Every classic motorcycle enthusiast knows that units of measurement are as basic as our ABCs. Need to talk to a buyer or seller about an engine? Need to get a price for a replacement tire? You name it—whenever you have a conversation about motorcycles or their parts, you’ll talk about units of measurement at some point. But what do all those units of measurement mean, and what are they applied to?

Although not exhaustive, the following list details most of the common measurement units found within classic motorcycling and where they are applied.

CC (cubic centimeters)

Example:

• Typically used to size an internal combustion engine's swept volume. Volume is ascertained by: area of base (the cylinder diameter) x the vertical height (the piston movement in one stroke).
• Yamaha TZ250 has a 5.4 cm x 5.4 cm bore and stroke and two cylinders. Area of base = 2.7² x 3.14 (p): 22.89 sq. cm's. Swept distance x area of base = 5.4 x 22.89: 123.6. Total cc, therefore, is: = 123.6 x 2 (# of cylinders) = 247.

Cu. inch

Example:

• Cubic inches are generally used to size an internal combustion engine in the United States. Volume is ascertained by: area of base (the cylinder diameter) x the vertical height (the piston movement in one stroke) x the number of cylinders.
• The Harley Davidson 883 has a 3" x 3.812" bore and stroke with two cylinders, and is rated at (53.9 cu. Inches).The engine's swept volume is ascertained by: area of base (the cylinder diameter) x the vertical height (the piston movement in one stroke) x the number of cylinders. Area of base = 1.5² x 3.14 (p): 7.065 sq. inches. Swept distance x the area of the base =3.812" x 7.065: 26.93 cu. in. Total volume in cu. inches, therefore is: 26.93 x 2 = 53.86 cu. in. (or 883 cc).

Chain Size.

Example:

• Chain measurement utilizes ?" increments. The first part of the identification number is the pitch between the link centers. The second part is the distance between the side plates.
• A 520 chain has 5 lots of ?" between the link centers: 5 x ?" = ?". The 20 represents 2.0 lots of ?" between the plates: 2 x ?" = ¼". Therefore, the 520 chain measures ?" x ¼".

Micron

• A micron is a unit of measurement typically used to define the hole size in fuel and oil filters. One micron is one millionth of a meter, or 0.00003937 inches. A good quality fuel filter, for example, would have around a 40 micron size rating.

Oil Capacity

• Motorcycle oil capacities are expressed in either SI units or liters.

Examples:

• A 883 Harley Davidson requires 2.5 quarts (5 pints) during an oil change.
• A Suzuki GSXR750 has a combined engine and gearbox oil capacity of 3.5 liters (3500cc) - 3.8 with a filter change.

PSI (pounds per square inch)

• Typically used on motorcycles for tire pressure ratings, gas shock (damper) pressures, and cylinder cranking pressures. PSI is generally used in the United States, whereas the unit bar is used in European countries. Note: I bar = 14.696 psi.

Tire Size

Example:

• As motorcycles were developed, so too were the measuring systems. Initially in inches, the tire width and aspect ratio (a percentage of the tire's width in relation to the side wall height) have changed to include metric sizing. However, the wheel diameters have kept the SI units.
• A 1974 Ducati 750 SS has 3.5 x 18 front and rear tires. At this time, tires were produced with the same width and side wall size. Therefore, the 3.5" tire tread width on the Ducati would also have a 3.5" side wall height. The wheel rim diameter in this case is 18".
• Later tires were manufactured with different tread widths to side wall size (the side walls being expressed as a % of the tire width), and incorporated both inches and mm's. For example, the rear tire for the RZ500 Yamaha has a 130/80x18. This tire has a width of 130mm, and a side wall height that is 80% of the width (104mm) but is fitted to an 18" rim.