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Get to Know Metric Bolt Sizes

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Nuts and bolts are essential fastenings, critical in the safe assembly of machinery, equipment, furniture and much more. In most circumstances, it’s absolutely vital that the right size of fastening is used. In this brief guide we provide an introduction to standard ISO metric sizes for nuts and bolts which will hopefully help dispel any confusion.

BS3643 Parts 1&2

BS3643 defines the limits and tolerances for ISO screw thread gauges. The standard is divided into two parts. Part 1 is based on ISO 965/1 and ISO 965/3 providing the principles and basic data for specifications of ISO metric screw threads. 

And BS3643 part 2 provides tabulated diameters and tolerances for coarse and fine pitch threads, based on ISO 965/2.

Metric Bolt Sizes

Metric nuts and bolts are commonly referenced using ‘M’ sizes, for example: M3, M8, M12. But the size of a metric fastener is more accurately specified using diameter, pitch and length dimensions, in millimeters. For nuts the size dimensions used are simply diameter and pitch.

Here’s an example metric bolt specification: M8-1.0 x 20

This bolt specification breaks down like this:

  • The letter ‘M’ indicates that this bolt uses a metric thread.
  • The number ‘8’ refers to the nominal diameter of the bolt shaft in millimeters.
  • The number 1.0 is the thread pitch of the bolt, which is the distance between threads, in millimeters.
  • And the final figure, 20, is the length in millimeters.

But it’s very common to see metric bolts specified like this: M12-50.

This abbreviated format omits the pitch definition which means that the bolt has a coarse thread. Whenever the pitch dimension is omitted from a metric bolt specification then the bolt is always coarse threaded. This example (M12-50) has a diameter of 12mm and a length of 50mm.

Metric Thread Pitch

bolt thread pitch

As noted, the specification of metric bolts includes the definition of thread pitch, but if this is omitted then a coarse pitch is assumed. Metric fasteners are available with either coarse or fine thread pitches with some metric bolt sizes offering extra fine thread pitch options. It is worth noting that metric bolts with a coarse thread pitch have more threads per inch (they are more closely spaced) than comparable imperial bolts.

List of common metric bolt sizes.
Metric Coarse Metric Fine Metric Extra Fine
M1.4-0.3    
M2-0.4    
M2.5-0.45    
M3-0.5    
M3.5-0.6    
M4-0.7    
M5-0.8    
M6-1.0    
M7-1.0    
M8-1.25 M8-1.0  
M10-1.5 M10-1.25 M10-1.0
M12-1.75 M12-1.5 M12-1.25
M14-2.0 M14-1.5  
M16-2.0 M16-1.5  
M18-2.5 M18-1.5  
M20-2.5 M20-1.5  
M22-2.5    
M24-3.0 M24-2.0  
M30-3.5    
M36-4.0    

Metric Bolt Lengths

The length of a metric bolt is measured and defined in exactly the same way as imperial, inch-based bolts and fasteners. Socket head, pan head, button head and hex head bolts are measured from the underside of the bolt head to the end of the shaft. For flat head bolts, the length includes the bolt head height and for dome head bolts the length is measured from the thickest, highest point on the curved head.

Metric bolts and fasteners are manufactured in conformance with standards set by the International Organisation for Standardisation (ISO) and the German Institute for Standardisation (DIN). Here’s a list of the most common DIN and ISO numbers and what they describe.

Descriptions of DIN and ISO fastener standards.
Standard No. Description
DIN 84 Slotted Cheese Head Machine Screw
DIN 85 Slotted Pan Head Machine Screw
DIN 94 Cotter Pin
DIN 125A Flat Washer (without Chamfer)
DIN 125B Flat Washer (with Chamfer)
DIN 127B Split Lock Washer
DIN 137B Spring Wave Washer
DIN 315 Wing Nut
DIN 439B Machine Screw Hex Nut, Finished Hex Jam Nut
DIN 911 Long Arm Hex Key
DIN 912 Hex Socket Head Cap Screw
DIN 916 Hex Socket Set Screw
DIN 931 Hex Head Cap Screw Partially Threaded
DIN 933 Hex Head Cap Screw Fully Threaded
DIN 934 Finished Hex Nut
DIN 939 Double End Stud
DIN 960 Hex Head Cap Screw Partially Threaded
DIN 961 Hex Head Cap Screw Fully Threaded
DIN 963 Slotted Flat Head Machine Screw
DIN 964 Slotted Oval Head Machine Screw
DIN 965 Phillips Flat Head Machine Screw
DIN 966 Phillips Oval Head Machine Screw
DIN 975 Threaded Rod
DIN 980V Hex Steel Prevailing Torque Lock Nut
DIN 985 Hex Nylon Insert Lock Nut
DIN 6334 Rod Coupling Nut
DIN 6797A External Tooth Lock Washer
DIN 6797J Internal Tooth Lock Washer
DIN 6912 Hex Low Socket Head Cap Screw
DIN 6921 Hex Flange Head Screw
DIN 6923 Hex Flange Nut
ISO 7379 Hex Socket Head Shoulder Screw
ISO 7380 Hex Button Socket Head Cap Screw
DIN 7985 Phillips Pan Head Machine Screw
DIN 7991 Hex Flat Socket Head Cap Screw

Hole Sizes for Metric Bolts

The defined diameter of metric bolts is actually slightly larger than the actual diameter of the bolt shaft. So an M8 bolt would have a shaft diameter that’s slightly under 8mm which means that the bolt should fit through an 8mm hole. But it’s common practice to drill clearance holes that are slightly larger to allow for misalignment.

When it’s necessary to cut a thread then the hole size needs to be a smaller diameter to allow for the thread to be tapped. The necessary tapping hole size can be easily calculated by deducting the thread pitch from the metric bolt diameter. For example, an M8 bolt with a standard coarse thread pitch of 1.25mm would require a hole of 6.75mm diameter to be tapped. But if the M8 bolt had a fine pitch thread of 1mm then the diameter of the hole required for tapping would be 7mm.

The following table provides examples of clearance hole sizes, standard or coarse thread pitches along with fine thread pitches and their corresponding tapping hole sizes.

Metric clearance and tapping drill hole sizes.
Size Clearance Hole mm Thread Pitch (std) Tapping Drill (std Pitch) Thread Pitch (fine) Tapping Drill (fine pitch)
M5 5.5 0.8 4.2 mm    
M6 6.5 1.0 5.0 mm 0.75 5.25 mm
M8 9 1.25 6.75 mm 1.0 7 mm
M10 11 1.5 8.5 mm 1.0 or 1.25 9.0 or 8.75 mm
M12 14 1.75 10.25 mm 1.5 10.5 mm
M16 18 2.0 14 mm 1.5 16.5 mm
M20 22 2.5 17.5 mm 1.5 20.5 mm
M24 26 3.0 21 mm 2.0 22 mm
M30 32 3.5 26.5 mm 2.0 28 mm

Standard Spanner and Allen Key Sizes

Using the correct tool for the job is always essential. It’s important to use the right size spanner or allen key in order to avoid slipping and potentially causing damage to the nut or bolt by rounding off the edges.

Here’s a short list of standard spanner and allen key sizes that apply to the most commonly used metric nuts and bolts.

Standard metric spanner and allen key sizes.
Size Spanner Allen Key
M5 8 mm 4 mm
M6 10 mm 5 mm
M8 13 mm 6 mm
M10 17 mm 8 mm
M12 19 mm 10 mm
M16 24 mm 14 mm
M20 30 mm 17 mm
M24 36 mm 19 mm
M30 46 mm  

 

If you have any questions or uncertainty regarding the fasteners you need then remember that we are here to help. Give us a call on 01273 475500 and we’ll provide you with free, expert advice and guidance.

This message was added on Friday 19th June 2020

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