How we choose to measure things is important. Most of the world uses the metric system of measurement. While the metric system is based on a standard meter, for mechanical uses such as on bicycles, it is will help to understand metrics better if you consider the millimeter the basic unit. The metric system breaks down on a system of 10, making it easy to understand. Using the millimeter, ten millimeter will comprise a centimeter. One thousand millimeter will comprise a meter. One thousand meters yields one kilometer. Going smaller then the millimeter the system is also simple. Units become simply decimal. 0.8mm is smaller then 1mm. 0.08 is smaller still. In the metric system, you will not find a part or sizing listed as "1/8th-millemeter." This would be stated as a decimal, 0.125mm
In the USA, and a few other countries, the British Imperial system is still in use. This is also sometimes called SAE system (Society of Automotive Engineers), or the Fractional System. The basic unit is the " imperial yard," which is scribed on a bar kept at the Office of the Exchequer in the United Kingdom. For most mechanical work, the inch is the most useful. 36 inches compose one yard, and 12 inches make one foot. Going smaller then one inch, the SAE system typically uses fractional sizing. 1/2-inch is smaller then 1-inch. 7/16-inch is larger then 3/8-inch. This is the system of sizing in the SAE system, for box and open end wrenches, for hex wrenches, and for thread and taps sizing. In theory, it would be possible for engineers and machinists to produce thread size of 0.16-inches. However, you will not find this. Instead, there is a 5/32-inch thread, which turns out to be 0.15625-inches. Common threading for smaller machinary such bicycles is broken down in sizes based on 1/16th of an inch. That small increment allows enough choices in threading. For some purposes you will also see sizing down to 1/32-inch.
While there has been some movement toward making the USA go metric, for now it is important to understand both systems. For most Americans, the SAE system is ingrained since birth. For example, the bars on a child's crib are likely to be sized in the inch-system, such as 3/4-inch doweling. A sheet of paper in U.S. school is 8.5 inches by 11 inches. In the USA, you are surrounded by the inch system. Learning fractions becomes an early and important math lesson in school system. Still, how many of us can quickly determine, which is larger, 13/64ths, or 3/16th? (The former is, but only by 1/64. Wouldn't you rather have had 5.15mm compared 4.8mm?)
When Americans are taught the metric system, it is often through conversion equations. One inch equal 25.4 millimeters. With that, you can figure out the rest. However, you will still not know the metric system. For a mechanic, you need to know tactilely the millimeter. Take this test. Grab some paper and fold it over repeatedly until you think it is a millimeter. Now, get a caliper. If this is 80 bond paper, it will take 10 folds to get to a thickness of 1mm.
How precise does one need to be? Again, it becomes important, especially for bicycle mechanics, to know and be comfortable with both systems. On one bicycle, you will see both the metric and SAE system. Most threading is metric, but not all. Axles on most bikes will use a 9mm threading for the front axle, and 10mm for the rear axle. Bottom brackets are commonly 1.375 inch with thread pitch of 24 threads per inch. Components will be fitted with metric fasteners using metric wrench sizing, but the chain is a one-half inch pitch. Ball bearings on a bike are inch sized, from as small as 1/16th-inch up to 5/16-inch.
There are several occasions where there is close sizing between the metric and SAE systems. This can sometime be useful, but it can often prove troublesome. If the wrench is not the correct size for the part, it will have poor "purchase" on the fittings. The load will be mainly on the corners of the nut or the hex bit, and this may damage the part. Here are some examples of poor interchangeability and workable fits.
- A 3mm hex wrench may seem to fit inside a 1/8-inch socket, but the 1/8-inch socket would actually need a 3.175mm. The 3mm hex wrench is a sloppy fit, and will likely cause bolt head to be ruined.
- A 4mm hex wrench, however, is very close to a 5/32-inch sizing. The 5/32-inch wrench is 3.97mm, which is with in the tolerance of many bolts.
- A 9/16th box wrench is 14.29mm. It will seem to fit on a 14mm nut. Worse yet, a 5/8-inch wrench is 15.65mm, and would be a poor choice for a 15mm nut.
The so called "gauge system" is an very poor application of measurement. Gauges are sometimes used for thickness of wire and of sheet metal. A gauge is not a direct measurement of the size. The primary use of gauges is in the electrical field, where the larger the wire, the lower the resistance to electricity, and hence the lower the gauge number. Relatively small wire has a greater resistance to electricity, and gets a higher gauge number. Smaller gauge numbers are thicker, and bigger gauge numbers are thinner. Make sense? For electricity, yes, but for bicycles, it does not.
The English system of measurement will be here for the foreseeable future. USA factories make basic stock materials in inch sizing. Taps, drills and other tooling in the USA is less expensive and easier to obtain in inch sizing. Fasteners available in the USA are less expensive and more readily available in then metric sizing. So, we will have both systems in the USA for as long as anyone who reads this is around.