Measuring Critical Components NOTE: Critical engine specifications charts can be found at the beginning of this section.
CYLINDER HEAD VOLUME (Squish-Gap Method) To check the squish gap, a micrometer and a heavy piece of solder will be needed. 1. Remove the spark plugs from the engine. 2. Insert the solder piece down through the spark plug hole and push it up against the inner cylinder bore towards the MAG-side of the engine.
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2. The trueness (out-of-roundness) is the difference between the highest and lowest reading. Maximum trueness (out-of-roundness) must not exceed specifications.
3. Pull the recoil rope and crank the engine over several times while the solder is being held firmly in place. 4. Remove the solder from the cylinder. Using the micrometer, measure the very end of the squeezed solder piece. Record the reading. NOTE: If the solder hasn’t been squeezed by the piston, a larger piece of solder must be used. Repeat procedure.
5. Using the opposite end of the solder piece, insert it down through the spark plug hole towards the PTO-side of the engine. Push on the solder until it contacts the inner cylinder bore. 6. Pull the recoil rope and crank the engine over several times. Remove the solder from the cylinder and measure the squeezed end with a micrometer. Record reading and refer to the chart. NOTE: Measure from PTO to MAG-side of the piston to accurately measure the squish gap. Never measure across piston, exhaust to carburetor side, as the piston will rock and the reading won’t be accurate.
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PISTON SKIRT/CYLINDER CLEARANCE 1. Measure each cylinder front to back about 2.5 cm (1 in.) from the bottom of each cylinder. 2. Measure the corresponding piston skirt diameter at a point 1 cm (0.394 in.) above the piston skirt at a right angle to the piston-pin bore. Subtract this measurement from the measurement in step 1. The difference (clearance) must be within specifications.
Readings may vary from side to side. Make sure the smaller reading is within the specifications listed. CYLINDER TRUENESS 1. Measure each cylinder in the three locations from front to back and side to side for a total of six readings.
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