6.2 Crankshaft & Main/Rod Bearing Service

6.2 Crankshaft & Main/Rod Bearing Service

The crankshaft converts the reciprocating linear motion of the pistons into rotational torque delivered to the drivetrain. Operating under violent combustion impact loads, the crankshaft relies on a continuous film of pressurized engine oil between its journals and precision sleeve bearings (main and connecting rod bearings). Servicing the crankshaft and lower block assembly requires exact dimensional metrology, bearing oil clearance verification, housing alignment checks, and thrust end play adjustment.

---

Crankshaft Visual Inspection & Dimensional Metrology

Crankshaft failure or excessive journal wear leads to immediate loss of oil pressure and catastrophic engine destruction. Inspection begins with a thorough visual check and precision micrometer measurements.

Visual Defect Analysis

• Scoring & Grooving: Deep ridges along journal surfaces caused by contaminated oil or metal debris.
• Heat Discoloration (Blueing): Dark blue or purple discoloration on journals caused by localized friction temperatures exceeding 600°F (315°C) during severe oil starvation. Blued crankshafts lose heat treatment hardness and must be magnetic-particle tested for fatigue cracks or scrapped.
• Stress Cracks: Micro-cracks originating at journal oil holes or shoulder fillet radii. Cast iron and forged steel crankshafts must undergo Magnetic Particle Inspection (Magnaflux).

Journal Measurement (Out-of-Round & Taper)

Using an outside micrometer readable to 0.0001 in., measure each main journal and connecting rod journal at four points:

1. Measure diameter across Position A (Thrust axis).
2. Measure diameter across Position B (90° to Thrust axis).
3. Repeat measurements at both the front edge and rear edge of each journal.

$ext{Journal Out-of-Round} = | ext{Position A Diameter} - ext{Position B Diameter}|$ $ext{Journal Taper} = | ext{Front Edge Diameter} - ext{Rear Edge Diameter}|$

Service Limit: Journal out-of-round and taper should typically not exceed 0.0002 in. to 0.0005 in. (0.005 mm to 0.013 mm). Readings exceeding these limits require crankshaft grinding.

---

Measuring Crankshaft Straightness (Runout)

A bent crankshaft causes binding, severe main bearing wear, block saddle distortion, and oil seal leakage.

Runout Test Procedure

1. Support the crankshaft at its front (#1) and rear main journals on precision V-blocks (or in the block saddles supported only by the end bearing shells).
2. Mount a Dial Indicator (DTI) on a magnetic base with the indicator plunger perpendicular to the center main journal.
3. Zero the dial indicator and rotate the crankshaft slowly through 360°.
4. Record the total movement of the indicator pointer. This is the Total Indicator Reading (TIR).

ext{Crankshaft Bentness (Runout)} = rac{ ext{Total Indicator Reading (TIR)}}{2}

Specification: Maximum allowable crankshaft runout is typically 0.001 in. to 0.002 in. (0.025 mm to 0.050 mm). If TIR is 0.003 in., the actual shaft runout is 0.0015 in. Shafts exceeding limits must be straightened using a hydraulic press or replaced.

---

Crankshaft Machining, Undersize Bearings, & Fillet Radius

When journals are worn, scored, or out-of-round, the crankshaft is reground on a precision crankshaft grinding machine to standard undersizes: -0.010 in., -0.020 in., or -0.030 in. (0.25 mm, 0.50 mm, 0.75 mm). Matches of undersize bearing inserts are installed during reassembly.

Fillet Radius Maintenance

The fillet radius is the smooth curved transition between the flat journal surface and the vertical counterweight cheek.

• CRITICAL REQUIREMENT: When grinding journals, the grinding wheel must be dressed to match the exact original factory fillet radius.
• Consequence of Incorrect Grinding: If a machinist grinds a sharp 90° square corner at the journal shoulder, it creates an extreme stress riser (stress concentration point). Flexing forces under load will concentrate at this sharp corner, causing the crankshaft to snap clean in half during operation.
• Oil Hole Chamfering: After grinding, journal oil holes must be chamfered (deburred) and polished to prevent sharp edges from scraping bearing material.

       CRANKSHAFT FILLET RADIUS GEOMETRY
  Correct Fillet Radius             Incorrect Sharp Corner
     (Distributes Load)               (Creates Stress Riser!)
   |                             |
   |                             |
   \____ Journal Surface         |_____ Journal Surface
    (Smooth Curved Radius)        (Sharp 90° Square Corner - FAILS!)

---

Main Bearing Saddle Alignment & Line Honing

The main bearing housing bores (saddles) in the engine block must be perfectly aligned along a single axis and machined to precise diameter tolerances.

Causes of Housing Bore Distortion

• Severe engine overheating or thermal warping.
• High-RPM block flex and heavy detonation.
• Over-tightening or stretching main cap bolts.
• Installing aftermarket main studs or conversion to 4-bolt main caps.

Alignment Inspection & Line Honing

1. Remove bearing inserts. Install main caps and torque cap bolts to specification.
2. Place a precision straightedge along the bottom of all main bearing saddles.
3. Attempt to pass a feeler gauge between the straightedge and saddle surfaces, or measure saddle diameters using a dial bore gauge.
4. If saddles are out-of-line by more than 0.0015 in. (0.038 mm), the block requires line boring or line honing.

• Line Honing Process: A small amount of material is ground off the mating faces of the main bearing caps (lowering the caps). The caps are torqued onto the block, and a long rigid line honing mandrel is passed through all main saddles, honing them back to true circular alignment and standard diameter.

---

Bearing Oil Clearance Measurement with Plastigage

Precision sleeve bearings require a specific oil film clearance. Excessive clearance causes low oil pressure and metallic knock; insufficient clearance causes oil starvation, bearing wipe, and journal seizure.

Plastigage Measurement Procedure

1. Clean all main journals, main cap saddles, and bearing insert shells.
2. Install bearing shells into the block and caps. Ensure bearing shells and journals are completely clean and DRY. Do NOT apply oil to Plastigage.
3. Carefully lower the crankshaft into the block saddles.
4. Place a strip of calibrated Plastigage (flexible plastic thread) across the width of each journal, approximately 1/4 in. off-center.
5. Install main caps and torque bolts to factory specification in the correct sequence.
6. DO NOT ROTATE THE CRANKSHAFT. Rotating the shaft will smear the Plastigage strip and destroy the reading.
7. Remove the main caps. The Plastigage strip will be flattened.
8. Compare the width of the flattened Plastigage strip against the graduated scale printed on the Plastigage envelope to read clearance directly in thousandths of an inch or millimeters.

Clearance Specifications & Diagnostic Effects

• Standard Bearing Oil Clearance: Typically 0.0010 in. to 0.0025 in. (0.025 mm to 0.063 mm) — roughly 0.001 in. clearance per inch of journal diameter.
• Excessive Clearance Effects: Oil flows out the sides of the bearings too rapidly (oil throw-off). This causes low engine oil pressure at hot idle, hydraulic lifter noise, lower block thudding/knocking, and over-saturation of cylinder walls with oil throw-off.
• Insufficient Clearance Effects: Prevents formation of a hydrodynamic oil film, causing localized overheating, bearing metal transfer (wiping), and crankshaft lockup/seizure.

---

Thrust Bearing & Crankshaft End Play

The thrust bearing (equipped with vertical flange faces on the sides of one main bearing or separate thrust washers) controls fore-and-aft (axial) movement of the crankshaft.

Measuring Crankshaft End Play

1. Mount a Dial Indicator on the front of the block with its plunger touching the front snout of the crankshaft or flywheel flange.
2. Using a pry bar, gently pry the crankshaft completely backward, then zero the dial indicator.
3. Pry the crankshaft forward and record the total reading on the dial indicator.
4. Alternatively, insert a feeler gauge between the thrust bearing flange and the machined crank counterweight cheek.

End Play Specifications & Wear Causes

• Standard End Play: Typically 0.003 in. to 0.007 in. (0.075 mm to 0.18 mm).
• Causes of Excessive End Play:
  • In manual transmission vehicles: Constant pressure from heavy clutch pressure plates riding against the thrust bearing when the clutch pedal is depressed.
  • In automatic transmission vehicles: Ballooning torque converters pushing forward against the crankshaft flexplate.
  • Operating with contaminated engine oil or missing thrust washer inserts during previous rebuilds.