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Understanding Main and Rod Bearings in Automotive Engineering

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Learn about measuring bearing clearance with Plastigauge, bearing properties, inserts, spread, crush, locating lug, oil groove, undersizes, inspection, excessive loads on bearings, and thrust bearing wear.

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Understanding Main and Rod Bearings in Automotive Engineering

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  1. CHAPTER 11 Main and Rod Bearings • Page 271 Classroom Manual • Page 273 Lab Manual

  2. BEARINGS • Check bearing clearance with Plastigauge. • Cut Plastigauge full width of journal, 1/4 off center. • Green Plastigauge range is .001” to .003” • Red Plastigauge range is .003” to .006”

  3. MEASURING CLEARANCE • Doubling oil clearance will through five times as much oil onto the cylinder wall. • This increases oil consumption. • Excessive oil clearance will cause low oil pressure at idle.

  4. BEARING PROPERTIES • Bearing surface has four properties. • Surface Action • Embedability • Conformability • Fatigue Resistance Page 272

  5. BEARING INSERTS • Bearing shells are a steel back. • Bearings have babbit, copper-lead, or aluminum alloy. • Some bearing have multilayered lining material. Page 272

  6. BEARING SPREAD, CRUSH • Bearings have a property known as spread. • Bearings have a property known as crush. Page 272

  7. LOCATING LUG, OIL GROOVE • Bearings are positioned in the bore by a tang (most), or a dowel (some). • Main bearings with a hole are upper bearings. • Some upper bearings are also grooved. Page 272

  8. BEARING UNDERSIZES • Are marked on the back of the bearing. • For some engines a .001” is available • .010” • .020” • .030” Page 275

  9. BEARING INSPECTION • Keep bearings in order during disassembly. • Main cause of short bearing life is DIRT. • Check for metallic particles on bearing surface.

  10. EXCESSIVE LOADS ON BEARINGS • Pressure from the flame front against the piston. • Centrifugal force from rotating weight of the rod. • Inertia from the up-and-down motion of the piston & rod. • These loads increase with RPM.

  11. EXCESSIVE LOADS ON BEARINGS • Upper rod bearings have the most load. • Lower main bearings have the most load. • Using high viscosity oil can fatigue bearings. • Dry starts. • Excessive drive belt tension.

  12. THRUST BEARING WEAR • Improper clutch adjustment, no free play. • Riding the clutch, improper driving habits. • Damaged pilot hole in crankshaft.

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