Metric or ISO screw threads are the most commonly used type of general-purpose screw threads worldwide. They were one of the first international standards agreed when the International Organization for Standardization was set up in February 1947 after the second World War.  The M used in describing a bolt or set screw is best explained as mm, for example M8, simply means 8mm.  Metric thread measurements comes in two sizes, like SAE bolts with UNF and UNC you have Metric Fine and Metric Coarse.

Metric thread callouts are fundamentally different because the thread pitch (distance between each thread, in mm) is specified instead of threads per inch or per mm. For example, an M10x1.5 (coarse) thread has a 1.5mm pitch, while an M10x1.25 (fine) thread has a 1.25mm pitch.

Can you tell if a bolt is Metric or Imperial by looking at the head?

Have you ever noticed that bolt heads have lines or numbers on them and wondered what do those mean? Nuts and bolts are graded on the material that they are made of. The larger the grade the stronger the bolt/nut. Imperial bolts or SAE have a different grade rating system than their metric, or ISO, counter parts, however the same holds true for metric, the larger the grade the stronger the bolt/nut. Also, the higher the grade, the higher the torque that can be applied.  Sometimes there are lines on the bolt head. 

Have you ever wondered what the markings mean or the grade of the bolt?

The lines on the head of a bolt indicate its grade. SAE J429 (Society of Automotive Engineers) bolts have radial lines, whereas ASTM (American Society for Testing and Materials) uses a combination of letters and numbers to indicate the grade.

A gold color zinc plated bolt has 6 hash markings on the head this represents grade 8.8 refers to the tensile strength which is generally 116,000 psi, grade 10.9 is generally 150,000 and grade 12.9 is generally 175,000 psi.

The standard American bolt is defined by the inch system designation. The bolt is identified by its diameter in inches, the pitch or number of threads per inch, the thread series designation, and the bolt length. ... However, metric bolt head size is measured in millimetres.

If you have ever looked at a metric hex bolt you might notice the markings 8.8, 10.9 or 12.9 this does not mean grade 8,10 or 12. The grade 8.8 refers to the tensile strength which is generally 116,000 psi, grade 10.9 is generally 150,000 and grade 12.9 is generally 175,000 psi.

Grade 4.6: The first digit relates to the ultimate strength of the material, whilst the second is the ratio of yield stress to ultimate strength. Thus grade 4.6 bolts have an ultimate material strength of 400N/mm2 and the yield (or proof) stress is 60% of the ultimate strength.

Grade 8.8: The high tensile steel is often referred to as the structural grade for bolts. It is the most common form of high tensile material and is usually stocked in plain finish or zinc.  8.8 bolts exhibit good tensile strength and perform well in most environments.

Grade 10.9: These metric bolts are manufactured using medium carbon steel, medium carbon steel alloy and low carbon boron steel that has been quenched and tempered. Used in automotive applications and for bolting large and forged parts, metric 10.9 bolts exhibit high tensile strength and good wear resistance.  A tempered medium carbon steel. Class 10.9 is stronger than class 8.8, and is commonly found in high strength automotive applications. Class 10.9 is similar to grade 8.8. A low carbon steel for general use.

Grade12.9: This grade of steel means that a component manufactured form this grade of steel has 12X100 N/mm^2 tensile strength and 9 is a multiplier means 90% of 1200 = 1080 N/mm^2 is the yield strength of the grade. 

How do you measure a bolt?

Bolts and screws lengths are generally measured from the point where the surface of the material will be when the fastener is installed.  Thus, fasteners with heads that sit above the surface are measured from under the head to the end, while fasteners that sit flush with the surface (countersunk) are measured from the top of the head to the end of the fastener.

Can different metals or bolts react with one another?

Various metals can react differently with one another and care should be taken when fixing them together.  Stainless steel is reactive with aluminium, and when a stainless steel screw is in contact with an aluminium base metal, the aluminium is likely to corrode.  Due to a phenomenon known as galvanic corrosion, some commonly used metals can cause accelerated corrosion when used with ZINCALUME® steel or COLORBOND® steel. Copper and lead are two such metals.... 

Stainless steel fixings and fixings containing copper should also not be used with ZINCALUME® steel or COLORBOND® steel.

How do lead and zinc nitrate react when put together?

The reaction won't occur since metals will only displace the ones below it from a solution according to the reactivity series, zinc would replace copper ions, but copper does not replace zinc ions. Zinc and lead nitrate react to produce zinc nitrate and lead.

Galvanized steel screws, however, are plated with a corrosion-resistant coating, usually consisting of zinc, that is not nearly as reactive with aluminium. The zinc plating prevents the underlying steel from coming into contact with the aluminium, and the risk of corrosion of the aluminium is reduced significantly.  See the chart below with metals that do and don't react with one another.

Low: No significant galvanic action is likely to occur.

Medium: Galvanic corrosion may occur under certain circumstances over a long period of time.

High: Galvanic corrosion is likely so avoid direct contact.

What are the three hand taps for cutting threads?

The three types of taps are Taper, Intermediate and Bottoming.  These three are the most common configurations for cutting threads by hand and are required in making the perfect female thread when cutting.  They’re available in the following thread types: Coarse, Fine, UNC, UNF and BSW.

What is the Taper tap used for?

The Taper Tap has quite a lot of chamfered threads to help it ease into cutting gradually and the taper also acts as a guide in starting the cutting action for the hole.   Typically, the first 8 to 10 threads are chamfered, with these taps being the most common type (often called “starting” taps) they are typically what you’d have in a Tap and Die Set.

What is the Intermediate tap used for?

The Intermediate Tap (sometimes called a “plug” tap) is in between the Bottoming and Taper Taps because it has 3-5 threads chamfered,  which is more than a Bottoming Tap but less than the Tapering Tap.  They are most widely used in through holes and where there is sufficient room at the bottom in blind holes.

What is the Bottoming tap used for?

The Bottoming Tap (often called the “finishing” tap) has almost no taper at the end because it is designed to thread all the way to the bottom of its reach.  Only 1 to 1.5 threads will be chamfered.  These are useful for threading blind holes, however it’s desirable to thread most of the hole with a Taper and intermediate taps first.

What do the rings on hand taps mean?

The rings are there to quickly establish which tap you’re picking up and using in sequence.  The Bottoming Tap in most cases has no rings; however can sometimes have three being the third tap required in the process of cutting threads.  Then on the other end of the three; you have the Tapering Tap being the first required tap needed when cutting, it has one ring.  The Intermediate Tap has two rings indicating the second tap being used during cutting the perfect thread. (See diagram above)

What is the correct drill size for taps?

See a few examples below:

(If you require more specs or something more specific, like BSW; get in touch with one of our trained staff)

How do I work out Rivet sizing?

When working with rivets for the first time it can sometimes seem confusing understanding what the numbers mean, so therefore we thought we’d break it down to make it simpler.  Rivets are measured in inches based on the diameter of the rivet head and the maximum thickness of the material being riveted.  Therefore the first number refers to the diameter of the rivet in 32^nds of an inch. The second number refers to the maximum grip length in 16^ths of an inch. Therefore a size 45 rivet is 4/32" (1/8") diameter with a 5/16" maximum grip range.  (See diagram below)

What Metric drill bit would you use for a 4-5 Rivet?

We recommend a 3.2mm metric drill bit. (See diagram below)

Work Shop Hints

How work with Buffing Compounds?

Dress the mop with compound across the rotating face for a few seconds taking care not to overload the mop - it is far better to apply a little compound more frequently than to overdress the mop.  After applying the compound, bring the workpiece to the wheel and with light pressure move back and forth across the job.  After the job is finished to your satisfaction, remove any compound residue with mineral turpentine or methylated spirits.

Makeshift Countersinking Tip?