want to use this type of wrench for a wheel stud or for a Formula 1 head stud is that this is firstly a metrological tool. If you want to get the right result when using it, you just have to understand and follow a few basic rules.

You can be a professionnal mechanic or a sunday morning one but please, don’t tell me that you never ever overtightened a bolt on one of your vehicles, sometime with big consequences … remember that feeling when you went so far with your wrench of “just a little” 🙂 ?

1/ Units and calculations :

From a mechanical point of view, we can apply loads on an object in 2 ways : apply a FORCE or apply a TORQUE.

A FORCE can be represented as an arrow whose tip indicate the direction and whose size indicates its value. Imagine an arrow pushing a box on the ground : the box is being pushed in the arrow direction with a force directly related to its size !

A TORQUE is rather used to rotate an object instead of pushing it in a linear way. Example : the torque of an engine allows to rotate its crankshaft ! In fact, a torque is a FORCE applied with a specific distance from the rotation axis of the part we would like to rotate.

The best example is a DOOR. The closer to the hinge you apply a force with your finger the smaller the torque applied on the door to start its rotation (because of the short distance). Conversly, the farther from the hinge you apply the force the bigger the torque applied on the door and the easyer its rotation.

Units : a FORCE will be expressed in NEWTON (represented by a N letter). We can apply a force of 100N on our box laying on the ground to try moving it.

A TORQUE as we just said before is a force applied at a distance from the rotation axis. Let’s talk about our door again : if we apply a force of 100N at a hinge distance of 0,5m, then the applied torque on the door is 100×0,5 = 50N.m (you should read it as 50 newton meters)

So, does it sound familiar to your hears ? You start to remember that a N.m value is the kind of stuff you’ve seen for … your wheel nuts for example ? It is the same for your head studs, spark plugs … everything were you need to have a control over your tightening.

Something important : NEWTONS and METERS are widely used all over the world except in some countries like the US. If you have some forces expressed in pounds (Lbs) or distances expressed in Inch, all principles are exactly the same. Only the values are a little bit different and you may have to convert them.

2/ Metrological principle :

If you want to clamp two thick metal sheets together with a bolt and nut assembly, you will notice that the needed “effort” to tighten the nut on the bolt will increase regularly all the way to the desired result. This is absolutely normal as all the parts will interacte together, friction will increase and so the amount of force you need to apply on the assembly to overcome the ‘resistance’.

So, tightening a bolt or nut with “the right amount of torque” means that you will know and manage the correct amount of torque you need to apply in order to overcome friction and resistance to get your assembly as you want it for different reasons.

“Hey, so why should I use this torque wrench ? I can just go by hand and add a little bit more tightening when I think I’m done to be sure, and this will not move at all right ?”

And … no. You can’t just do this because the purpose of a bolt and nut assembly can be way different between two applications :

  • Wheel studs : you need to apply a torque to be sure that the wheel is well maintained, but also because the right torque will ensure that the nut will not loosened during your drive !
  • Head stud : the manufacturer choosen torque will not only maintain the head over the cylinders of your engine, it will also ensure that the sealing with the headgasket is good, that there is no vibrations between head and cylinders, and that the head can hold the high detonation pressure when the engine is running.
  • Parts made out of different materials : you can have an assembly done by two bolts which seems exactly identical. But in fact, if you have different materials (for example stainless steel and alloy) you will have big troubles tightening at the same torque. It will be ok for the stainless steel one but you will abolutely need to reduce torque applied on the alloy one because threads will be weaker. Or you’ll just destroy the alloy bolt …
  • Etc …

So yes, if your user manual tells you to tighten a bolt or a nut to a precise torque, there is a very good reason to do so … 🙂

3/ Hardware :

On the market, you can find different types of torque wrench :

  • Direct reading : you can read directly the torque being applied on the bolt or nut through a needle pointing the value on a display of the wrench.
  • Automatic triggering : a sound is triggered by the wrench to alert you when you reached the predetermined value of torque on the wrench.

We can say that a direct reading wrench is perheaps more precise than a trigering one, but the bigger concern about precision comes more from the wide range of torque wrenches available on the market : the more expensive the more precise, yes. But how can you decide what’s the right suitable wrench for your application ?

So first question amongst all : is there a real difference between a 30 € wrench and a 300 € one ? YES !

Tools manufactured by great brands will be made out of greater materials, have more precise triggering systems or reading gauges, they will be delivered with a certificate of calibration indicating that they have been precisly prepared and checked for their job …

But the other question coming as fast as the first one is : should I buy only an expensive wrench if I want to get a good result when using it ? NO !

The real first criteria to consider is the frequency of use : if you own a repair shop and use this kind of tool every day, you know that buying chinese sh** is buying three times the same sh**. So you will go for the right brand like FACOM or another one.

BUT, if you’d like to get a nice tool but knowing that it will be used only a couple of times every 6 months, you have to search and try to find middle range prices and evaluate the quality of the brand you find.

Please, don’t go for shitty cheap tools … you will be SO disappointed with it !

At the time I was looking at a new wrench, I found a brand called KS Tools. Correct prices and good feedbacks made my decision and I had no problem at all since I own my 2 wrenches (around 6 years of use).

Here is a picture of the wrench I use mostly (covering a 20 N.m – 200 N.m range) :

torque-wrench-001

And yes, you will have to determine what range you mostly want to cover with your wrench as they do not cover all torques. If you usually work on your bicycles or on small engines you will not buy the same covered range as if you own a truck and have heavy tightenings to do for wheel bearings or stuff like that.

  • The first one I bought covers 20 > 200 N.m allowing to work on a LOT of requested torques for classic cars and motorcycles.
  • The second one I bought recently is a 1 > 25 N.m because I had more and more small jobs to do.

Remember to check what you really needs for torques 😉

4/ Read bolts manual and follow rules !

Unless you are able to design and build your own engines and gearbox from scratch, you will probably have access to every technical sheets and manuals you need to move forward on your projects.

Please, don’t think that you are smarter than the group of engineers who designed and tested the assembly you’re rebuilding ! What you need to be aware of is that some bolts and nuts assemblies NEED to be greased or oiled … but some other DON’T !!

It will be too long to describe why some assemblies should be oiled, greased or not but what you have to remember is that you should never take any initiative by changing what is indicated in your user manual.

Remember, you always have to respect the following TRIO for a correct torque tightening : A SPECIFIC ASSEMBLY + OILED / GREASED ASSEMBLY OR NOT + A SPECIFIC TORQUE.

5/ THE MOST IMPORTANT : the right gesture when using the wrench

Speaking of a metrological way the person in charge of doing the tightening with the wrench is called an operator. You should know that a torque wrench, if wrongly used by the operator, can result in a final torque gap of +/- 50% on your assembly ! Somehow it can be ok for one assembly, not very important.

But the biggest problem comes when you have to repeat a set of identical tightenings on a part, for example when rebuilding an engine : the head needs to be tightened very precisly and this has to be done the same way on each of the 8 or 10 head studs …

So let say that you have to rebuild your head and each stud should be reinstalled with a 180 N.m torque. If you’re going the wrong way with your wrench, you will probably get as a result :

  • Stud 1 = 145 N.m
  • Stud 2 = 187 N.m
  • Stud 3 = 152 N.m
  • Stud 4 = 201 N.m
  • And so on …

Yes, the quality of your torque wrench is important BUT the way you use it is FAR MORE important :

  • Be sure that the part or assembly you’re working on is FIXED and will not move during your operations. If your engine is installed on an engine stand or on the workbench ask for some help and let your friends hold it to avoid any misstightening.
  • Prepare your torque wrench ! Yes I think that you read it right, I said please prepare your torque wrench before using it … let’s talk about our head again. We should tighten each stud at a 180N.m torque. If you didn’t use the wrench since a long time, think about setting it on a low torque value like 50 N.m and use it a few times on a wheel stud. By doing so it will help the wrench mechanism settle in and be more precise on your head torque.
  • Try to approach your bolts or nuts by hand and evaluate how much you’ll need to rotate the wrench. In fact, the secret of a good tightening is to be able to maintain a slow and constant speed all the way to the “click sound” of your wrench. If you can’t and have to tighten in 2 or 3 times, try to have the final sequence leaving you enough movement.

If you want, you can regularly watch people doing the wrong things with a torque wrench at a trackday for example. You see someone getting out of its car to change wheels between two sessions, working fast.

He needs to tighten the wheels and grabs a torque wrench, taht’s good. But in fact the guy triggers the wrench on a stud and just a second after, triggers it again fast on the same stud. And he do so with every studs on each wheel of the car. Baaddd …

By doing so the real torque applied on each stud will not be the one settled at the beginning but it will be 30 N.m “higher” … in this situation, the good way is to install the wheel, approach each nuts by hand and triggers the wrench with a regular movement one time only on each stud.

Last advice but not least is to “UNLOAD” your torque wrench when you store it. In fact, it concerns mainly the triggering wrenches. They are often made with a spring load based triggering system and by unloading it you help the inside spring aging better.

For example, if your wrench covers a 20 > 200 N.m, remember to unload it after use to a low value like 20 or 30 N.m.

6/ A few pictures of GOOD HABITS to have with your wrench (and something you should NOT DO)

Always use a SHORT SOCKET if you can. Of course sometime you will need to use a long one or one with an extension but remember to keep this principle in mind. The shorter the socket the more precise the tightening. Try to find the balance with your need to rotate the wrench enough.

torque-wrench-002

Here I have the shortest possible socket BUT this is not so good as I will have troubles rotating the wrench on the full operation. I can take a long extension but I will need to hold it with my hand and this is not good AT ALL : it will distort the triggering of my wrench. Please watch the picture below : this is what you should NOT DO !

torque-wrench-003

In fact I have in my toolbox a shorter extension that I installed on my socket. It is shorter enough to avoid the use of my second hand holding the extension. As you can see, I am short enough to get a precise triggering but long enough on the socket to have enough room and rotate the wrench regularly :

torque-wrench-004

torque-wrench-005

Another example on my own wheel studs. The socket is long enough to clear wheels but as you can see, it is also short and keeps a great precision on the tightenings.

torque-wrench-006

Conclusion

This is a short tutorial and this is also one written in english by a french native, so please be cool with my writing :).

A torque wrench is a well known tool and one used by many people working on their own vehicles. But this is also one of the most missused tool …

I hope that those few advices will help you understand why tightening with the right torque is important, and how to do it !

Category: DIY : Do It YourselfHomemade

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Article by: yogi