Cylinder Block, Crankshaft, and Bearings Diagnosis and Repair
What the candidate must know
This domain covers the block, cylinder bores, pistons, rings, crankshaft, main and rod bearings, and thrust bearing. Know how to measure cylinder taper, out-of-round, and ridge; how to check crankshaft journal size, taper, and out-of-round; how to read bearing wear patterns (fatigue, scoring, wiping, misassembly); and how to verify bearing clearance with plastigauge or micrometers. Understand thrust bearing end play, main bore alignment, and the role of oil clearance in maintaining oil pressure.
Traps in questions
- Bearing wear pattern questions are common. A bearing worn at the parting line suggests a bent or twisted rod or excessive clearance; a bearing worn in the center suggests a bent crank or misalignment. Upper rod bearing wear is normal (load side); lower rod bearing wear suggests excessive clearance.
- Thrust bearing failure shows as excessive crankshaft end play and can cause converter or clutch damage. Do not blame the transmission first.
- Taper and out-of-round are measured at the top, middle, and bottom of the bore. A ridge at the top means the ring travel has worn the bore; questions often confuse ridge with taper.
- Plastigauge reads clearance only if the journal and bearing are clean and the cap is torqued to spec - questions sometimes describe checking clearance with the wrong torque.
- Oil pressure low at hot idle can be excessive bearing clearance, a worn oil pump, or a stuck relief valve. Bearing clearance is confirmed by measurement, not by guessing.
Worked example
A rod bearing shows wear concentrated at the parting line on one side. That pattern points to a bent or twisted connecting rod, not simply a worn journal. The rod should be checked for straightness and the journal measured for taper before reassembly.
Rule of thumb
Measure, don't guess: cylinder taper over about 0.005 in, or out-of-round over about 0.003 in, generally calls for boring or replacement. Always verify journal size and clearance with a micrometer and plastigauge.