
The No. 8 screw is a common screw size, measuring 4mm in diameter. The size of the wall plug to be used with a screw is usually 2mm larger than the screw diameter. Therefore, a 6mm wall plug is often used with a No. 8 screw. The corresponding drill bit for a 6mm wall plug is 5mm or 6mm, with the latter being more common.
Screw gauge and length
The gauge of a screw refers to the diameter of its shank or body, and this is only relevant for imperial-sized screws. The higher the number, the larger the diameter. For example, a number 4 screw has a much smaller diameter than a number 12 screw.
It's worth noting that there is no direct relationship between the head size of an imperial screw and its gauge. It's simply a coincidence that, for screw gauges 6 and above, the gauge is close to twice the head diameter.
When it comes to screw length, the measurement given refers to the length that is buried in the material (e.g., wood) and does not include the length of the screw head for raised or domed-headed screws.
In the metric system, the diameter of the screw (major diameter) is given in millimetres, and the length is also measured in millimetres. As a coincidence, the gauge of an imperial screw roughly equals the screw head size in millimetres. So, a 4-gauge screw will typically have a head that is approximately 4mm wide.
To calculate the relationship between the imperial screw head size and the gauge, you can use the following formula:
Head diameter in sixteenths of an inch x 2) - 2 = Gauge
For example, a screw head with a diameter of 5/16 inches:
5/16 x 2) - 2 = 8.
So, the gauge for this screw would be 8.
Metric vs. Imperial
The metric system is generally considered simpler, as it measures the diameter and length of screws in millimetres. However, when purchasing screws online, retailers often omit the metric size due to character limitations in product titles.
Pilot Holes and Wall Plugs
When using screws, it's important to consider the size of the pilot hole and the necessary wall plug. The pilot hole should be slightly smaller than the screw shank to ensure a tight fit. For wall plugs, the size will depend on the screw gauge and the material you're fixing to.
For example, when fixing into masonry, an 8-gauge screw would typically use a red (6mm) or brown (7mm) wall plug.
Understanding screw gauge and length is crucial for selecting the right screw for your project. The gauge refers to the diameter of the screw shank, and length excludes the screw head for raised or domed-headed screws. The metric system simplifies measurements by using millimetres, but imperial screws are still commonly used, and their gauge can be roughly converted to millimetres.
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Screw head size
The head size of a screw is the width of the top of the screw, which is the part that is not threaded. The head size of a No. 8 screw is approximately 8mm wide. This is a coincidence, as the size of an imperial screw head usually corresponds to the gauge of the screw.
No. 8 screws are available in a variety of head styles, including button head, flat/countersunk, and pan head. The head style of a screw is important because it determines the type of screwdriver or bit that is needed to drive the screw.
No. 8 screws are commonly used in wood screws, which are designed for connecting items to a wooden base. They are also available in machine screws, which are used for fastening metal parts together. The length of a No. 8 screw can vary, with common lengths including 1 inch, 1.1/4 inch, and 1.1/2 inch.
When choosing a screw, it is important to consider the length and diameter or gauge size. The length of the screw is usually measured in millimetres, while the width can be measured in either millimetres or gauge depending on the length measurement. It is also important to select the appropriate type of screw for the project, such as wood screws for connecting items to a wooden base or sheet metal screws for projects involving metal.
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Wall plug size
Wall plugs are essential for installing shelving, doors, and other lightweight fixings, providing extra strength and durability. They come in various sizes, and choosing the right one depends on the screw size and the material of the wall. Here's a comprehensive guide to help you select the appropriate wall plug size for your project.
Understanding Screw Sizes
Before selecting a wall plug, it's crucial to understand screw sizes, as they are closely related. Screws are sized by their gauge and length. The gauge refers to the diameter of the screw, and the larger the number, the wider the diameter. For example, a number 4 screw has a smaller diameter than a number 12 screw.
It's important to note that the head size of an imperial screw doesn't necessarily correspond to its gauge. Although, coincidentally, for screws with gauges 6 and above, the gauge is often close to twice the head diameter.
When measuring screw length, remember that the specified length is the portion of the screw that will be buried in the material, excluding the head for raised or domed-headed screws.
Wall plugs, also known as plastic wall plugs, are commonly available in three sizes: 8mm (often brown), 6mm (often red), and 5mm (often yellow). However, the colours may vary depending on the manufacturer. The size of the wall plug corresponds to the drill size needed for creating the hole in the wall.
The relationship between wall plug size and screw size is flexible. Here's a general guide:
- 8mm wall plugs are suitable for 5-6mm screws.
- 6mm wall plugs are typically used with 4-5mm screws.
- 5mm wall plugs can accommodate 3-4mm screws.
It's recommended to use the smallest wall plug that can fit the screw or the largest screw that fits the wall plug for optimal results.
Selecting the Right Wall Plug
When hanging items on solid masonry brick walls, it's essential to use screws and wall plugs. To determine the appropriate wall plug size, start by measuring the diameter of the screw. For instance, if the screw is 5mm, you'll need a 6mm or 8mm wall plug.
The length of the screw should be slightly longer than the wall plug, including the thickness of the bracket or mounting piece. The screw should go entirely through the wall plug and protrude slightly at the back.
For a 5kg load, such as a speaker, any size screw and wall plug will be sufficient.
Drill Bit Size
When using wall plugs, it's crucial to select the correct drill bit size. As a general rule, use a drill bit that is 1mm smaller than the wall plug size. For example, for an 8mm wall plug, use a 7mm drill bit. This ensures a snug fit without enlarging the hole.
It's worth mentioning that some wall plugs and screws are sold in sets, making it easier to ensure compatibility. Additionally, the packaging on wall plugs often indicates the recommended drill bit size.
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Drill bit size
When selecting a drill bit, it is important to consider the type of screw, the material being drilled into, and the desired fit. For a #8 wood screw, the recommended pilot hole diameter is 5/32 inches, with 7/64 inches for hardwood and 3/32 inches for softwood.
The appropriate drill bit size for a #8 screw depends on the material being drilled. For hardwood, a 7/64-inch drill bit is suitable, while for softwood, a 3/32-inch bit is recommended. These measurements correspond to the thickness of the screw's centre, excluding the threads.
It is worth noting that #8 screws have limited shear strength, so alternative fasteners may be preferable in certain applications. Additionally, it is crucial to verify the correctness of the drill bit size before drilling and to pay attention to the depth of the pilot hole.
When drilling into masonry, the size of the wall plug and the length of the screw should be considered. For an 8mm wall plug, a 6mm or 7mm drill bit is typically recommended. The length of the screw will determine how deep the wall plug needs to be inserted into the wall.
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Screw types
There are many different types of screws, each designed for specific applications and materials. Using the wrong type of screw for a project could be disastrous. Here are some of the most common types of screws and their uses:
Wood Screws
Wood screws are one of the most commonly recognised types of screws. They have a smooth shank and tapered point that make them ideal for penetrating wood. They typically require a pilot hole for installation. The most common type is the Phillips drive, which has an X-shaped head. Hardwood screws have tighter threads than softwood screws to prevent splitting.
Machine Screws
Machine screws differ from other screws in that they do not drill through materials themselves. Instead, they are used with pre-drilled and tapped holes that have threads cut into them. When inserted into a tapped hole and secured with a nut, the screw stretches, forming a strong and stable bond. Machine screws are commonly used in the assembly of machinery, electronics, appliances, and vehicles.
Sheet Metal Screws
Sheet metal screws are designed for connecting thin metal sheets or other materials like plastic, wood, and aluminium. They are categorised by their head styles and three numerical values representing the screw's diameter, length, and thread count. The diameter, or the size of the screw's shank, is indicated by a number between zero and 24, with common sizes ranging from 6 to 14.
Drywall Screws
Drywall screws are specifically created for installing drywall panels. They have bugle heads to protect against tearing the paper surface and a sharp point for easy penetration. They can be found with coarse threads for wood studs or fine threads for metal studs.
Concrete Screws
Also known as masonry screws, concrete screws are designed to fasten materials to concrete, brick, and block. They have a high-low thread design for easy installation and strong holding power. Concrete screws require a pre-drilled hole and typically have a hex, bugle, or flat head.
Deck Screws
Deck screws are designed for outdoor wood applications such as decking, fencing, and railing. They are made from corrosion-resistant materials like stainless steel and have a bugle head for flush installation. Deck screws also have a sharp point for easy penetration into wood.
Self-Drilling Screws
Self-drilling screws, commonly referred to as TEK screws, are ideal for the quick and efficient installation of metal components. They create their own pilot hole, eliminating the need for pre-drilling. Self-drilling screws come in various head styles, including hex, pan, and flat head shapes.
Lag Screws
Lag screws, also known as lag bolts, are heavy-duty fasteners used for connecting heavy lumber or other materials bearing intense loads. They have a hexagonal head and coarse threads, providing excellent grip and holding power. Lag screws typically require a pre-drilled pilot hole and a wrench or socket for installation.
Set Screws
Set screws are unique in that they have no head, allowing them to be fully embedded into a pre-drilled hole. They typically have a hexagonal socket and are tightened with an Allen wrench. Set screws are used to secure one object within another and come in various thread types and point styles, such as cup, cone, and flat point.
Tamper-Resistant Screws
Tamper-resistant screws are designed to deter unauthorised access to protected areas or equipment. They have unique head designs, such as pin-in-torx or pin-in-hex, that require specialised tools for installation and removal. Tamper-resistant screws are commonly used in security, electronics, and other applications where vandalism or theft is a concern.
Metal Roofing Screws
Metal roofing screws are designed to securely fasten metal panels to a building's underlying structure. They have a sharp self-piercing point, eliminating the need for pre-drilling and preventing damage to the sheet metal. Metal roofing screws provide secure connections, durability, and weather resistance.
Self-Tapping Screws
Self-tapping screws are used to join two pieces of material together. They have a pointed tip and sharp threads that create their own mating thread as they are driven in. Self-tapping screws come in various sizes and materials, such as steel, stainless steel, and brass. They are commonly used in sheet metal fabrication, woodworking projects, and automotive repair.
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Frequently asked questions
Imperial screw sizes are determined by two numbers: the gauge of the screw, which refers to the diameter, and the length. The larger the number, the larger the diameter. The length given for a screw is the length that is buried in the wood or other material and does not include the head of a raised or domed-headed screw. Metric screws are measured in millimetres, which is the diameter of the screw (or bolt) in millimetres.
A No. 8 screw is 4mm wide, so you will need a red wall plug with a 6mm or 7mm hole.
You will need a drill bit that is 1mm smaller than your wall plug. So, for a red wall plug with a 6mm or 7mm hole, you will need a 5mm or 6mm drill bit.