Motorcycle Precision Measurements - Part 2
A mechanic should consider the relationship of the size of fasteners to the parts they connect. Various sizes of fasteners, such as studs and bolts, are chosen for their ability to apply a certain force to hold parts in place. This force depends on both the size and quality of the bolts and nuts. The quality of many kinds of bolts is indicated on their heads by marks or "designs." A good mechanic comes to understand the relationship between the size of a bolt and its torque capacity.
Another indication of a mechanic's ability, perhaps an even more important one, is how well he or she is able to apply the principles of measurement. If you can use all of the basic measuring tools, you're way ahead of the game and have the ingredients that mark the difference between a successful mechanic and a shade tree butcher.
In "my day" really good motorcycles are designed both in inches and centimeters or millimeters (hundredths or thousandths of a meter). At this time, however, most motorcycles use the metric system rather than the English system of measurement. A common conversion problem is torque. If your torque wrench says inch-pounds or foot-pounds, but the book says kilograms and centimeters, what do you do? Conversion charts either printed out or online calculators will save you a lot of trouble and your bike a lot of damage.
One of the most useful tools in the mechanic's box is a 6-inch steel rule. A Metric-English version, available from the parts shop, stationery or hardware store, is indispensable. The graduations should be as fine as 1/32 of an inch or in millimeters. A rule that's somewhat flexible is even more useful.
The Vernier caliper can make both inside and outside measurements up to 6 inches with an accuracy of 1/1000th of an inch. This accuracy is accomplished by a principle Pierre Vernier, a French mathematician, invented. His system has the number of lines on the Vernier scale differing from the number of lines on the regular scale of the rule by one unit. For each thousandth of an inch that you open the slide, the next mark on the Vernier scale will align with a mark on the base scale: Simply add the Vernier scale number to the number indicated on the base scale at zero (0). This gives your precision measurement to the closest thousandth. The correct reading for the first setting between the jaws of the caliper would then be 1 inch and 402 thousandths (written as 1.402 inches or 1.402").
With a little practice you can develop good consistency and accuracy when using the Vernier caliper. A small magnifying glass helps to determine which number on the Vernier scale lines up with a number on the base scale.
Another measuring tool, the dial indicator with its many holding and positioning devices, as it is one of the easiest instruments for a mechanic to use and provides accuracy to 1/1000th of an inch. Its common uses in motorcycle mechanics include setting two-stroke timing, truing crankshafts and, occasionally, truing wheels.