Standard Screw Diameter Dimensions For 308 Size

If you're looking for a screw with a diameter of 308, you're likely looking for a heavy-duty construction screw. In the US, you'll be using the imperial system and will need to follow the ASME/ANSI standard, also known as the Unified Thread Standard (UTS).

In the UTS system, a #12 screw has an outside diameter of 0.216 inches. Anything larger than this will be labelled according to the outside diameter, so a screw with a diameter of 308 will be labelled 308.

If you're using the metric system, you'll be following the ISO screw thread standard, where screw sizes are labelled with an M and the diameter in mm, so you'll be looking for an M308 screw.

Screws are sized using the imperial system in the US and the metric system elsewhere

Screws come in a variety of sizes and measurements, which can be confusing when trying to select the right one for your project. The US uses the imperial system for screw sizes, while most of the world uses the metric system.

In the metric system, both the diameter and length of screws are measured in millimetres. The first number in the imperial system is the gauge of the screw, which refers to its diameter – the larger the number, the larger the diameter. The length is represented similarly in the imperial system, but it is denoted in inches rather than millimetres.

For example, a #6 screw is used for small crafts, hinges, drawer slides, children's furniture, toys, and light-duty jigs and fixtures. It is available in lengths ranging from 1/2" to 1-1/2". On the other hand, a #10 screw is suitable for general construction, heavy-duty furniture, outdoor projects, decks, and lawn furniture. Its typical lengths range from 3/4" to 4".

Conversion charts are available to help you find the correct screw size and make conversions between the imperial and metric systems. These charts provide information on the decimal (inch) and metric (millimetre) equivalents, as well as the nearest fractional size for US screw sizes.

It is important to note that different screw size standards exist because they were developed to meet specific regional or industry requirements. Therefore, it is crucial to refer to the appropriate standards and conversion charts when selecting the right screw size for your project.

The numbers on screws refer to their diameter, with higher numbers indicating larger diameters

The numbers on screws, such as #6 or #8, refer to the diameter of the screw. Higher numbers indicate larger diameters. For example, a #10 screw has a greater diameter than a #8 screw. Screw sizes are usually indicated by a number and a measurement of length, for example, 6 x 3/4. The first number is the diameter, and the second is the length.

Screw gauges are a measure of the head size and shaft size, and while they are roughly linear, there is not a 1:1 relationship. For instance, a #4 screw is suitable for small crafts, jewellery boxes, and attaching hinges, while a #10 screw is more appropriate for general construction, heavy-duty furniture, and outdoor projects.

When choosing a screw, it's important to consider the specific requirements of your project, such as load-bearing capacity, material compatibility, and environmental conditions. It's crucial to select the right screw size to ensure a secure fit and prevent damage to the material.

Additionally, the length of the screw is a crucial factor. As a general rule, the screw should penetrate about two-thirds of the way into the secondary piece it is being fastened to. However, this ratio can be adjusted based on the thickness of the materials involved.

The US uses the Unified Thread Standard (UTS) sizing system

UTS defines a standard thread form and series, along with allowances, tolerances, and designations for screw threads. The standard is used for bolts, nuts, and other threaded fasteners.

UTS threads are defined by their major diameter and pitch. These threads are symmetrical and have a 30° flank angle, which is the angle from the crest to the groove. Therefore, a UTS thread has a 60° profile, similar to an ISO metric screw thread. The difference is that UTS measures pitch as an inch fraction, while ISO measures pitch in millimeters.

A standard UTS designation has a number that indicates the major diameter, followed by a second number that indicates the threads per inch (TPI). This pair of numbers is then followed by an indication of the thread's coarseness: Unified Coarse Thread (UNC), Unified Fine Thread (UNF), or Unified Extra Fine Thread (UNEF).

For example, a #1-64 UNC bolt has a major diameter of 0.0073 inches and a TPI of 64. In this case, the "1" indicates the major diameter, and the "64" indicates the TPI. The "UNC" indicates that this is a Unified Coarse Thread.

• Unified Coarse Thread (UNC): UNC bolts are typically used for mass production and applications that require rapid installation. The extra space between the threads allows for more variation in manufacturing and application without failure.
• Unified Fine Thread (UNF): UNF threads are used for applications that require tight and fine threads, such as aerospace and automotive applications.
• Unified Extra Fine Thread (UNEF): UNEF threads are used for tapped holes in hard materials, thin walls, and thin materials.

UTS also has different tolerance classes that indicate how tightly the threads fit together:

• Class 1: These threads fit loosely and are suitable for easy assembly or dirty work environments.
• Class 2: These threads are free-fitting and have greater tolerance. They are the most common and are designed to maximize strength.
• Class 3: These threads are used for closer tolerances in high-quality work.
• Class 4: This class is now obsolete.
• Class 5: These are interference threads, meaning they require a wrench to screw and are meant for semi-permanent or permanent installations.

The UTS system measures in inches, including the screw's gauge, threads per inch, and thread standard

The Unified Thread Standard (UTS) is the main standard in the United States and Canada for the form and series of bolts, nuts, and other threaded fasteners. The UTS system measures in inches and includes the screw gauge, threads per inch, and thread standard.

Screw Gauge (Major Diameter)

The screw gauge is the diameter of the outside thread in inches, also known as the "major diameter." Anything less than 1/4" will be labelled from #0 to #12. #0 has an outside diameter of 0.06 inches, while #12 has an outside diameter of 0.216 inches. After #12, the gauge size is labelled according to the outside diameter. For example, if the screw has an outside diameter of 0.25 inches, the gauge size will be 1/4".

Threads Per Inch (TPI)

TPI stands for the number of threads per inch. It is specified right after the screw gauge. For example, a screw with a #8 gauge will have 32 threads in a one-inch section of a coarse screw (#8-32 UNC). If the screw is from the fine series, it will have 36 threads per inch (#8-36 UNF).

Thread Standard

There are two thread standards under the UTS: Unified Coarse Thread (UNC) and Unified Fine Thread (UNF). The extra-fine screw thread standard (UNEF) is also available but is not as commonly used.

The standard UTS designation has a number that indicates the major diameter, followed by a number that indicates the TPI. Following these two numbers is an indication of the thread's coarseness: UNC, UNF, or UNEF.

The tolerance class number indicates how tightly a screw will fit

Tolerance class numbers are part of a thread tolerance designation, which dictates how much a thread can deviate from its basic profile while still achieving a proper connection. Thread tolerance data provides manufacturers with the information they need to ensure the threads they produce are within an acceptable range.

For metric thread tolerance, the designation follows these rules:

• Letters: Capital letters are used for internal threads, while lowercase letters are for external threads. These letters also indicate tolerance position. For example, 'A' indicates external, and 'B' indicates internal for a UTS screw thread.
• Numbers: These indicate the tolerance grade.
• Order: Pitch diameter tolerance is listed before major diameter tolerance.

The tolerance position is the distance of the tolerance from the basic size of the thread profile. It is indicated by letters, with upper case letters for nut threads and lower case letters for bolt threads.

For metric threads, the numbers 3-9 are used to designate tolerance class. The higher the number, the looser the tolerance. There are four groups of tolerance grades:

• Internal thread minor diameter: Grades 4, 5, 6, 7, and 8.
• External thread major diameter: Grades 4, 6, and 8.
• Internal thread pitch diameter: Grades 4, 5, 6, 7, and 8.
• External thread pitch diameter: Grades 3, 4, 5, 6, 7, 8, and 9.

For UTS threads, there are five classes, but only three are commonly used:

• Class 1: Loose fit threads for easy use, even in dirty environments.
• Class 2: Most common class that maximises strength.
• Class 3: Medium fit for high-quality work.

The standard tolerance classes for metric internal threads are 6H and 6g, while for metric external threads, it is 6g.

Frequently asked questions