/DIY Car Repair – Understanding Torque Values

DIY Car Repair – Understanding Torque Values

In some cases when torquing a fastener, whether it is a nut or a bolt, it is very important to achieve a level of accuracy. If you are attempting to repair your car yourself there are some basic techniques that will save you both time and, in the long run, money. By understanding and applying some basic car repair techniques the average person is going to increase their chance at performing a successful car repair.

Car repair tools are something to consider – they can either provide a solution or, if used improperly, or in the case of Torque related tools, if not used at all, can make the car problem worse. You may not realize it but you may already own torque related tools. Have you ever wondered why the 10mm wrench is shorter than a 14mm wrench? Why does the ¼” drive ratchet typically only have sockets up to about 15mm? There is a good reason for both. If you use a 10mm wrench to tighten down a 10mm fastener, and aren’t over “torquing”, you are going to get the proper torque. If you used a wrench that is twice as long to do the same thing your chances of over torquing, breaking or compromising the fastener are vastly increased. Using a ¼” drive ratchet to tighten a 10mm fastener is much safer than using a ½” drive ratchet to tighten the same 10mm fastener. This is a simple matter of understanding the forces of leverage and torque, the longer the tool the more leverage you get and the easier the torque is applied. As a rule of thumb, use a ¼” drive ratchet to tighten fasters if the bits are available. Generally speaking there is no reason to use a larger ratchet to tighten the fasteners below 15mm.

There is another car repair tool that will get you the proper torque every time; provided the tool works properly. It’s the torque wrench. What does a mechanic have that you don’t? Well, for the most part, experience. Gaining a feel for how tight a bolt needs to be is the first step to getting the proper torque. This is simple to practice, and it is easily gauged with a torque wrench. There are certain instances that using a torque wrench is incredibly important. Rotating parts is one of them. You don’t want a spinning part to loosen the nuts or bolts that hold it in place. So torque your axle nuts and flywheel bolts. Any time there is a gasket or a nut or bolt that is a “Torque to Yield” it is very important to get out that torque wrench to ensure proper installation and operation.

Make sure you replace a the bolt or nut if it is recommended by a Car repair manual. This is because that bolt or nut has to be, and has been, torqued to yield. This means that the fastener is actually getting stretched to it’s proper torque. It also means that when that particular bolt/nut is removed you will not be able to reassemble using that previously torqued bolt and achieve the same required torque that assures both fit and function. Now when we are talking torque, let’s consider some other factors. When the car repair manual says to oil the bolt before torquing they are further modifying the manner in which that bolt gets torqued. The friction is reduced from the addition of the oil and this means that more force is now being applied to the bolt for that set torque value. Now let’s think about a dry bolt at the same torque – it is not going to get turned enough to do what the manufacturer intended to do at that torque. What is even worse is if there is a bunch of dirt, rust or any contaminants on the threads. So now that there is a bunch of dirt on the threads, you are increasing the friction and ultimately under-torquing your bolt. Do that on a new hub bearing assembly and you are going to have a separated wheel bearing that has too much play.
To get a better understanding of the importance of following torquing guidelines let’s consider a situation like replacing a gasket that holds oil or coolant – a good example is a valve cover gasket; the factory wants you to apply 10 foot pounds of torque to “most” valve cover gaskets. What happens if you over-torque? You squish the gasket too much and that creates the potential for oil to escape, this compromises the gasket and the required seal.

Here are a couple of charts to give you an idea of the relative torque values that are typically applied to different sizes of bolts. These are guidelines only and this information has not been verified.

Bolt Assembly Torque (ft-lb)
Values are based on the use of lubricated threads bolt size Grade 8.8 Grade 10 Aluminum

M6 5 10 4
M7 9 14 7
M8 17 25 14
M10 33 50 25
M12 60 85 40
M14 90 133 65
M16 140 200 100
M18 200 285 135

The above chart is for lubricated threads and only provides typical torque values – This chart is not to be followed for any torque to yield bolts or nuts and does not provide proper torque values for a bolt or nut that is holding a gasket surface. Gaskets are made of different materials and the manual pertaining to the exact application is the only source that should be used in determining the proper torque for these gaskets.

Note: size: 10mm does not refer to the bolt head it refers to the diameter of the shaft. Typical 10mm bolts have a 13mm or 14mm heads.

The following chart shows slightly different torque values, and is not based on lubricated threads. M8x1.25 is an 8mm diameter with a 1.25 thread pitch – the lower the number the finer the threads.

Hex head Cap Screw
bolt size Grade 8.8
M4 x 0.7 1.65
M5 x 0.8 4.13
M6 x 1.0 7.1
M7 x 1.0 12.0
M8 x 1.0 18.0
M8 x 1.25 17.0
M10 x 1.0 39.0
M10 x 1.25 36.0
M10 x 1.5 34.0
M12 x 1.25 65.0
M12 x 1.5 63.0
M12 x 1.75 59.0
M14 x 1.5 102.0
M14 x 2 94.0

Anther consideration is that fact that torque specification depends on the type of material used – 8.8 grade steel is the most common, but stainless steel fasteners are a stronger alternative, whereas, aluminum is a much softer metal and will require a different torque. Always check the manufacture’s manual for proper torque specs. No chart that is provided in this document has been validated for accuracy. Most factory service manuals will show a torque specification chart simply based off the size of fastener, however this should only be referred to if the specific torque application is not provided.

After reading this you should feel a little more confident and a little more worried. Continue to do your homework and take on the car repair jobs within your means. Ultimately, it is the experience that is required and some failures can be great learning experience so practice first and take your time.

Shane White has over 30 years’ hands on experience in the Auto Repair and Aftermarket Auto Parts industry. As a fully licensed mechanic Shane ran a successful garage for over 10 years. Over the past 9 years Shane has focused on the managerial side of the Auto Repair and Replacement Auto Parts industry. Currently Shane is Vice President Operations with Prime Choice Auto Parts a Factory Direct to consumer, online store, specializing in High Quality – High Value Aftermarket Auto Parts like Hub Bearing Assemblies, Complete Strut Assemblies, Brake Parts, Car Starter Motors and Alternators for all makes and models.

Buy – Strut Assembly from Prime Choice Auto Parts

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