
The difference between metric and SAE screw sizes is essentially the difference between the metric and imperial systems of measurement. Metric screw sizes are measured in millimetres, while SAE screw sizes are measured in inches. Metric screw sizes are also communicated differently, with the pitch and major diameter determining the size. For example, a 4mm screw with a pitch of 1mm would be M4 x 1mm. On the other hand, SAE screw sizes are determined by the nominal major diameter and the threads per inch. While the U.S. primarily uses the SAE system, the country is now producing fasteners using the metric system.
Characteristics | Values |
---|---|
Measurement System | Metric: Decimal-based system, measurements based on metric units (e.g. millimetres) |
SAE: Imperial system, measurements based on inches | |
Thread Specification | Metric: Specified by pitch and major diameter |
SAE: Unified National Coarse Thread (UNC) or Unified National Fine Thread (UNT) | |
Thread Measurement | Metric: Pitch measured in millimetres (mm) |
SAE: Threads per inch (TPI) | |
Head Size Measurement | Metric: Measured in millimetres (mm) |
SAE: Measured in inches or fractions of inches | |
Bolt Types | SAE: Unified National Fine Thread bolt, Unified National Coarse Thread bolt |
Bolt Markings | Metric: Marked with "ISO M" or "M" for compliance with ISO standards; may also be marked with single-digit numbers to indicate strength |
SAE: May have blue-painted heads (especially older GM metric fasteners) |
What You'll Learn
- Metric and SAE screw sizes: Metric uses a decimal-based system, while SAE uses imperial units
- Measuring screw sizes: Metric uses millimetres, while SAE uses inches or fractions of inches
- Screw head markings: Metric heads have numbers, while SAE heads have lines
- Thread pitch: Metric pitch is measured in pitch in millimetres, while SAE pitch is measured in threads per inch
- Bolt types: SAE has Unified National Coarse Thread and Unified National Fine Thread bolts
Metric and SAE screw sizes: Metric uses a decimal-based system, while SAE uses imperial units
Metric and SAE screw sizes refer to the different systems of measurement used for screws and other fasteners. Metric uses a decimal-based system, with measurements based on metric units such as millimetres. SAE, on the other hand, uses the imperial system, where measurements are made in inches. This fundamental difference means that metric and SAE screw sizes are communicated differently and require different tools.
The major difference between metric and SAE screw sizes lies in how threads are specified. SAE screw sizes are either Unified National Coarse Thread (UNC) or Unified National Fine Thread (UNF). Metric screw sizes, on the other hand, are specified by the pitch and the major diameter in millimetres. For example, a metric screw with a major diameter of 4mm and a pitch of 1mm would be sized as M4 x 1mm. In contrast, an SAE bolt with a nominal major diameter of 5/8" and 11 threads per inch would be sized as 5/8-11 (coarse).
Another distinction between metric and SAE screw sizes is in the measurement of the head size. Metric bolt head sizes are measured in millimetres, whereas SAE bolt head sizes are measured in inches or fractions of inches. This can lead to confusion when attempting to use one type of bolt with tools designed for the other system.
SAE screw sizes are primarily used in the United States, whereas metric screw sizes are used internationally and are favoured by international manufacturers. Even in the US, there is a growing trend towards the use of metric fasteners, with some American manufacturers adopting the metric system to facilitate exports.
Identifying the difference between metric and SAE screw sizes can be challenging, especially when dealing with older fasteners that may not be marked. One method is to use a thread gauge, which can quickly determine whether a fastener is metric or SAE. Another approach is to use calipers to measure the thread size, as SAE fasteners will conform to English measurements, while metric fasteners will match millimetre sizes. With experience, it becomes possible to identify metric and SAE screw sizes at a glance, even when markings are absent.
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Measuring screw sizes: Metric uses millimetres, while SAE uses inches or fractions of inches
The difference between metric and standard screw sizes is evident in how they are measured. Metric screw sizes are measured in millimetres, whereas standard screw sizes are measured in inches or fractions of inches. This is because metric screws follow the decimal-based metric system, while standard screws adhere to the imperial measurement system.
When it comes to head size, metric and standard bolts have distinct measurement methods. Metric bolt head sizes are determined in millimetres, which can lead to confusion if they are applied to standard systems that rely on inch-based measurements. On the other hand, standard bolt head sizes are measured in inches or their fractions.
The pitch of the thread is another differentiating factor. Metric screws use pitch in millimetres, which refers to the distance measured between the peaks of two adjacent threads. In contrast, standard screws express the pitch as threads per inch (TPI), where you count the number of threads covering a one-inch distance.
Standard bolts typically originate in the United States and are available in two types: Unified National Fine Thread (UNF) and Unified National Coarse Thread (UNC). Metric bolts, on the other hand, are designed according to the metric system, which offers the advantage of being easier to scale up and down compared to the imperial system.
To identify the strength of metric bolts, manufacturers sometimes mark them with single-digit numbers. Additionally, for hexagonal bolts, the "M" and strength grade may be embossed on the flats.
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Screw head markings: Metric heads have numbers, while SAE heads have lines
Screw head markings are engraved on the top or side of the bolt head and indicate the manufacturer, material, and dimension of the fastener. They are a helpful way for customers to identify what they are purchasing and using. When it comes to metric and SAE screw head markings, there are some distinct differences.
Metric Heads
Metric screw heads typically have markings that consist of two numbers separated by a dot, such as 5.8, 8.8, 10.9, or 12.9. These numbers are engraved on the top or side of the bolt head. The number before the decimal indicates the approximate minimum tensile strength of the bolt, while the number after the decimal represents the approximate yield strength percentage. The higher the numbers, the stronger the material of the fastener.
SAE Heads
SAE screw heads, on the other hand, are identified by lines or letters. For example, SAE grade 5, which is similar to metric grade 8.8, is marked with three radial lines. SAE grade 8, similar to metric grade 10.9, will have six radial lines. These lines indicate the grade of the fastener, which represents the material, hardness, and strength characteristics. The higher the grade, the stronger the material that makes up the fastener.
In summary, when identifying metric and SAE screw head markings, it is important to look for these distinct differences. Metric heads will have numbers, while SAE heads will have lines or letters. These markings provide valuable information about the fasteners, ensuring strength and reliability.
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Thread pitch: Metric pitch is measured in pitch in millimetres, while SAE pitch is measured in threads per inch
When it comes to screw sizes, there are two main systems of measurement: the metric system and the imperial system. The metric system uses metric units such as millimetres, while the imperial system uses inches. While the metric system is based on a decimal system, the imperial system uses a different set of units and measurements.
One important aspect of screw sizes is the thread pitch, which refers to the distance between adjacent threads on a fastener. In the metric system, the thread pitch is measured in pitch in millimetres. This means that the distance between the crests of two neighbouring threads is specified in millimetres. For example, if the distance between two threads is 1.6 mm, the thread pitch is specified as 1.6. Standard measuring tools such as a steel rule, caliper, or comparator are commonly used to determine the thread pitch.
On the other hand, the imperial system, also known as the SAE (Society of Automotive Engineers) system, measures thread pitch in a different way. In this system, the pitch is specified in threads per inch (TPI). To determine the TPI, one would count the number of thread peaks or crests within a one-inch segment of the bolt. For instance, if there are 20 thread peaks within that inch, the TPI would be 20.
The distinction between metric and SAE thread pitch measurements is essential, as it ensures that bolts and nuts can couple together properly. If the threading of a bolt and nut are incompatible due to different thread pitches, it can lead to seizing or stripping of the threads, resulting in an unusable connection. Therefore, it is crucial to have a clear understanding of the thread pitch and the measurement system used to avoid such issues.
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Bolt types: SAE has Unified National Coarse Thread and Unified National Fine Thread bolts
SAE bolts are classified as either Unified National Coarse Thread (UNC) or Unified National Fine Thread (UNF). UNC and UNF bolts differ in their thread pitch, with UNC bolts having a coarser pitch and UNF bolts having a finer pitch. This means that UNC bolts have a larger number of threads per inch compared to UNF bolts.
The coarser pitch of UNC bolts results in a larger cross-sectional size for each tooth, making them better suited for bearing larger tensile and impact forces. UNC bolts are typically used for mass-produced threaded fasteners such as bolts, screws, and nuts. They are commonly used in applications that require quick assembly or disassembly and are prone to corrosion and damage.
On the other hand, UNF bolts have a smaller pitch, resulting in a higher thread strength and better space utilisation. UNF bolts are generally used for positioning, adjustment, and fixation applications. They are commonly used in optical instruments, measuring instruments, and battery compartments of metal shell flashlights.
The choice between UNC and UNF bolts depends on the specific application and requirements. UNC bolts are preferred when higher tensile and impact strength is needed, while UNF bolts are better suited for applications requiring fine adjustments and improved sealing capabilities.
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Frequently asked questions
Metric screw sizes are measured in millimetres, whereas SAE screw sizes are measured in inches. Metric sizes are typically denoted by a number followed by "mm" (e.g. 8mm), while SAE sizes are often indicated by a number and a dash (e.g. #8-32).
Metric screws typically have numbers on their heads, while SAE screws usually have lines. Metric measurements are also based on a decimal system, so you'll see numbers like 5mm, 6mm, 8mm, etc.
Yes, the two basic types of SAE screws are Unified National Fine Thread (UNF) and Unified National Coarse Thread (UNC). UNF screws have finer threads and are used for precise applications, while UNC screws have coarser threads and are more common.
It is not recommended to mix metric and SAE screws and holes. Using the incorrect type can compromise the integrity of the connection and lead to structural failures. Always refer to the specific application's requirements and use the appropriate screw type.