Motorcycle Precision Measurements - Part 3
Micrometers, ranging in sizes from 1 inch to 4 inches capacity, comprise the set normally used by motorcycle mechanics. Large micrometers are also available of course. Measurement with a micrometer is based on the principle that an internal thread with exactly 40 threads per inch will allow the barrel to advance at a rate of 25/1000th of an inch per revolution (also written as 0.025"). Twenty-five accurate graduations are then radiated around the thimble scale of the micrometer to allow the reading of measurements to an accuracy of 1/1000th of an inch.
Feeler gauges are measuring tools made from precisely ground thin metal strips. If you can slide a flat feeler gauge in between two parts, you have an indication of this clearance. That is, you know there is at least the thickness of the feeler gauge between the two parts. Wire-type feeler gauges rather like probes are used in situations where the two contact points are not surfaces, but may be only small high spots.
Telescoping gauges are tools that are used to take an internal dimension in a tight spot, then transfer it outside where it can be measured accurately by a micrometer. The telescoping gauge is placed in position, then it is expanded to contact both surfaces. The expanded gauge is locked on that dimension and is carefully drawn out of the cylinder bore. The distance across the telescoping gauge is then measured accurately with an outside micrometer. When measuring a cylinder, this process is repeated several times in different directions and at different levels to get an accurate idea of the dimensions of the cylinder.
Plasti-gauge is a measuring device that is relatively inexpensive, but it is rarely used in motorcycle engine rebuilding. To determine the clearance between an insert bearing and the crankshaft journal, a small piece of Plasti-gauge is broken off the strip and placed across the journal surface. The bearing cap is then put into place and the bolts tightened to the correct torque. When the bearing cap and bearing are removed, the width of the Plasti-gauge is measured and the clearance is determined.
Volumes can be accurately measured by using many different devices and materials. For motorcycles, a simple baby bottle seems to serve most purposes such as measuring the amount of oil for the front forks or pre-mixing oil with gas for two-strokes. These bottles are available at many stores and are graduated in both cubic centimeters and ounces. Sometimes extreme accuracy is required, as in measuring the volume of a combustion chamber. These instances require a chemist's burette or a graduated cylinder.
If accurate measurement of the weight of an object is necessary, the proper measuring instrument would be either a shadowgraph or a balance scale. These scales measure weights down to 1/100th of a gram.
A quick look at various manufacturer's specifications will show that pressures are an important measurement for the motorcycle mechanic. A compression gauge is one of the most important tools available for assessing an engine's basic mechanical condition.