Introduction
The determination of cylinder taper (and out-of-roundness requires collecting 6 measurements--A1, A2, B1, B2, C1 & C2. See Figure 1 Six measurements.
Figure 1 Six measurements
The measurements must be orientated with the crankshaft, A1, B1 & C1 should be parallel with the crankshaft and A2, B2 & C2 are perpendicular to the crankshaft. The measurements must be taken at the top, bottom and middle of the ring travel, not the top, middle and bottom of the cylinder.
Different
combinations of these six numbers are used, depending on if out-or-roundness or
wear is being determined.
Taper/wear
Taper (wear) is determined by averaging the two measurements at each level and comparing the averages with the standard bore. If the difference between the averages and the standard bore is greater than the allowable wear, the cylinder should be resized.
Taper is caused by wear. The taper may occur in either the top half or bottom half of the cylinder, or along the entire length of the ring travel in the cylinder. Taper in the top half is cause by dirt ingestion through the air filter. Taper in the bottom half is caused by dirty oil.
Out-of-Roundness
Out-of-roundness is caused by unequal pressure on the piston.
Figure 2 Measuring for out of round
The maximum cylinder pressure occurs when the piston is past TDC. At this point the connecting rod is at a slight angle. This causes a side force on the piston, which causes more wear on the cylinder wall perpendicular to the crankshaft. This wear only occurs at the top of the ring travel cylinder.
Out-of-roundness is determined by subtracting each pair of numbers at the top, middle and bottom of the ring travel. If any of the three answers exceeds the allowable wear, the cylinder should be resized.
Sample Problem
The following measurements were collected for an engine. Determine the amount of taper and out-of-roundness for the cylinder. The standard bore is 2.187 inches and the allowable wear and out-of-roundness is 0.004 inches.
|
A1 |
2.190 |
|
A2 |
2.187 |
|
B1 |
2.187 |
|
B2 |
2.187 |
|
C1 |
2.189 |
|
C2 |
2.187 |
Is this cylinder acceptable? Explain. If not, what should be done?
