What Are Metric Fastener’s Standards Of Size And Rating?
Metric fasteners are sized according to the pitch and diameter of the fastener. The pitch indicates the distance between each thread which is more fine and, therefore, has more threads than a standard SAE fastener. The diameter is the width of the fastener’s threads.
Metric hardware can be identified on the head with two numbers separated by a decimal point: the first is the diameter and the second is the pitch. For example, a metric fastener that shows “M8x3.0'' has a diameter of 8mm and a pitch of 3 mm in length.
The strength rating of metric hardware or tools uses a system called “property class” and is stamped on its head. This indicates the machine's tensile strength and its ability to resist being pulled apart under tension. Higher property class numbers specify higher tensile strength which will hold up whether it is an older thread or a new thread. For example, metric fasteners or tools with a property class of 8.8 have a minimum tensile strength of 800 MPa.
MPa stands for megapascal, a unit of pressure or stress in the International System of Units (SI), and is used as a unit of measurement for tensile strength, compressive strength, or yield strength. One megapascal is equal to one million pascals. A pascal is the unit of pressure defined as one newton of force per square meter of area.
What Are SAE Fastener’s Standards Of Size And Rating?
SAE (Society of Automotive Engineers) fasteners and bolts are also identified by their diameter, thread count, and thread pitch. Their thread is less fine than a metric bolt and is measured by thread per inch (TPI), and the diameter is measured in inches. The pitch is the distance between each thread on the bolt.
SAE hardware and tools are rated for bikes according to their strength or grade which you will find on the head of bolts. The rating is listed with two or three numbers separated by a decimal point such as 8.0 which indicates it is a grade 8 bolt. The first number reflects the ultimate tensile strength of the bolts in thousands of pounds per square inch (KSI). If a second number is present, it indicates the yield strength in KSI, and a third number represents the type of material the connectors or tools are made from; for example, machine grade 5 in the third number is made from medium carbon steel while a grade 8 in the third marking has a longer life and is made of medium carbon alloy steel.
Older Harley-Davidson Motorcycles Used SAE Hardware
Metric hardware and tools weren’t common in America until the late 1980s and early 1990s, so a pre-80s Harley bike was manufactured almost exclusively with SAE hardware. Some examples of Harley-Davidson bikes that used almost exclusively this type of hardware include the Panhead and Shovelhead models produced in the 1950s through the 1970s as well as the Evolution Big Twin models produced from the 1980s through the 1990s.
Most Harley-Davidsons Use a Mix of SAE and Metric Hardware
Because many Harley bikes have had upgrades and because some parts and wheels are exclusively made overseas, it’s possible for any Harley bike to have a mix of both SAE and metric parts and tools. The specific fasteners and tools used can vary depending on the model and year of the motorcycle, so it’s always best to consult the owner's factory service manual or a qualified mechanic to determine the appropriate tool set and fasteners needed for your specific Harley-Davidson bike.
Harley Davidson Parts that Generally Tend to Use More SAE Hardware
- Frame parts like rear swingarms and tubes for front forks
- Engine parts like the cylinder heads and rocker boxes
- Transmission parts like the gears and bearings
- Brakes and brake parts like the rotors and calipers
- Electrical like the battery terminals and wiring connections
Harley-Davidson Parts that Generally Tend to Use More Metric Hardware:
- Wheels and tires
- Handlebars and controls
- Suspension components such as shocks and springs
- Exhaust components such as headers and mufflers
- Bodywork and trim pieces
Why Are American-Manufactured Harley Parts Sometimes Metric?
Some Harley-Davidson bike parts are made by third-party suppliers who often use metric hardware and metric tools as a standard in their own manufacturing processes. Harley-Davidson may choose to use the same connectors and tools in their bike designs in order to streamline the production process and reduce costs.
Also, many of these components for Harley bikes are designed to be interchangeable with bike parts from other suppliers including those from countries that predominantly use the metric system. By using metric hardware and tools, Harley-Davidson can ensure that its bikes are compatible with a wide range of aftermarket parts and accessories from around the world.
Lastly, the use of metric hardware in certain motorcycle parts is influenced by the design and function of those bike parts. For example, suspension components such as shocks and springs may be designed to fit within specific tolerances which are better achieved with metric connectors that offer finer gradations than SAE.
Metric Bikes and Tools vs. SAE Bikes and Tools: No Universal Solution
A "metric bike" typically refers to a motorcycle that is manufactured by a Japanese company like Honda, Yamaha, and Kawasaki that tends to use the metric system for its measurements, hardware, and tools. The term “metric cruisers” originated in the United States and was used to differentiate between current metric bikes manufactured by Japanese companies and those manufactured by American and European companies which tend to use the Imperial system for their bike hardware and tools.
But the term "metric bike" is largely inaccurate because most modern motorcycles manufactured by American and European companies, like Harley-Davidson and other bikes, also use the metric system for some of their measurements, parts, and tools.
That’s not the only problem, some Japanese motorcycles also use the Imperial system for certain components and tools. Ultimately, the use of the metric or Imperial/SAE system in a motorcycle's design, parts, and tools is determined by the manufacturer based on cost and the intended market for the motorcycle.
There appears to be no immediate solution for bike manufacturers or mechanics around the world to unify and adapt universal hardware, tools, and measurement systems for repairs. Standardization of tools and parts would require a significant shift in industry practices and would cost way too much money for manufacturers to implement.
Some motorcycle manufacturers of tools and parts are beginning to try and bridge the divide by incorporating both SAE and metric systems producing combination wrench and socket tools that have both measurements on them. However, for Harley owners today, it continues to be a frustrating game of “guess the measurement” between these two global measuring standards.