Understanding Bolts

In this article we will be looking an bolts and how we can understand them; this is not an in depth article as this is a vast subject and way above the average man in the street, instead we shall focus on the basic understandings of bolts and how we can use them in the average 4X4. This can also be considered as a grounding for people looking to fabricate their own 4X4 components such as underbody protection where the need to select the correct fastening is the difference between something working, or getting torn off and causing more damage.

Most people select a bolt by using experience or guesswork and often over specify a bolt, this is fine, but for someone inexperienced they can design something and underspecify a bolt and this can lead to a disaster, and this is what this article hopes to avoid by instilling a basic understanding of practical bolt selection, and which bolt suits our application.

Bolts come in something called grades, and there are thousands of grades of bolts and fasteners so we need to narrow this down, we will work with something called “machine bolts” which will be a standard metric coarse pitch and are classed as “high tensile bolts” under the European ISO classification and the lowest grade of these bolts are the most popular type of bolt sold in the UK and Europe.
Grade 8.8 is the lowest grade and the largest seller, and every machine will be fitted with these, and there are also grade 10.9 and grade 12.9 which are even stronger and subsequently more expensive, and these three grades are the most popular types of high tensile fixings. Cars are machines, 4X4’s are machines, and even your washing machine is a machine which means any 4X4 must be fitted with a high tensile bolt of at least grade 8.8 and this is the grade we will focus on. Most suppliers of bolts who retail bolts and fastenings as a part of a larger range of unrelated products will often only sell grade 8.8 bolts, but always check your bolt supplier to see if they are selling grade 8.8 bolts as here in the UK large multinational suppliers of bolts such as Toolstation only stock grade 8.8 bolts.
Nuts are also a consideration as they too are graded and they come as a grade and the simple rule of thumb is to fit a nut of the same grade as the grade of bolt you are using, therefore you use a grade 8 nut on a grade 8.8 bolt, a grade 10 nut on a grade 10.9 bolt, and a grade 12 nut on a grade 12.9 bolt. If your chosen retailer only sells grade 8.8 bolts then they will only sell grade 8 nuts.

Why is this important? This is important because every bolt of a specific grade has something called a “proof load” and this is the minimum load the bolt will hold for a specific size, for an individual bolt; and to test them there are other criteria to be met. Each bolt must have the correct grade of nut fitted and this must be a full nut and not a half nut or locking nut; and each bolt has a specific torque requirement which is the tightening force which must be applied to it, and this torque or tightening force must be applied correctly for a bolt to meet its designed “proof load” or loading. If you overtighten a bolt it will stretch the thread and reduce its proof load and it will fail at a much lower loading, under tighten a bolt and it can move in its hole and wear both the hole material and the bolt and reduce its proof load. So, what are these proof loads in simple measurements that everyone can understand in simple terminology?

Bolt Size Proof Load Tightening Torque

M6 1160Kg 11.8 Nm
M8 2120Kg 28.8Nm
M10 3370Kg 57.3Nm
M12 4890Kg 99.8Nm
M16 9100Kg 247.5Nm

From this chart we can conclude that a standard grade 8.8 bolt which is a standard coarse pitch thread and a 6mm diameter bolt, and fitted with the correct grade 8 full nut which is correctly tightened or torqued to 11.8Nm will carry a load if 1160 Kg or 1.16 tonnes, in a word, YES. Things aren’t that simple as this only applies to a fixed position range of materials which are static or rigid such as steel frames for warehouses or even roof trusses and are not subjected to shock loadings or constant vibrations, so what if they are subjected to shock loadings or vibrations? We down rate the bolt.
Vehicle manufacturers of cars down rate their bolts by 10% because roads are not smooth as they are often full of pot holes, and engines constantly vibrate, and you get idiots who bump and bounce their vehicles up kerbs, and over speed humps and this down rating allows a margin for shock loadings that these types of vehicle drivers create through their bad driving habits.

4X4’s are a different matter as your 4X4 driver generally drives their vehicle more sensibly than your average car driver, and as your average 4X4 is heavier than an average car, and they encounter many more off road shock loadings your average 4X4 designer down rates the bolt proof loadings by 20% and this now gives a practical chart with practical bolt sizes to work to, and this is:

Bolt Size Proof Load Tightening Torque

M6 928Kg 11.8 Nm
M8 1698Kg 28.8Nm
M10 2696Kg 57.3Nm
M12 3912Kg 99.8Nm
M16 7280Kg 247.5Nm

Now we have actual practical figures to work with and we will note that the torque or tightening figures remain the same, therefore we have the same M6 bolt fitted with a full nut on a standard coarse thread which is correctly tightened to 11.8Nm figure, which will carry a load of 928Kg loading for a single bolt with a contingency for shock loadings built in to handle the rigours of off road driving under all conditions.

These figures are the figures we work to when we are designing components which we are going to manufacture or fabricate for our vehicle, if we want some rock sliders for example, and our vehicle weighs 3.2 tonnes (3200Kg) fully loaded with all our kit, and we fabricate our own rock sliders we can see that if we install 2 mounting bolts of 10mm we can have a mounting load of 2 X 2696Kg or 5392Kg or 5.3 tonnes. If our rock sliders are of sufficient construction strength we can jack our vehicle up on them safely, and with some margin to spare.

If we intend making or buying an under vehicle water tank we can make a four bracket mounting system, if our tank weighs 10 Kg and holds 80 litres of water at 1Kg per litre, its total weight will be 90Kg and we can downgrade to M6 bolts to secure our mounting brackets to our chassis instead of using M8 bolts and save some money as M6 bolts of a specific length are cheaper than M8 bolts of the same length.

Similarly, if we intend making a cross member to support serious underbody protection and we intend lifting off this crossmember as we regularly ground out or high centre our vehicle, and we make substantial chassis mountings to mount our crossmember onto, we can use a single grade 8.8 M8 bolt at each end of our crossmember to mount it. M8 bolts have a load rating of 1698Kg per bolt, and one at each end of our crossmember means we can lift 3396Kg without out bolts failing.

One serious benefit of understanding bolt loadings is weight which is followed by cost, the smaller diameter the bolt, the cheaper it is and if we can fit a crossmember with two M8 bolts it is cheaper than using four M 10 bolts, so we are also saving weight on our modifications.

Kg Kilogrammes
Nm Newton metres
1000 Kg = 1 metric tonne

Tightening Torque

11.8Nm 8.7 Ft/Lb
28.8Nm 21.2 Ft/Lb
57.3Nm 42.2 Ft/Lb
99.8Nm 73.6 Ft/Lb
247.5Nm 182.5 Ft/Lb

What do we look for:

Always use the grade 8.8 bolt as these are the most common and cheapest bolts, always ensure you use the matching grade 8 nut so they match the bolt and then you can refer directly to the above down rated chart for the correct tightening torque and minimum bolt loadings.

Threaded Bar

Many people look at the option of using threaded bar or threaded rod for many applications and the best thing I can say is DON’T because most threaded bar is not a high tensile material and most of it is only grade 5 at best, much of it is an even lower grade than this, so avoid using it on vehicles except in an emergency situation. Threaded bar was introduced into engineering in the construction industry to hang things from ceilings of portal framed structures such as overhead pipework, cable trays for factory trunking for wiring, or to suspend other very light items such as heating and ventilation ducting and extraction ductwork, basically light equipment.