1) Fault symptoms and what they mean
- Typical signs: deep “rod knock” at mid–low rpm that changes with load, sudden loss of oil pressure, metal filings in oil filter/sump, blue smoke from burning oil, seizure.
- Theory: the big-end rod bearing supports the crank journal on a pressurized oil film. When that film collapses (bearing wear, contamination, low oil pressure or bolt failure) you get metal-to-metal contact between crank journal and rod; that makes the knocking noise, generates debris, destroys clearance and oil pressure and rapidly worsens to crank/journal damage or seized piston.

2) Diagnosis and inspection (why each check matters)
- Oil-pressure test: confirms whether low oil pressure is primary or secondary. Low pressure points to pump/clearances; normal pressure with knock points to bearing/cap looseness or local damage.
- Visual inspection (sump/oil screen/filter): metal flakes/copper/bronze color on bearings show bearing failure source and direction of wear.
- Remove oil pan and inspect rod bearings in situ: scoring, embedded metal, overheated discoloration identify failed bearings vs crank damage.
- Measure crank journals and bearing bores: determine whether journals are within roundness/diameter spec (if out of spec, undersize bearings or crank regrind needed).
- Check rod straightness and cap alignment: bent or twisted rods cause eccentric loading and accelerated bearing wear.

3) Decide repair strategy (how the repair choice fixes the fault)
- Replace rod bearings only if journals and rods are sound: new bearing shells restore correct clearance and allow a stable hydrodynamic oil wedge, eliminating metal-to-metal contact and knock.
- Machine crank (undersize journals) plus matching undersize bearings if journals are scored beyond spec: restores cylindricity/diameter so bearings run on a smooth, properly sized journal and rebuilds correct clearance and oil film.
- Replace bent/cracked rods or recondition rod caps if alignment is off: restores correct bearing geometry and load distribution so bearings don’t fail prematurely.
- Replace rod bolts/nuts: bolts stretch/lose preload during failure; new fasteners restore correct clamp load so the oil film geometry and bearing retention are reliable.

4) Ordered repair procedure with theory (concise, in-order)
1. Safety & prep: disconnect battery, drain oil and coolant. Rationale: prevents fire/electrocution and clears oil for inspection.
2. Remove external items to access bottom and top of engine (intake/exhaust as needed, turbo piping if blocking). Rationale: gives full access to heads, pistons and oil pan.
3. Remove cylinder head(s) or at minimum free pistons for removal from the bore. Rationale: you must be able to push pistons out the top if rods are removed. Removing the head also lets you inspect piston crowns/top compression rings for secondary damage.
4. Remove oil pan and windage tray; remove oil pickup and screen; inspect for debris. Rationale: clears path to rod caps and allows inspection of metal debris source.
5. Mark rod/piston orientation and cylinder numbers. Rationale: rods and pistons are matched to their journals; orientation affects clearances and small-end bushings.
6. Rotate crank to bring each rod journal to a convenient position; unbolt and remove rod caps (work in order). Rationale: gives access to bearing shells and allows safe piston retention/removal.
7. Inspect bearings and journals visually; remove bearing shells and note color/pattern (scoring, embedded material, fatigue). Rationale: confirms cause and extent of failure.
8. Measure: use micrometer to measure crank journal diameters, bore gauge for journal roundness, plastigage or micrometer + bore readings for big-end clearance, and check rod big-end inside diameter. Rationale: determines whether bearings can be replaced or machining is required; correct clearance is essential for hydrodynamic lubrication.
9. If journals within limit and rods straight: fit new OEM-spec bearing shells (use correct upper/lower orientation), lubricate with assembly lube/engine oil, assemble caps and torque/fasten new bolts to factory torque or stretch spec. Rationale: replacing bearings and restoring clamp preload recreates the correct bearing geometry and oil film thickness so the hydrodynamic bearing can carry load.
10. If journals damaged: have crank re-ground to undersize and fit matched undersize bearings, or replace crank. Rationale: a smooth, round journal is required for a stable oil wedge; grinding removes damage and undersize bearings restore clearance.
11. If rods bent/cracked or caps misaligned: replace rods or have them line-bored/stud-honed and check cap fit. Rationale: correct alignment prevents eccentric loading and oil starvation at high-load areas.
12. Re-check clearances with plastigage or dimensional measurement after final assembly but before final torque check. Rationale: ensures the target oil clearance is achieved; incorrect clearance will fail again.
13. Reassemble oil pickup, pump (if removed), windage tray, oil pan; prime oil system before initial start (pre-oil the bearings via crank rotation with starter disabled or use pump priming tool). Rationale: prevents dry start—dry bearings cause immediate damage.
14. Start and run at idle while monitoring oil pressure and listening for abnormal noise; re-check torque on accessible fasteners after warm-up if procedure calls for it; change oil and filter after a short initial run and again at break-in interval. Rationale: early monitoring verifies the repair worked and removes any residual particles; fresh oil prevents contamination.

5) Measurement and tolerances — what matters (theory, not numbers)
- Key quantity: radial oil clearance between rod bearing inner surface and crank journal. That clearance establishes the hydrodynamic oil wedge. Too large → low oil pressure, vibration, noise; too small → metal contact and seizure.
- Bearing surface finish and roundness are critical: roughness or taper disrupts the pressure distribution in the oil film and concentrates loads.
- Rod bolt clamp force: bearings rely on correct cap preload to keep shells accurately seated and maintain clearance under load. Bolts must meet torque/stretch spec.
- Oil supply (pickup, pump condition, screen) must be good — bearings depend on sufficient flow and pressure to generate the film.

6) How the repair actually cures the fault (mechanics)
- Replacing worn bearings (and restoring journal geometry) re‑establishes the hydrodynamic lubrication regime: at running speed oil is drawn into the converging gap between journal and bearing creating a pressure field that supports the load and separates metal surfaces. That eliminates knocking, removes high local temperatures, and stops metal debris generation.
- Correct rod alignment and bolt preload ensure even load distribution and that the oil film thickness is stable across the bearing width, preventing eccentric wear.
- Repairing the oiling system and removing contamination prevents re‑abrasion of new bearing surfaces and preserves pressure/flow required for the wedge.

7) Final checks and break-in
- Initial run: monitor oil pressure and listen; inspect for leaks and check for metal in the oil filter after first run.
- Change oil/filter after break-in interval to remove any machining/assembly debris.
- Recheck oil pressure and road-test under varying load; if noise persists, stop and re-inspect.

Concise cautions
- Use correct replacement parts and adhere to factory specs for clearances and fastener torque/stretch.
- Never reuse rod bearings or rod bolts if the manual calls for replacement after removal.
- If journals are scored beyond repair, continuing without machining or replacement will rapidly fail the new bearings.

This sequence and the underlying measurements restore bearing geometry, oil film formation and rod/crank alignment — the physical conditions that stop rod knock, restore oil pressure, and prevent recurrence.
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