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Motorcycle Precision Measurements - Part 4

Updated on November 30, 2009

Normal procedure is to loosen the spark plugs one turn, then spin the engine over to blowout carbon at the inner surface of the plug before any compression reading is taken. After the dirt is blown away from the outside, the spark plug can be removed from the cylinder head. The correct adapter is chosen to make sure the gauge fits properly in the spark plug threads. With the compression gauge in place, the engine is then kicked through at least five times and a pressure reading is taken. This procedure is accomplished with the choke and throttle fully opened, and with the engine at normal operating temperature.

In multi-cylinder engines this procedure is followed for each of the cylinders, making sure that the spark plugs are removed from the cylinders initially so that correct readings are obtained. After the first compression gauge readings are taken, a small amount of oil (approximately a teaspoonful) should be placed in each cylinder, then the engine turned through several times. New compression readings should be taken for each cylinder and the difference from the original readings if any, noted. If the oil increases compression pressure above 25 psi, we may assume that the rings are worn and need to be replaced.

If a four-stroke engine's compression (both wet with oil and dry) is much lower than shop manual specifications, the mechanic should look at the valves. Low compression pressures might also be caused by blown head gaskets, damaged pistons or cylinder walls, or stuck or broken rings.

The most frequently used pressure gauge should be the tire gauge. Tire pressure plays a very important role in handling, tire wear, and overall motorcycle performance. Gage pressures are given in pounds per square inch (psi) or kilograms per square centimeter (kg/cm2). The conversion factor is: 14.25 psi = 1.0 kg/cm2. Tire pressure should be checked daily and increased for heavy load or high-speed driving conditions.

The valve-spring tension gauge is used to measure the force on this spring. The required condition of a spring may be stated in different ways. The specifications may require a certain length of free height, or the resistance of a spring at a stated height may be required. Valve springs are often measured for resistance at both installed and open heights. An example is the specification of 80 pounds seat pressure required to compress the valve spring to a 1.50-inch installed height.

The measurements of rotation, angles, and arcs are shown in degrees of a circle, one full rotation equalling 360°. In a motorcycle engine the crankshaft is indexed at "zero" when the piston reaches the top of its travel. The amount of turn in one direction or another is given in degrees. The position of the crankshaft at the precise time of ignition is often stated in "degrees before top dead center." This is called "timing advance." Determining the degree of advance is accomplished by measuring the difference between index marks on the rotating crankshaft or attached flywheel and a stationary mark on the engine case. Degree scales are commonly used to set engine ignition timing, camshaft timing, and port timing on two-stroke engines. Degree scales are also found on machinery such as lathes, and valve grinders, where they are used to indicate the angles at which various parts should be machined.

Continued In Motorcycle Precision Measurements - Part 5

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