Understanding Deep Thread Screw Sizes: A Comprehensive Guide

Deep thread screw size refers to the dimensions of the threads on a screw, which is important for understanding how it fits into a threaded hole. Screw sizes can be identified by screw diameter and pitch. The Unified Thread Standard (UTS) system uses an inch-based measurement for screw threads, while the metric system uses millimetres (mm). For inch thread sizes, the diameter is specified by a number (e.g. #10 or No.10) and threads per inch. For metric thread sizes, the diameter is specified by the letter M followed by the number of millimetres (e.g. M6 for 6 mm). Deep thread screws are commonly used in woodworking applications as their aggressive threads maximise holding power in softwoods, particleboard, and MDF board.

Deep Thread Screw Characteristics and Values

Screw thread terminology

Screw threads are the sloped helices spiralling down the surface of a cylinder. The thread can be external, as on a bolt or screw, or internal, as inside a nut. The two most common thread types used in the manufacturing industry are spaced, designed to form threads within a pre-cut hole, and machine screw, designed to fit a pre-formed thread in a nut or hole.

• Major diameter: The major diameter is the largest diameter of the thread, including the height of the raised helix. It can be measured on the crests with a calliper rule or slot gauge.
• Minor diameter: The minor diameter measures the diameter of the "root," or the innermost part of the screw, not including the crests of the helix. This measurement requires specialised equipment.
• Effective diameter: The effective diameter is halfway between the major and minor diameters. It measures halfway up the helix crest and requires specialised equipment for accuracy.
• Pitch diameter: This is the middle diameter between the major and minor diameters of screw threads.
• Pitch: The pitch is the distance between two identical threads. It is the distance from a point on the screw thread to the same point on the next thread.
• Flank: The flank is the angle at which the helix is raised to form a crest on the thread.
• Flank angle: This is the angle made by the intersection of the two thread flanks.
• Crest: The crest is the height at which an external thread is raised, or the depth to which an internal thread is indented. Screws and bolts are typically measured at the crests, while nuts are measured at the roots.
• Root: The root is the deepest part of the grove that corresponds with the minor diameter.
• Thread angle: The thread angle of a screw is the angle between the threads.

When shopping for a replacement, you can use the above terms to read and understand a screw thread callout. Here are the steps to interpreting a thread callout:

• Look at the numbers in the callout.
• Understand the first number in the callout – This indicates the major diameter. Unified threads (in inches) express the diameter as a fixed number #0 through #10. Anything larger than #10 is listed in fractional inches. Metric threads express the diameter with M followed by the diameter in millimetres.
• Understand the second number in the callout – This indicates the distance between threads. It can be expressed as the number of threads per unit or as the distance between identical threads (the pitch). Unified threads measure threads per inch. Metric threads measure millimetres per thread.
• Read the length – This is the number that follows the x. Unified threads measure the length in inches, expressed as a decimal or a fraction. Metric threads give the length in millimetres.
• Be mindful of other nomenclature – You may see additional specifications in a callout. Tolerance classes include numbers 1-3; these refer to how loose or tight a screw fits. The letter A indicates an external thread and B indicates an internal thread. 2A and 2B are the most common classes. The abbreviations UNC (unified coarse) and UNF (unified fine) specify thread series.

Screw thread measurement systems

Screw thread size refers to the dimensions of the threads on a screw, which determines how it fits into a threaded hole. Understanding screw thread size will help you choose the right screw for a stable, secure connection. There are several different systems of measurement used for screw thread sizes. These include the Unified Thread Standard (UTS), generally known as Imperial, and the metric system, which is used in most countries outside the US.

Unified Thread Standard (UTS)

The UTS system uses an inch-based measurement for screw threads, and includes different thread series, such as the coarse thread series (UNC) and fine thread series (UNF), which are finer, tighter threads. Threads are measured in threads per inch, or TPI. You simply count the number of thread peaks along a one-inch length. You also need to measure the major diameter of the screw. These two measurements will combine to give you the screw size you need. For instance, if your major diameter is 1/4" and you have 20 threads per inch, then your screw size is 1/4" x 20.

Metric System

The metric system uses millimetres (mm) to measure screw thread sizes. To get your screw size, you’ll need to measure the screw’s major diameter and the thread pitch. While determining threads per inch is straightforward, thread pitch is trickier due to the minute spacing involved. Use a caliper for accuracy. To calculate thread pitch, measure the distance between two adjacent threads at their peaks. Then take the major diameter of the screw. Let’s say it’s 6mm, which is represented as M6. Now let’s say the pitch is 1mm. Your screw size is M6 x 1mm.

Identifying Screw Threads

It is important to be able to identify screw threads, as there are multiple thread types and sizes that could make two pieces incompatible. The first step is to determine the type of thread. External threads, known as male threads, are visible on the outside of a bolt, screw, or threaded post. Internal threads, known as female threads, are threads inside of a part such as a tapped hole or a nut.

Measuring Screw Threads

Threads have three critical dimensions: diameter, thread spacing, and the length of the threads. To measure the thread diameter, hold the fastener in question against a ruler, and measure from one side of the thread crest to the other thread crest. It is important to try to keep the head of the fastener parallel with the ruler for the most accurate measurement.

Next, identify the number of threads per inch (TPI) or pitch. Threads per inch and pitch are the inverse of each other. So measuring one way or the other can yield either TPI or pitch by simply dividing one by the measured size and converting to the proper units.

Comparing Measurements

Once both diameter and TPI or pitch are known, compare the information against a thread chart. In the U.S., imperial fasteners are most common. However, if the fastener dimensions don’t match up well, compare it to a metric thread chart, pipe thread chart, or consider if the fastener is worn. Older fasteners will sometimes wear down with time, resulting in a slightly smaller diameter than when they were new, so consider the next size up if the fastener might be an older one.

Screw head styles

Screws and bolts each have a unique head designed for its intended function. There are two basic head designs: countersunk and non-countersunk. Non-countersunk heads, where the head is fully exposed, offer the largest variety of styles. This style includes binding, button, cheese, fillister, flange, hex, pan, round, socket, low socket, square, and truss heads. Countersunk designs, on the other hand, typically consist of flat, oval, and bugle heads.

Non-Countersunk Heads

The non-countersunk style is quite versatile, with a wide range of head types to choose from. Here are some of the most common types:

• Binding Head: Also known as a binder head, it is often used for electrical applications. The undercut area beneath the head allows space for wire connections.
• Button Head: This type features a rounded head and is commonly used in socket-driven screws. It is well-suited for confined spaces and light-duty applications.
• Fillister Head: Fillister heads have a smaller diameter and higher profile than round or pan heads, allowing for a deeper slot.
• Hex Head: Hex heads, also known as hex bolts, have a six-sided hexagonal head. They are commonly used with a washer and hex nut assembly or a tapped hole, allowing for greater torque than screws with circular heads.
• Pan Head: This is the most common type of rounded-top screw head. It is used in many applications requiring a flat-bottomed screw and can be substituted for similar head styles.
• Round Head: While less common today, round heads offer an appearance alternative to other rounded-head screws.
• Truss Head: Truss heads, sometimes called mushroom heads, provide a larger bearing surface and a lower profile compared to pan or round heads.

Countersunk Heads

Countersunk screw heads are designed to sit flush with the surface of the material, resulting in a flat and smooth finish. Here are the most common types:

• Flat Head: This is the standard countersunk screw head, typically available with an 82-degree angle. They are suitable for applications requiring a strong and flush fastener.
• Flat Undercut Head: This variation is used for some short screw lengths, allowing for shallower countersinking than standard flat heads.
• Flat 100 Head: Featuring a 100-degree angle, these screws are designed for thinner materials that require a flat head.
• Flat 100 Undercut Head: Used for some short screws, this style allows for a slightly longer thread grip.
• Oval Head: Oval heads have a decorative rounded finish at the top and are often used for switch coverings.
• Bugle Head: Primarily used in drywall and wood decking, bugle heads automatically create their own countersunk hole during installation.

Drive Styles

The drive style, or drive recess, refers to the type of tool required to install or remove a screw. Common drive styles include slotted, Phillips, combination, hex, hex socket, square, Torx®, and spanner. The choice of drive style can impact security, with less common styles like square socket and Torx® offering a degree of protection.

Screw drive types

The type of screw drive used depends on the specific application and the level of torque required. Here are some common screw drive types:

• Slot Drive: This is the most basic type of screw drive with a single horizontal indentation (the slot) in the fastener head. It is driven by a flat-bladed screwdriver and was the first type of screw drive developed. While it is simple and cheap to manufacture, it is not suitable for power tools as the driver can often cam out of the slot, causing damage.
• Phillips Drive: This type of screw drive was created to address the problems with slotted screws. It has a cross-shaped recess in the head and can handle higher torque levels without camming out. Phillips drive bits are designated by the letters "PH" followed by a size code (0000, 000, 00, 0, 1, 2, 3, or 4).
• Pozidriv Drive: The Pozidriv is an improved version of the Phillips drive, allowing for more torque and greater engagement. It is similar in appearance but has radial indentations (tick marks) at a 45-degree angle from the main cross recess. Pozidriv screwdrivers are designated by the letters "PZ" followed by a size code (0, 1, 2, 3, 4, or 5).
• Robertson Drive: Also known as a square or Scrulox screw drive, this type has a square-shaped socket in the screw head and a square protrusion on the tool. Robertson screws are commonly used in Canada and are advantageous because they can be easily driven one-handed.
• Hex Socket Drive: This type of screw drive has a hexagonal recess and can be driven by a hex wrench, also known as an Allen wrench or key. Metric sizes of hex sockets are defined by ISO standards.
• Torx Drive: The Torx drive, often referred to as a star drive, uses a star-shaped recess in the fastener with six rounded points. It is designed to resist cam-out and extend bit life. A tamper-resistant version, known as Security Torx, has a small pin inside the recess.
• Triple-Square Drive: This type of screw drive has 12 equally spaced protrusions, forming a 12-pointed star shape. It is similar to the Robertson drive but can be engaged at more frequent angles by the driver bit. Triple-square drive fasteners are commonly found in German vehicles.
• Polydrive Drive: Also known as RIBE, this type of screw drive has a spline-shaped recess with rounded ends in the fastener head. It resists cam-out and is commonly used in the automotive industry in high-torque applications.
• One-Way Screw Drive: One-way screws are designed to be turned in only one direction and are often used in commercial restroom fixtures and vehicle registration plates to prevent vandalism. They can be installed with a standard flat-blade screwdriver but are difficult to remove with conventional tools.

Screw thread pitch

The pitch of a screw thread refers to the distance from a point on the screw thread to the same point on the next thread. This is also known as the thread pitch or threads per inch (TPI). The thread pitch is used to calculate the difference between two threads.

Thread pitch is measured differently for US fasteners and metric fasteners. US fasteners use TPI, which is the number of thread peaks along a one-inch length. For example, a bolt described as 1/4″-20 has a diameter of 1/4 inches and 20 threads per inch.

Metric fasteners, on the other hand, use thread pitch instead of TPI. The distance between threads is expressed in millimetres and measured along the length of the fastener. For instance, a metric fastener described as M8 x 1.25 has a diameter of 8mm and a thread pitch of 1.25mm. This means there is a distance of 1.25mm between one thread and the next.

Fine thread fasteners have a tighter helical structure, while coarse thread fasteners have larger, deeper threads. Coarse thread fasteners are more forgiving, meaning that if the threading gets lightly damaged, they may still work. Fine thread fasteners, meanwhile, have a higher tension strength due to their larger tensile stress areas.

Frequently asked questions