Choosing The Right Screw: Understanding Head Size And Variations

Screw head size is an important consideration when selecting the right screw for a project. The size of a screw head is typically indicated by a number, such as #6 or #8, which corresponds to its diameter. For example, a #6 screw has a smaller diameter than a #8 screw. Screw head size can also be measured in inches or millimetres, with the specific measurements depending on the type of screw head. Round head screws, for instance, are about 15% smaller than flat or oval head screws. Understanding screw head sizes is crucial for ensuring the compatibility and structural integrity of your project.

Screw Head Size Characteristics

Screw gauge and shaft length

The three basic measurements of a screw are its gauge, threads per inch, and shaft length. These three parameters are essential in defining the size and functionality of a screw.

The gauge of a screw refers to the diameter or thickness of the screw, including the head and shaft. It is a rough linear measurement, with a #8 screw, for instance, being nearly twice the diameter of a #4 screw. However, there is no precise conversion system between gauge and calibrated measurements. As a result, it is advisable to refer to a screw gauge table to determine the correct size.

The shaft length of a screw is the second detail in its label and refers to the portion of the screw that drives into a surface. For countersinking screws, the length is measured from the top of the head to the tip, whereas for non-countersinking screws, it is measured from directly under the head to the tip.

For a non-countersinking screw, the screw length is the length of the shaft. The shaft is the part of the screw that goes into the surface. Some screws have heads that can be driven completely into the surface. This means that the head of the screw is part of the screw length.

Screws that can be driven completely into a surface are known as countersinking screws. Some screws, like truss-, round-, hex-, button-, and pan-head screws don’t go all the way into the surface. These screws are known as non-countersinking.

The length measurement for a countersinking screw is the distance from the top of the head to the tip. This goes for flat-head, bugle-head, trim-head, and any other countersinking screw where the head can be driven beneath a surface.

For a non-countersinking screw, it's the distance from the bottom of the head to the tip. So for hex-, pan-, button-, round-, and truss-head screws, length is measured from directly under the head to the tip.

Screw head size

The numbers on screws (#6, #8, etc.) refer to the diameter of the screw, with higher numbers indicating larger diameters. Screw gauges are a measure of the head size and shaft size, and while they are roughly linear, there is not a perfect 1:1 relationship. For example, a #8 screw is a little less than twice the diameter of a #4 screw.

There are a few rules of thumb for estimating the size of a screw gauge. One is to measure the diameter of the countersunk head in inches, subtract 1/16", and then count the number of 1/32" remaining. So, a screw head with a diameter of 1/4" would be a 6 gauge, 5/16" would be an 8 gauge, and 3/8" would be a 10 gauge. Another method is to measure across the screw head in 1/16ths, double it, and then subtract 2. So, if the measurement is 5/16ths, you would double it to get 10 and then subtract 2 to get 8.

In addition to gauge, screws have two other basic measurements: threads per inch and shaft length in inches. For example, a screw labelled 6-32 x 1 1/2" would be a #6 diameter with 32 threads per inch and an inch and a half long. When the middle number is absent (e.g., 6 x 1 1/2"), the screw has the "normal" number of threads per inch for that size and type of screw.

When selecting a screw, it is important to consider the specific requirements of your application. Different types of screws are designed for different purposes, and using the correct type of screw can help prevent splitting or damage to the material being fastened.

Screw thread pitch

The thread pitch of a screw is the distance between the threads. It is usually defined under various categories, such as standard, fine, or extra fine. The thread pitch is critical as it ensures the screw matches the nut it will be used with. If the wrong pitch is used, the screw may not be securely fastened.

Thread pitch is often referred to as TPI or threads per inch. This is a count of the number of threads per inch measured along the length of a fastener. TPI is used with American fasteners. The TPI measurement is affixed to the diameter of the thread to form a common imperial thread size, e.g. "1/4-20", which would denote a 1/4-inch diameter and 20 threads per inch.

For metric fasteners, the thread pitch is used instead of TPI. The distance is measured in millimetres. For example, a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm.

To measure thread pitch, use a caliper and measure from the peak of one thread to the next. You can also calculate thread pitch by measuring the entire length of the thread of a screw and counting the total number of threads. You can then divide the thread length by the total number of threads to calculate the thread pitch.

Coarse and fine thread series are the most common designations for general application bolts and nuts. Coarse threads are beneficial as they are less likely to cross-thread and are more tolerant in adverse conditions. Fine threads are used in precision applications as they have higher tension strength.

Screw length

• Screws that are flat on the bottom of the head (non-countersunk) are measured from the flat bottom of the head to the tip.
• Screws that are flat on the top of the head (countersunk) are measured from the flat top of the head to the tip.
• Oval head screws are measured from the widest point of the head to the tip, as they have no flat side.
• Hex and hex washer head screws are measured from the bottom of the head, despite being flat on both the top and bottom.
• Self-drilling screws are measured to the end of the drill point tip.

The general rule of thumb is that the screw should enter at least half the thickness of the material it is being screwed into. For example, a screw should enter at least 3/4" into a 2 x 4 piece of wood. However, this can vary depending on the type of connection and the thickness of the material. For instance, when joining metal to wood, there is a minimum embedment required for the screw to perform as expected. When the fastener is fully seated against the metal panel, there should be a minimum of three fully developed threads into the wood.

For thicker materials, longer screws may be needed to ensure a firm hold. Conversely, shorter screws are more suitable for thinner materials. The screw should always go through the thinner piece and thread into the thicker piece, with about 2/3 of the screw threaded into the secondary piece.

Screw types

There are many different types of screws, each meticulously engineered for use with different materials and surfaces. Using the wrong type of screw for a project can be disastrous.

Screws are a type of mechanical fastener that typically has three main parts: the head, the shank, and the threads. The head is the top part of the screw, which provides a surface for tools to grip and apply torque during installation or removal. The shank is the long, cylindrical part of the screw that extends from the head to the tip and can be fully threaded or partially threaded. The threads are the helical grooves that wrap around the shank and provide the screw with its grip.

• Wood Screws: Wood screws are one of the most commonly recognised types of screws. They are used to join two pieces of wood together and are designed with large threads that cut into the wood to provide a strong and secure connection. The most common type of wood screw is the Phillips drive, which has an X-shaped head.
• Machine Screws: Machine screws differ from other screws in that they do not drill through materials themselves. Instead, they are used in conjunction with pre-drilled and tapped holes. Machine screws are commonly used in situations where precision and strength are required, such as 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 fully threaded and pointed and are categorised by their head styles and three numerical values that represent the screw's diameter, length, and thread count.
• Drywall Screws: Drywall screws are specifically designed to install drywall panels. They have bugle heads that protect against tearing the paper surface, as well as a sharp point for easy penetration. They can have either coarse threads for wood studs or fine threads for metal studs.
• Concrete/Masonry Screws: Concrete screws, also known as masonry 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. They typically have a hex, bugle, or flat head and require a pre-drilled hole for installation.
• 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 and a sharp point for easy penetration.
• Self-Drilling Screws: Self-drilling screws, also known as TEK screws, are designed for quick and efficient installation of metal components. They create their own pilot hole, eliminating the need for pre-drilling. They 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 that require a strong connection. They have a hexagonal head and coarse threads, providing excellent grip and holding power. They are typically used with a pre-drilled pilot hole and require a wrench or socket for installation.
• Set Screws: Set screws are designed to secure one object within another, such as a gear on a shaft. They have no head, allowing them to be fully embedded into a pre-drilled hole. Set screws typically have a hexagonal socket and are tightened with an Allen wrench.
• Tamper-Resistant Screws: Tamper-resistant screws are designed to deter unauthorised access to protected areas or equipment. They have unique head designs that require specialised tools for installation and removal. They 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 onto a building's underlying structure. They have a sharp self-piercing point that eliminates the need for pre-drilling and prevents damage to the sheet metal.
• 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. They are commonly used in sheet metal fabrication, woodworking projects, and automotive repair.

Frequently asked questions