SOS Mechanic Certificate – Engine Repair Practice Exam

Using plastigage is a common method to measure the clearance between a bearing insert and the journal of a crankshaft. This tool consists of a thin strip of plastic that is compressed between the bearing surface and the journal when the engine is assembled and the bolts are tightened. After disassembly, the width of the flattened plastigage strip indicates the clearance between the two surfaces. The correct choice, pertaining to the journal taper, stems from the fact that if the journal is not perfectly cylindrical (which includes being tapered), the plastigage will show uneven wear along its length after being compressed. A tapered journal can result in inconsistent clearance measurements when the plastigage is used, illustrating that the journal does not maintain a uniform distance from the bearing throughout its entire circumference. By observing how the plastigage has spread, mechanics can determine if there is a taper issue that needs to be addressed to ensure proper engine function and longevity. The other choices relate to characteristics that plastigage does not measure directly. It does not indicate the presence of debris in the oil (that requires a visual inspection), it cannot determine oil pressure, and while it can provide some insight into wear through clearance measurement, it does not specifically show a wear pattern on the bearing itself.