1) Purpose and symptom → theory in one line
- Function: connecting rods transfer combustion loads from pistons to the crankshaft and keep piston/rod geometry correct.
- Typical faults: rod knock, low oil pressure, metal debris, seized rods or broken rod bolts, bent rods. These come from excessive bearing clearance, lack of oil, bearing fatigue or impact. Repair restores correct clearances, alignment and preload so hydrodynamic lubrication is re‑established and loads are carried without metal‑to‑metal contact.

2) Preparation and safety (why it matters)
- Drain oil/coolant, remove ancillaries and cylinder head or separate block as required so you can access pistons/rods.
- Theory: internal contamination or debris during work will accelerate bearing failure; cleanliness and degreasing preserve bearing surfaces and oil film formation.

3) Marking and orientation (order first)
- Mark piston-to-rod and cap orientation and journal indexing before removal. Rod caps must return to their original rod and orientation.
- Theory: rod design and chamfers set bearing crush and oil clearance; swapping or rotating caps changes geometry and can create step or misalignment that breaks the hydrodynamic oil wedge.

4) Cylinder positioning and piston pin removal (if needed)
- Rotate engine to bring piston to service position (typically near bottom or top as specified). Remove circlips and wrist pin if piston removal is required.
- Theory: freeing piston ensures no undesired loads on the rod or pin during cap removal and prevents scoring.

5) Remove rod caps (ordered sequence)
- Loosen rod bolts/nuts in an even sequence, remove caps, keep bolts and caps with their rods. Inspect for torque‑to‑yield bolts — replace if specified as one‑time use.
- Theory: rod bolts are critical preload members. Torque‑to‑yield bolts plastically stretch to set clamp; reusing them reduces preload and leads to loosening and fatigue.

6) Inspect rod, cap and crank journal (what to measure and why)
- Visually check for discoloration, scoring, pitting, or carbon. Measure big‑end journal diameter with a micrometer and check roundness; check rod big‑end bore for ovality with calipers or bore gauge. Measure crankshaft journal taper and runout with dial gauge.
- Theory: bearings carry a pressurized oil film; wear changes geometry (journal diameter, roundness) and thus changes oil wedge thickness and pressure distribution. If journals are out of spec the bearings will see edge loading and fail.

7) Inspect and measure bearings (clearance check)
- Remove old shells, check for embedded debris, scoring, and measure bearing shell thickness. Use plastigauge or micrometer measurements to determine bearing clearance between journal and bearing shell. Compare to OEM spec. Typical rod bearing clearance for many diesels is in the 0.02–0.05 mm range but use the manual.
- Theory: correct clearance provides an oil film of the right thickness. Too large → low oil pressure, knocking and metal contact; too small → high friction, seizure.

8) Decide repair path (replace vs regrind)
- If crank journals are within limits, replacing shells with correct undersize inserts may suffice. If journals are scored/tapered beyond spec, crankshaft regrind or replacement is required. Bent rods must be replaced. Rod bolts often replaced.
- Theory: replacing only bearings when the journal is badly damaged will reintroduce immediate failure because the underlying geometry that supports the oil film is compromised.

9) Parts and tolerances (what to obtain)
- Obtain OEM rod shells, new rod bolts if required, and pistons/rods if bent. Get the workshop manual torque specs, bearing selection chart (match shell crush and clearance), and crankshaft undersize chart.
- Theory: manufacturers specify shell thickness and bolt preload to create predictable clearances and bolt stretch for fatigue life. Using correct parts restores designed oil wedge and clamping.

10) Assembly measurements before final tightening
- Install new shells dry to check fit, then use plastigauge across joint and torque cap to specified initial torque or sequence to measure actual clearance. Replace shell size if clearance out of spec.
- Theory: plastigauge simulates the oil film thickness and lets you confirm the bearing crush/clearance that will exist when running. Final preload of bolts determines cap seating and thus bearing crush.

11) Lubrication and final torque procedure
- Before final assembly, coat bearings liberally with assembly lube. Tighten rod bolts in the exact sequence and to the prescribed torque or torque+angle if torque‑to‑yield. Replace bolts that are one‑time stretch. Recheck torque where required.
- Theory: assembly lube preserves the oil wedge on first start; correct bolt preload keeps bearing caps aligned and prevents relative motion which causes fretting and fatigue.

12) Check axial/crank endplay and rod side clearance
- Measure crankshaft endplay and rod side clearance if the engine design requires. Ensure rod small‑end and big‑end clearances are within limits.
- Theory: axial movement beyond spec can lead to oil clearance changes and bearing overload.

13) Reassembly, prime oil system, and break‑in
- Reassemble remaining components, prime the oiling system (turning with starter disabled or using an oil pump priming tool) so oil pressure is present before first start. Run engine at low load for initial break‑in, then bring to normal operation. Recheck oil pressure and listen for noises.
- Theory: priming prevents initial dry start-up scoring; controlled break‑in lets the bearing conforms and bedding establish a stable oil film.

14) How the repair fixes the fault (concise)
- Replacing worn/damaged bearings and restoring journal geometry returns the designed radial clearance, which restores hydrodynamic lubrication — oil is dragged into the wedge between journal and bearing, generating a pressure cushion that carries combustion loads without metal contact. Correct bolt preload and cap alignment maintain that geometry under load, preventing motion and impact that cause knocking and debris. If the crankshaft or rod was damaged, correcting those removes misalignment that focused loads and caused premature bearing failure.

15) Final verification and warning
- Verify oil pressure and absence of knock, check for metal in the oil after first hours. If abnormal noise or low pressure persists, the crankshaft or block may be beyond repair and needs machining or replacement. Always follow OEM torque and bearing selection data. Improper tightening, incorrect clearances, or contaminated assembly will cause rapid re‑failure.

Tools/measurements to have: torque wrench (angle gauge if required), micrometer, bore gauge, dial indicator, plastigauge, feeler gauges, straight edge, clean solvent and lint‑free wipes, assembly lube, parts manual.

End.
rteeqp73

Надежный или неудачный? Разбираем все проблемы дизел... Двигатель F9Q, появившийся в 1997 году на Renault Megane, стал первым французским дизелем с непосредственным ...

Разборка Mitsubishi - Space Star, 1.9 (VIN: XMCLNDG4A2F033203) 36938 Разобран Mitsubishi - Space Star Основные характеристики: Год выпуска: 2001 Двигатель: F9Q1D4192T4C042684 Объём ...

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