The numbers on spark plugs indicate properties such as spanner width and design, heat rating, thread length, construction features and electrode distances. Different manufacturers use similar coding, which makes it easier to interchange brands if need be. Measurements are usually metric.
Using NGK spark plug codes as an example, the code numbers may be BCPR6ES-11 or PFR6A-11A. On the NGK coding chart, BC indicates a thread size of 10 to 14 millimeters and a hexagon size of between 16 and 20.8 millimeters. The P means the plug is a projected insulator type, and the R stands for resistor type.
The number 6 indicates the heat rating, which is mid-range on the scale. The E is the thread reach, in this case, 19.0 millimeters. The S indicates a conical seal type of firing end construction, and the 11 is the spark gap, in millimeters.
On the second example, the P means the plug is a platinum electrode type. The F is for the metal shell casing size. For this plug, the measurement is 14 millimeters by 19 millimeters with a hexagonal measurement of 16 millimeters.
The R is for resistor construction. Again, the 6 represents the heat rating, and the A indicates the plug has a special design. The 11 indicates the spark gap, and the A indicates this plug has no gasket.