Main and Rod Bearings

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.