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Motorcycle Precision Measurements

Updated on November 30, 2009

Did you ever wonder why two identical bikes may perform so differently, even though both seem to be tuned identically? With the same timing, valve adjustment, gearing, jetting, and rider weight, why will one bike outperform the other? Most likely the answer lies in the clearances and tolerances with which the engine and transmission were assembled. The measurement, tolerances, and clearances used when an engine is assembled are major factors in performance and reliability.

When an engine is designed, every part has its size stated in distances, radii, bore, thread size, or other dimensions with a numerical value. These dimensions are the ideal or designed size. When the part is manufactured however, it is not always the exact size stated by the designer. Instead, it can be within a narrow range of allowable sizes. This range is referred to as a tolerance. Since machining parts to an exact size would be very expensive, they are machined to a tolerance instead.

These dimensions for a part are stated in fractions of an inch, thousandths of an inch, and millimeters. The designer also decides how much distance, or clearance, is needed between two moving parts to allow for lubrication and for expansion when the parts become heated. Too little clearance or too much heat expansion forces lubrication out, causing metal-to-metal contact to occur, which often results in seizure.

The designer could increase piston clearances to a point where seizure would be impossible, but this would cause poor sealing, and result in a loss of power and piston slap. A thousandth of an inch can make a difference in both power and reliability.

Any performance difference could have been the result of many different problems, most of them relating to measurement. Of course, running too fast, too hard, too soon are also critical factors. Designers often specify close clearances between moving parts so that some desirable wear-in or break-in is possible. Until that initial mating occurs, clearances are relatively tight and require moderate driving.

Almost every dimension of every part is designed and manufactured to precise tolerances. In the motorcycle world measurements are given in either metric or the English standard system. In many cases, measurements are given in both systems, allowing the mechanic to use the kind of measuring equipment he has available. Where this is not the case, the mechanic must be able to convert the measurements from one system to another.

Distances, such as length, diameters, or thicknesses are measured with scales, calipers, micrometers. Pressures, tensions, and torque settings are measured with spring type scales such as the torque wrench. Vernier scales on instruments, such as calipers or micrometers, allow even greater accuracy in measurement, by indicating fractions of basic measurement units. Instruments such as depth gauges and telescoping gauges provide a means of measuring internal dimensions that normal measuring instruments cannot measure directly.

Other measuring instruments used in motorcycle diagnosis and repair include: dial indicators, feeler gauges, wire gauges, and screw-pitch gauges. The performance and condition of parts or groups of parts can be measured with instruments such as compression gauges or electrical meters. These various measuring tools allow the mechanic to determine the condition of the different parts of his machine.

Continued In Motorcycle Precision Measurements - Part 2


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