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Posted by: offroad on Tuesday, September 14, 2010 (15:23:17)
To many people, nuts are bolts are little metal things used to stop pieces falling off their car. When they're old they seize up and won’t come off. Generally causing much frustration, cursing and usually some skin to be removed from the owners knuckles while vainly trying to budge them. The opposite can also be true. Sometimes they come off too easily, all by themselves in fact. Usually at an inconvenient time while allowing an important part of the car to escape the torture of being part of a buggy.

To an Engineer, a nut and bolt is part of the fastener family. If correctly specified, installed and maintained a nut and bolt will do its job perfectly. If it breaks then it was the wrong size or strength. If it falls off then it probably wasn’t installed correctly. If it seizes up then it either wasn’t maintained or wasn’t the right material for the environment to which it was subjected.

So with this article, I hope to begin to explain the complex world of bolts. Let’s start with the name. I’m going to use bolts as that is the common name (the engineer in me will keep trying to type fastener though). Technically a bolt and a screw can be almost the same thing. A bolt is generally used with a nut. A screw which can cut its own thread (like a wood screw) is quite different from a bolt. But there are screws that are essentially bolts although they are designed to work by screwing into a tapped part, rather than by working with a nut. I will leave this area of ambiguity as it’s not too important to us car enthusiasts.

So before we start, I better explain a few things (maybe an article on metals would have been a good lead up to this one). Bolts are made of different metals and different metals have different properties. The most important property of a bolt is its strength. Strength is (metrically) measured in a unit called a Megapascal (MPa), which itself if a measure of stress. Stress is calculated by dividing a force by an area. The force is measured in Newtons and is calculated by multiplying a mass by an acceleration. A 1kg mass under the acceleration of earths gravity (which is 9.81m/s2) gives you 9.81N of force. 1N of force loaded onto 1mm2 of area will give you 1MPa of stress.

Normal old mild steel is relatively weak. There are two main measures of the strength that you will see used. The yield strength is the measure of when the metal starts permanently changing shape, or deforming. The ultimately tensile strength (UTS) is the measure of when the metal will break, or fail. Generally when the strength is low, the yield strength will be a lot lower than the tensile strength. In this case the metal is ductile, i.e. it will bend quite a lot before it breaks (like a paper clip). When the strength is made very high the yield strength generally starts to become close to the UTS. In this case the metal is brittle, i.e. it won’t bend much at all before it breaks (like drill bits!). If all of this doesn’t make sense then don’t worry, just remember that stronger is generally better.

This all means that you can calculate what load any given material will handle. For example, you can calculate that you could hang up to 3.2 tonnes from a 10mm class 4.6 bolt (which has a UTS of 400MPa) before it breaks. Similar you can calculate that you could hang up to 6.4 tonnes from a 10mm class 8.8 bolt (which has a UTS of 800MPa) before it breaks. Actually those calculations aren’t quite right as a 10mm bolt has a minimum diameter in the thread area of a bit less than 9mm, but I’m sure you get the picture.

I’m not going to try to cover all the bolt grades and markings, as there are simply too many. Many countries have their own standards for bolts and they all like to do things slightly differently. Main standards are;

· ISO – International Standards Organisation – Mainly Metric

· AS – Australian Standards – Mainly Metric (for bolts we are based on ISO)

· ASTM – American Society for Testing and Materials – Mainly Imperial

· SAE – Society of Automotive Engineers – Mainly Imperial

Each of these standard organisations have many standards for each fastener type, this means that there are many different individual fastener standards out there. In the end you basically end up with metric steel, metric stainless steel, imperial (UNC/UNF) steel and imperial stainless steel. I will cover the first 3 bolts in this article. Figure 1 is a little scale that I created to show the different bolts that you can expect to find, and the relative strengths of each;

Figure 1 – Related Bolt Strengths of Common Imperial and Metric Bolt Grades

A better description of these 3 groups of bolts is given in Table 1 - Table 3 below. Again I have only covered the more common grades, there are actually a lot more than I have shown here. If your interested in the others just have a poke around on the internet (look up screw in wikipedia), but be careful as there are a lot of little mistakes in the online info.

Table 1 – Imperial Steel Fasteners

Head Marking
Imperial Bolt Grade
Steel Type



Grade 1
Low Carbon Steel

Grade 2
Low or Medium Carbon Steel

Grade 5
Medium Carbon Steel Quench and Tempered

Grade 8
Medium Carbon Alloy Steel Quench and Tempered

Table 2 – Metric Steel Fasteners

Head Marking
Metric Bolt Class
Steel Type



Class 4.6
Low or Medium Carbon Steel

Class 8.8
Medium Carbon Steel Quench and Tempered

Class 10.9
Alloy Steel Quench and Tempered

Class 12.9
Alloy Steel Quench and Tempered

Table 3 – Stainless Steel Fasteners

Head Marking
Stainless Steel Composition
Alloy Group
Property Class

Steel Type
Ultimate Tensile Strength (MPa)


Where the A2-70 is split up as follows;

A is the Steel Composition

2 is the Alloy Group


70 is the Property Class

500 (soft)

700 (cold-worked)

800 (high strength)

450 (soft)

600 (cold-worked)

500 (soft)

700 (hardened and tempered)

1100 (hardened and tempered)

800 (hardened and tempered)

1200 (hardened and tempered)


Are you still reading, or have you fallen asleep by now? Hopefully you were paying enough attention to notice a few things. First thing is that like all things from the imperial world of measurement their bolt marking and strength designation system makes absolutely no sense what-so-ever! 3 lines on the bolt head for grade 5, 6 lines for grade for grade 8, the 5 and 8 don’t really relate to anything. What were they thinking!

Metric bolts are much better. For metric steel bolts the 8.8 tells you all about it. The 8 at the front is roughly the UTS in hundred of MPa (i.e. 8 indicates a UTS of 800MPa). The .8 is roughly the yield strength as a percentage of the UTS (i.e. .8 indicates a yield of 0.8x800 = 640MPa). The head is marked with the bolt property class (i.e. 8.8 means property class 8.8). Simple really. Just remember the imperial and metric system don’t relate to each other. A grade 5 imperial bolt is a similar strength to a class 8.8 metric bolt.

The metric stainless steel fasteners get a bit more complicated. For A2-70 the A2 is the just the type of stainless steel used (A2 relating to 321/347). The 70 is approximately the UTS in tens of MPa (i.e. 70 indicates of UTS of 700MPa). You’ll mainly come across the A2 and A4 types of bolt as they are the most common. A4 is made from 316 stainless steel, which is commonly referred to as marine grade stainless steel because it resists corrosion from sea water. The interesting thing about stainless steel is the way it increases in strength as it is cold worked. With the steel bolts you need a better and better steel composition to achieve the higher strength classes. But with stainless steel bolts you get different strengths using the same material composition by simply cold working the bolt (i.e. A4-50, A4-70 and A4-80 are all 316 stainless steel material, but the amount of cold working is increasing). This behaviour of stainless steel is easily noticed in fabrication. If you bend stainless steel wrong it doesn’t like bending back as the bend part has cold worked and increased in strength, so when you try and flatten it the nearby un-work-hardened material (the flat part) will try to bend back instead!

I’m going to confuse the matter a little now. Those bolt markings are great if you go down and buy some bolts from the local fastener store, but car manufacturers sometimes decide that they know better. They like to make their own bizarre bolt markings. Table 4 shows the bolt marking system used by Toyota. Sometimes the bolts have just the numbers on them. The fact that they use both 6 and 9 as bolt head markings shows how strange the Japanese can be (a 6 and a 9 look the same when you don’t know which way is up!). They then mix up the number markings with line markings that look similar to the imperial line markings (and use them on metric bolts). Anyway, once you indentify which bolt you have, you then look up a torque specification table that has a different torque values for each bolt torque class – for any given bolt size. The higher the torque class the higher the tightening torque for that bolt.

Table 4 – Toyota Bolt Markings (other Japanese Manufacturers Similar)

Ok, so now you know there are lots of different types of bolts to choose from. The thing to remember from all this is that bolts aren’t simply bolts. Most bolts on your car will be at least equivalent to a class 8.8 bolt. In general metric allen head bolts (aka socket head cap screws) are class 12.9 and therefore very strong. So don’t go bolting your steering box onto your front beam using a non-marked bolt from bunnings warehouse, as it’s not going to be strong enough for the job. A common unmarked hardware bolt would only have a yield strength in the region of 200-250MPa, which clearly won’t work very well if the original bolt was a class 12.9 bolt that had a yield strength of 1100MPa.

I also recommend ignoring marketing hype. A good example is VW head studs. Judging by the torque values supplied by Volkswagen their head studs are at least equal to class 8.8. Some aftermarket suppliers sell 4140 chrome moly head studs and claim they are twice as strong. The problem is that 4140 in it’s hardened and tempered condition is only equivalent to class 10.9 (10.9 certainly isn’t twice as strong as 8.8). Chrome moly is also a family of steel alloy’s, so chrome moly’s aren’t chrome moly’s. ARP is a reputable fastener manufacturer that uses 8740 chrome moly which is capable of reaching strengths equivalent to class 12.9 fasteners (if correctly heat treated).

Well, that’s enough about bolts for this issue. If the readers are keen we could have a part two and discuss important issues such as the advantages of thread lubricants, the importance of not using old rusted bolts, and the interesting world of bolt tensioning methods.

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