Bolts and fasteners 101

Bolts, they all look the same. But then people start talking about UNC =, Metric, grade 8. It can get confusing and a little daunting sometimes. (Do a little spiel on what bolts and fasteners are, pretty obvious but you need sort of an intro.CAN you remember back to a time you were learning about bolts and how to apply them? Was it confusing? How did you learn?


Tensile strength

Grade, class otherwise known as……..  tensile strength varies widely, from mild steel( made from low grade carbon steel )up to high tensile alloy heat treated fasteners.High tensile fasteners are heated and quenched to get their tensile strength to a standard where you know the bolt is going to be reliable. (how are these made?) can t assume people know, why are they so good?


What is tensile strength? Tesnsile strength is the maximum load or force a fastnener can support BEFORE it fails. For example in imperial bolts grade 2 (mild steel) can take 60000psi of force before failure whereas a grade 8 bolt can take 150000psi of force before failure.

ADD the difference between imperial and metric bolts

Tensile strength in imperial and metric is measured differently.

Imperial bolts are measured using the term ‘grade’ for example a mild steel bolt is grade 2 and the highest tensile strength fastener is grade 9.

Metric however uses the term ‘class’ starting at class 4.6 as the lowest tensile strength and going up to 12.9 highest tensile strength.


Imperial threads originated in the US around the 1870’s and became a standard thread when suppliers received government contracts. In the early 1900’s the US and Canada adopted a standard which included UNC UNF UNEF NPT.

UNC Unified National Coarse

UNF Unified National Fine

UNEF Unified National Extra fine

UNS Unified National special (usually finer than extra fine)


(WHAT DOES THIS MEAN??? You need to have long form at least once then you can abbreviate)



How do I tell what tensile strength my bolts are?

Each bolt has a specific marking that gives you the tensile strength. The markings are found at the top of the bolt.


Imperial bolts

3 strokes from the center to the outside is a grade 5

6 strokes from the centre to the outside is a grade 8 (ARE these the only two grades?)

Tensile or load strength between grade 5 and grade 8 is significant, for example a grade 5 ½ inch unc bolt has a load rating 9038 psi whereas a grade 8 ½ inch unc bolt has a load rating of 12788 psi.(CAN you give some examples of applications of each of these psi to make it more interesting?)


Metric will have them on stamped on.

Lower strength 4.6 Higher 12.9


(insert bolt diagram here)




A great resource for fastener information is the Fastener black book. (hyperlinked to our product)




Coatings are not relevant at all to the  tensile strength of a bolt.  Its an easy trap to fall into because most people think that zinc plated bolts are only mild steel. Zinc plated or galvanised coatings were first used a lot in the building industry  where tensile strength isn’t as critical. This is the primary reason that most people think that coated fasteners are not as strong as uncoated or black bolts.

Some common coatings are galvanised,(are there any interesting features about each of these and why they are used?) zinc and zinc yellow.


Coatings are used in places where high corrosion resistance is required, under water, in high temp areas or in harsh environments like underground coal mines. (CAN you give a storyish example or familiar example)

Uncoated or Black bolts are the same thing and are used in areas where there isn’t a high chance of corrosion or the bolts will be replaced periodically, eg plough or ground engaging bolts where the bolt will wear away before it will corrode away. There is no point using a zinc bolt on a grader cutting edge when the coating will be worn away immediately and the bolt will wear away and have to be replaced before any type of corrosion can occur. CAN you give an everyday example of where these would be used that the reader can relate too.



Commonly zinc coatings are between 2 and 5  microns thick. It seems thin but a thin coating is all that’s needed to prevent corrosions and still maintain a good tolerance and fit between the bolt and nut.


Zinc plated bolts for resist corrosion but are a low cost option compared to stainless steel. More on stainless steel bolts here.(( EXPLAIN stainless steel and its applications and superiority is it is)


When it comes to threads, basically the finer the thread the more load you can apply to that fastener before it will fail.

Imperial commonly has 2 types, UNC (see above)(WHAT IS THE LONG NAME FOR THIS?) and UNF. There are others like UNS and UNEF. Today we will cover the 2 most common. UNC is definitely the most common and will perform best in most non critical situations.


Threads in imperial sizes are measured by TPI or threads per inch that is how many peaks on the thread per inch of thread.


(diagram here)


UNF is a fine thread, it has more more threads per inch and is used for high torque applications required on the bolt where you  need reliability and as secure torque. (Story, this is why alot of cylinder head bolts you find are all fine thread, they need to be torqued up to an accurate level and stay tight) Fine threads can be tightened up more than coarse threads is there is more surface area that is in contact with each other when the bolt is threaded into the nut or threaded hole. For example clamping load on a ½ unf bolt is around 14400 lbs whereas clamping load on a ½ unc bolt is 12700, around a 20% difference when using a fine threaded bolt.

UNC is the standard.

Metric Fasteners

Metric fasteners are measured by pitch, Pitch is the distance between peaks on thread  for example m10 bolt 10 mm diameter  x 1mm pitch imm between peaks therefore would be finer than say a 1.5mm pitch.




Metric fasteners work on the same principal as imperial whereby the finer the pitch the more load you can place on the fastener.

Metric fasteners come in many different pitches, the most common are listed below. Metric is becoming the standard fastener in industry as manufacturers move to a standardised thread they can use the world over.




(Fastener balck  book link here)