6.5 Crankshaft Inspection and Bearing Wear Evidence

Crankshaft Inspection, Bearing Failure Analysis, Select-Fit & Clearances

In a medium- or heavy-duty gasoline truck engine, the crankshaft and its hydrodynamic plain bearings must support tens of thousands of pounds of combustion force without metal-to-metal contact, supported by a hydrodynamic film generated by journal motion, with pressurized supply replenishing the oil; diametral clearance is not the same as loaded film thickness. Rebuilding a durable lower end requires meticulous crankshaft journal metrology, forensic diagnosis of old bearing wear patterns, and exact control of bearing crush, oil clearance, and thrust endplay.

Crankshaft Inspection & Journal Metrology (Task C.6)

Cleaning Oil Passages, Crack Inspection & Fillet Radii

1. Drilled Oil Passage Brushing: During engine operation, centrifugal force inside the spinning crankshaft throws heavy metallic particles and carbon sludge outward against the walls of the diagonal oil passages drilled between the main journals and rod journals. Chemical washing softens this packed centrifugal sludge deposit; if a technician fails to run a stiff nylon rifle brush with solvent through every crankshaft oil hole, that loosened plug of grit flushes straight into the new connecting rod bearings on the very first startup.
2. Crack & Surface Inspection: Inspect all main and connecting rod journals for deep circumferential grooves, ridging, babbitt transfer, or thermal bluing from oil starvation. Perform wet fluorescent magnetic particle inspection (Magnaflux) across the entire crankshaft, paying close attention to the oil hole edges and the transition zones between journals and counterweights.
3. Journal Fillet Radii: Where each horizontal main or rod journal meets the vertical crankshaft cheek/counterweight, the transition is machined with a precision curved fillet radius (often deep-rolled or induction-hardened on heavy-duty truck crankshafts) to distribute bending and torsional loads. If a crankshaft grinder dresses the grinding wheel with too sharp a corner, a severe stress riser is created and the crankshaft will snap across the web under heavy towing loads; conversely, if the fillet radius is left too large, the outer edges of the bearing shell ride up on the radius (fillet ride), pinching off oil flow and wiping the edges of the bearing.

Four-Point Crankshaft Journal Measurement

Using an outside micrometer calibrated to 0.0001 in., measure every main journal and connecting rod journal at four points: vertically (Axis A, parallel to crank throw) and horizontally (Axis B, 90° to crank throw) near both the front/left edge and the rear/right edge of the journal:

• Journal Out-of-Round = |Vertical Diameter (A) - Horizontal Diameter (B)| at the same side of the journal (typical service limit: 0.0005 to 0.0010 in. / 0.013 to 0.025 mm). Rod journals wear out-of-round faster than main journals because peak combustion load always hammers the top of the rod journal at the exact same rotational angle.
• Journal Taper = |Front/Left Diameter - Rear/Right Diameter| along the same axis (typical service limit: 0.0005 to 0.0010 in.).

Piston, Rod-Bearing & Main-Bearing Wear Pattern Failure Analysis (Task C.7)

Never discard old main and rod bearing shells without reading their wear patterns first. The location of babbitt overlay wear pinpoints hidden geometric distortion in the connecting rods, crankshaft, or block main tunnel:

Applied Diagnostic Check

Keep the main caps in their original locations and orientation during housing checks. The block and caps were machined as an assembly; a cap that bolts down can still create an incorrect bore. Record any fretting at the mating surfaces and verify the specified bolt condition before torque. Where the bearing pattern suggests a distorted bore, measure with the inserts removed and the cap assembled according to the procedure. Journal dimensions and housing dimensions are complementary evidence. Neither an attractive wear pattern nor a selected bearing color proves that the assembled support line and clearance are correct.

Accepting the Repaired Journal

Check the finished journal at multiple positions and verify surface finish, oil-hole edges and fillet geometry. A correct nominal diameter does not prove the journal is round, parallel or compatible with the selected shell. The repair record should identify the final undersize, bearing part and actual installed clearance. Reinspect any unusual contact pattern during trial assembly rather than relying on a machining-size label alone.

Making Journal Measurements Repeatable

Clean the journal and micrometer faces, verify instrument zero and allow the part to reach the required measuring temperature. Measure away from an oil hole or obvious damage, then record the same axial positions and rotational axes on each journal. A single measurement cannot show taper or ovality. For example, two diameters of 2.1000 and 2.0994 inches at the same axial position differ by 0.0006 inch; the decision still depends on the supplied journal tolerance and allowed repair.

Wear patterns narrow the investigation rather than substituting for geometry. Check the rod big-end bore with the cap matched and torqued as specified, and inspect its small-end bushing and pin condition. Check the main housing line if several bearings show related edge loading. Do not use a thicker shell to hide a distorted housing. A ground journal also needs the required surface finish and fillet geometry, with a bearing that clears the fillet. Record the final dimensions and selected repair size so the clearance check can be interpreted correctly.