Below is a concise, ordered explanation of what it means to “do the crankshaft” on a Toyota HiAce (diagnose → remove → inspect/repair/replace → reassemble), with the engineering theory behind each action and how each repair step fixes the underlying faults. Consult the factory service manual for engine-specific torque values, clearances and any special procedures for your year/model.

1) Purpose and theory — why the crankshaft matters
- Role: converts piston reciprocation to rotation; journals carry bearings and transmit loads to the block; fillets and counterweights control fatigue and balance.
- Lubrication: bearings operate on a hydrodynamic oil film (wedge) — proper journal roundness and oil clearance create and maintain that film. If clearance, roundness, or oil supply is wrong, mixed/boundary lubrication occurs, causing metal contact, scoring, overheating and bearing failure.
- Common failure modes: bearing wear (excess clearance), scored journals, worn thrust surfaces (endplay), crankshaft cracks/twists, imbalance, oil gallery blockage, and metal contamination from other failures.

2) Symptoms that lead you to crankshaft work (diagnosis)
- Persistent rod/main knock that doesn’t change with oil or filter.
- Low oil pressure with metal in oil filter or visible metal in oil.
- Excessive crank endplay or lateral movement.
- Severe vibration or banging that indicates imbalance or broken counterweight.
- Visual evidence (magnet collects shavings) or borescope inspection reveals journal damage.
How these diagnose theory: Knocking + metal = boundary lubrication/metal contact; low oil pressure + metal = bearing clearance changed or bearing material worn away; endplay/vibration = thrust or balance faults.

3) Preparation and safety
- Disconnect battery, drain oil and coolant.
- Remove intake/exhaust and ancillaries blocking removal of timing cover, oil pan and accessories.
- Either remove the engine from vehicle (recommended for thorough inspection/repair) or support on an engine stand with the block accessible.
- Tools needed (typical): engine hoist, engine stand, torque wrench, micrometer (0.001 mm/0.0001 in resolution), dial bore gauge, dial indicator, plastigauge, V-blocks, bearing puller/installer, crankshaft oil seal installer, assembly lube, clean solvent, threadlocker, straightedge.

4) Remove lower-end components in order (logical sequence)
- Remove oil pan and pick up screen/strainer; inspect for metal debris (collect and note size/amount).
- Remove oil pump and pickup (inspect and prime later).
- Mark timing marks and remove timing cover, timing belt/chain and sprockets/gears (maintain crank/cam alignment).
- Remove pistons/connecting rod caps in firing order, keep caps matched to rods and orientation marks.
- Remove main bearing caps in sequence, keeping caps matched to their positions and orientation marks.
- Lift crankshaft free from block using an appropriate sling/engine stand support.

Theory: Removing ancillary systems and pistons lets you access the journals and bearings. Matching caps ensures reassembly maintains original journal alignments and reference surfaces.

5) Inspection and measurement (decide repair or replace)
- Clean crankshaft thoroughly.
- Visual checks: look for scoring, scratches, discoloration (heat), corrosion pitting, cracks at fillets, broken counterweights, keyway damage.
- Measure journal diameters with micrometer at multiple axial positions and radial orientations. Measure main bore diameters with dial bore gauge in block.
- Check journal roundness (diameter variation) and taper. Measure runout (place on V-blocks and use dial indicator to turn crank).
- Check crank thrust face flatness and width; measure endplay with dial indicator with caps torqued.
- Measure oil-clearance: either calculate from journal/bores values or use plastigauge with installed bearings.
- Magnetic particle or dye-penetrant crack test if suspect crack.
How this maps to theory: roundness/taper/runout determine whether hydrodynamic oil wedge will be stable. Scoring/corrosion reduces surface finish and destroys film. Endplay affects gear/shaft axial location and can damage seals/timing.

6) Repair options and theory of how each fixes faults
- Polishing: for light scoring/marring. Restores a smooth surface to help re-form oil film. Works only for shallow damage.
- Grinding/undercutting to undersize journals + fit undersize bearings: removes damaged metal and restores correct geometry; requires matched undersize bearings to restore proper oil clearance. Fixes excessive wear/scoring but requires precise machining.
- Welding (build-up) and regrind (rare, specialist): used for larger damage; requires post-weld heat treatment and careful machining; restores geometry but is less common for high-stress crankshafts.
- Replace crankshaft: when out-of-spec, twisted, cracked, or beyond economical repair.
- Replace bearings, seals, timing components, oil pump: worn bearings cause the original fault; replacing them with correct-size shells restores oil clearance and reestablishes the hydrodynamic film; new seals prevent leakage and contamination; new oil pump ensures adequate pressure/flow.
How it fixes the fault (summary): restoring journal concentricity, roundness and correct clearance recreates the oil wedge that keeps metal surfaces separated, reduces friction/heat, stops metal contact and noise, and restores oil pressure and longevity.

7) Machine-shop work (if applicable)
- Regrind journals to a standard undersize; machining must maintain straightness and correct taper.
- Dynamic balancing after machining if weight loss/redistribution occurs.
- Final polish to specified surface finish.
Theory: machining to a precise cylindrical geometry ensures predictable oil film thickness. Balancing reduces vibration and bearing loads.

8) Reassembly in order (with theory for each action)
- Clean block and oil passages thoroughly; remove all debris. Failure to clean causes immediate re-failure by debris embedding in bearings.
- Install new bearing shells in block and caps (correct orientation, tangs). Apply assembly lube on journals.
- Install crankshaft, fit main caps in their original location and orientation.
- Torque main cap bolts in specified sequence and increments (prevents distortion and ensures uniform bearing crush).
- Check main bearing clearances with plastigauge; verify they’re within specified range. If not, remedy (change bearing size or re-machine).
- Check crank endplay with dial indicator; fit thrust bearings or adjust if out of spec.
- Reinstall connecting rods with new rod bearings, torque caps to spec, and check side-to-side clearance on rods.
- Reinstall oil pump (prime it), pickup, oil pan, timing components, front/rear seals, and ancillary parts.
- Replace timing belt/chain, tensioners and water pump as recommended (timing components commonly replaced during bottom-end service).
Theory: correct bearing clearances ensure hydrodynamic lubrication; correct torque/method prevents cap distortion; priming oil pump and cleaning galleries prevents dry starts; replacing timing components reduces risk of secondary engine damage when reassembling.

9) Final checks and run-in
- Rotate the engine by hand through several revolutions to check for binding and correct clearances.
- Refill oil and coolant. Prime the oil system (crank without fuel to build oil pressure).
- Start engine and monitor oil pressure, knock, and leaks. Begin break-in with recommended RPM/load procedure (avoid high load/high revs initially).
How this fixes fault: priming prevents initial boundary lubrication at first start. Proper run-in allows bearings to bed to journals under controlled conditions.

10) Why each repair action addresses the root cause (brief mapping)
- Removing and inspecting root-causes contamination, finds damage pattern (e.g., bearing material in oil indicates bearing failure; scoring on journals indicates metal contact).
- Replacing bearings restores correct clearance and surface material that forms the hydrodynamic film; this stops metal-to-metal contact and knocking.
- Machining or replacing crank journals restores circularity and concentricity, reestablishing proper oil wedge and pressure.
- Correct endplay/thrust reestablishes axial position so timing and seals work and no axial impact damages components.
- Replacing oil pump and cleaning galleries restores flow/pressure preventing future starvation.
- Balancing reduces vibration that accelerates bearing and fatigue failure.
- Replacing timing, seals and gaskets prevents secondary failures and contamination that could re-damage the bottom end.

11) Common pitfalls and preventative tips (short)
- Never reuse bearing shells or seals; always use correct size/grade.
- Always verify oil clearances with plastigauge or micrometers—don’t guess.
- Cleanliness is critical; one grit particle can ruin new bearings.
- Always use factory torque and tightening sequence to avoid distortion.
- Replace the oil filter and change oil after a break-in interval; inspect for metal again.
- If unsure, have a reputable machine shop perform grinding and dynamic balancing.

End.
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