Fasteners

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Metric Thread Pitch

bolt thread pitch

Omit the thread pitch and the standard assumes a coarse pitch. Metric bolts come in coarse, fine, and extra-fine pitches. Coarse threads have more threads per inch than comparable imperial bolts.

Metric Bolt Lengths

Metric Bolt Lengths

Measure socket head, pan head, button head and hex head bolts from under the head to the shaft end. Measure flat head bolts including the head height and dome head bolts from their thickest point.

Metric bolts follow ISO and DIN standards. The list below shows common designations and their descriptions.

Descriptions of DIN and ISO fastener standards.

Clearance Hole Sizes for Metric Bolts

An M8 bolt has a shaft diameter slightly under 8 mm. The bolt fits through an 8 mm hole. Drill clearance holes slightly larger to allow for misalignment.

Cutting a thread requires a smaller hole. Subtract the thread pitch from the bolt diameter to find the tapping hole size. An M8 bolt with 1.25 mm coarse pitch needs a 6.75 mm hole; with 1 mm fine pitch, it needs 7 mm.

The table below shows clearance hole sizes, coarse and fine thread pitches, and corresponding tapping hole sizes. Metric clearance and tapping drill hole sizes.

Standard Spanner and Allen Key Sizes

Use the correct spanner or allen key size to avoid slipping and damaging the nut or bolt.

The list below shows standard spanner and allen key sizes for common metric nuts and bolts.

Metric Bolt Strength Grades

Class 8.8:

Head Marking: 8.8 Material: Medium carbon steel, quenched and tempered Size Range: All sizes below 16 mm Proof Load (MPa): 580 Min. Yield Strength (MPa): 640 Min. Tensile Strength (MPa): 800 Size Range: 16 mm to 72 mm Proof Load (MPa): 600 Min. Yield Strength (MPa): 660 Min. Tensile Strength (MPa): 830

Class 10.9:

Head Marking: 10.9 Material: Alloy steel, quenched and tempered Size Range: 5 mm to 100 mm Proof Load (MPa): 830 Min. Yield Strength (MPa): 940 Min. Tensile Strength (MPa): 1040

Class 12.9:

Head Marking: 12.9 Size Range: 1.6 mm to 100 mm Proof Load (MPa): 970 Min. Yield Strength (MPa): 1100 Min. Tensile Strength (MPa): 1220

Minimum Ultimate Tensile Load

Thread d (mm) Pitch P (mm) Nominal Stress Area As,nom (mm2) Property Class 4.6 4.8 5.6 5.8 6.8 8.8 9.8 10.9 12.9 Minimum Ultimate Tensile Load - Fm,min (N)

Proof Load

Proof load is the maximum tensile force a bolt carries without plastic deformation. The material stays elastic up to this load, typically 85-95% of yield strength. Acceptable clamp load is 75% of proof load.

Maximum recommended tightening torque for coarse threaded metric steel bolts

Drive Types

https://www.boltdepot.com/fastener-information/type-chart.aspx

Phillips The standard drive type for most screws. Abbreviated PH

Frearson Similar to Phillips. Requires a Frearson driver for installation.

Slotted A slot in the head. Abbreviated SL

Combination A combination of slotted and Phillips drives. Abbreviated Combo

Socket, Hex or Allen A hexagonal hole for use with an Allen wrench.

One Way Installs with a normal slotted driver but cannot remove without special tools.

Square Also known as Robertson drive. Abbreviated SQ or SD.

Star Also known as Torx. A six-pointed star pattern avoids cam-out and stripped heads.

Washer Types

Flat

A flat washer distributes load. Available in SAE, USS, and other patterns.

Fender

An oversized flat washer further distributes load on soft materials.

Finishing

A washer gives a finished look. Use with oval head screws.

Split Lock

The most common washer style prevents nuts and bolts from backing out.

External Tooth Lock

A washer with external teeth prevents nuts and bolts from backing out.

Internal Tooth Lock

A washer with internal teeth prevents nuts and bolts from backing out.

Square

A square shaped washer.

Dock

Dock washers have a larger outer diameter and are thicker than standard.

Ogee

Thick, large diameter cast iron washers have a curved or sculpted look. Use in dock and wood construction.

Sealing

A neoprene washer bonds to a metal backing. This seals out air and water and dampens noise and vibration.

Embedded Nuts in 3D printing

Embed a nut into a 3D printed part using a nut-shaped cavity called a nut boss. This method avoids material removal. Finding optimal dimensions takes several iterations; printing small test parts saves time and material.

Methodology for embedded nuts

Measure the nut. Standard dimensions for metric and imperial nuts are available online.

Include the nut profile in the CAD model. Start with a 0.2 mm offset around the nut (0.1 mm each side) for a loose fit. Increase for nuts larger than M12.

Select the cut-out depth, usually just below flush.

Add a drop of superglue on the nut's back side to hold it in place.

Inserts for 3D print assemblies

Use heat-set or tap-in inserts for 3D printing. They provide strong metal-on-metal contact and install easily. Drill an accurate pilot hole first.

Methodology for heat-set inserts

Align the insert with the pre-printed or drilled hole. Check the insert manufacturer recommendations for hole sizes.

Insert a soldering iron into the insert. Heat the insert and the surrounding material. Avoid overheating and melting the material.

Slowly apply pressure to push the insert into the hole to the desired depth.

Methodology for tap-in inserts

Align the insert with the pre-printed or drilled hole. Check the insert manufacturer recommendations for hole sizes.

Gently tap the insert into the hole with a hammer to the desired depth.

Related

• Screws - screws and threaded fastener types
• Failure Criterion - bolt strength grade failure
• Fatigue and Fracture - bolt fatigue under tensile loads
• Material Selection - fastener material grade selection
• Metallurgy - quenched and tempered bolt steel
• Stainless Steel - stainless steel fastener grades
• Gridfinity - embedded nuts and inserts in 3D prints
• O-Rings - sealing washers for fluid joints
• Bolt Torque Calculator - preload from applied torque