Ordered, theory-focused explanation of diagnosing and repairing cylinder walls on a Toyota 4Runner (how the repair fixes the fault). Concise, no questions.

1) Symptom → diagnosis theory
- Symptoms: low compression, blow-by, oil burning, white/blue smoke, poor oil pressure, noisy rings.
- Tests to localize fault: compression test shows low/uneven cylinder pressures; leak-down quantifies where loss occurs (rings vs valves vs head gasket); borescope inspection through spark plug hole reveals glazing, scoring, scratches, corrosion, or cracks.
- Theory: cylinders must provide a hard, smooth, dimensionally accurate surface so piston rings can seal compression and control oil. Wear, glazing or scoring breaks that seal and/or prevents oil film control.

2) Remove and measure (why)
- Remove head/pistons/rods to expose bores. Measure bore diameter, taper and out-of-round with bore gauge/micrometer; compare to service limits. Measure piston diameters and ring land clearances.
- Theory: repair choice depends on amount and pattern of metal loss. Minor glazing/wear may be corrected by honing; excessive wear/taper/score requires reboring, sleeving or block replacement.

3) Repair choices (theory overview)
- Light glazing/minor wear → honing (deglaze + crosshatch). Restores oil-retaining microtexture and proper surface finish for ring seating.
- Moderate wear/taper/out-of-round → rebore to next oversize and fit matching pistons/rings, then finish-hone (plateau hone). Removes distortion and restores concentricity and specified clearance.
- Severe scoring/cracks/thin wall → install a pressed-in sleeve (liner) or replace block. A sleeve gives a new, correctly sized, wear surface and is used when reboring would over-thin walls or when cracks exist.
- Theory: all repairs aim to restore correct cylinder geometry (diameter, roundness, concentricity, taper) and surface topography so rings can form an effective gas seal and control oil.

4) Honing theory and ordered actions (when honing is appropriate)
- Purpose: remove glaze and create a controlled microtexture (crosshatch) that retains oil and allows ring bedding. Crosshatch angle ~25–45°. Surface finish target ~20–50 µin Ra (≈0.5–1.3 µm Ra).
- How it fixes the fault: glazing polishes the bore so rings cannot bite and seal; honing freshens the surface peaks and valleys so rings wear in and seal, restoring compression and reducing oil consumption.
- Practical sequence (theory-ordered): choose appropriate abrasive and mandrel/hone size matched to bore; use controlled, light passes to avoid removing excessive material; rinse and remove abrasives; measure bore for correct final clearance; plateau-hone if rebore was done.
- Important: honing refines surface and minor size, but does not correct significant taper or out-of-round beyond the hone stones’ ability.

5) Reboring theory and ordered actions (when rebore is required)
- Purpose: restore geometry by removing a uniform cylindrical layer; then fit oversize pistons and rings.
- How it fixes the fault: restores roundness and diameter so piston-cylinder clearance is correct, enabling proper ring sealing and oil control where honing cannot.
- Sequence (theory-ordered): determine oversize needed from wear measurement; rebore to consistent diameter with professional boring bar; finish with a plateau hone to achieve the correct surface finish and crosshatch for ring seating; measure to confirm concentricity and size.
- Note: reboring removes more metal—if the walls become too thin, use sleeve or replace block.

6) Sleeving theory and ordered actions (when sleeving is required)
- Purpose: install a precision liner where the original bore cannot be restored safely. Sleeve can be wet or dry depending on design.
- How it fixes the fault: provides a new, hardened, dimensionally stable surface reset to OEM specs without over-thinning the block. Press-fit or shrink-fit ensures correct interference and heat-transfer characteristics.
- Sequence (theory-ordered): machine block to accept sleeve, press/install sleeve to specified interference, machine sleeve ID to final size, then finish-hone for final surface. Ensure correct protrusion/flushness and sealing if required (head gasket area).

7) Rings, pistons and clearances (why they matter)
- After any cylinder work install new rings and usually pistons sized to the finished bore. The axial and radial clearances (ring end gap and piston-to-wall clearance) must be within spec.
- Theory: correct clearances allow thermal expansion and ensure rings seat without scuffing; wrong clearances cause blow-by, oil consumption, or seizure.

8) Surface finish and break-in theory
- Surface finish (crosshatch angle, roughness) controls oil film thickness and ring seating. Plateau honing (light final honing) removes high peaks and polishes ring lands/mating surfaces so rings contact evenly.
- Break-in: controlled engine operation (varying RPM load for initial period) seats rings into the re-created crosshatch without overheating. Early oil change removes abrasive particles from initial wear. This seating restores compression and minimizes oil consumption.

9) Reassembly & verification (why each test)
- Reassemble to factory torque/specs (head bolts, bearings, etc.). Use new gaskets and follow proper lubrication procedures.
- Verify with compression and leak-down tests, check oil consumption and exhaust for smoke, and ensure oil pressure and operating temps are normal. Successful repair shows restored compression, reduced blow-by, and normal oil consumption.

10) Failure modes and when to choose replacement
- If cracks, severe warpage, excessive corrosion, or multiple past repairs exist, sleeve or block replacement is often more reliable. Repeated short-term failures indicate deeper issues (cooling problems, detonation, poor lubrication) that must be fixed to prevent recurrence.

Summary of how the repair fixes the fault
- Problem = loss of a correct bore geometry or surface microtexture so rings cannot seal or control oil.
- Honing fixes surface glazing and renews oil-retaining texture so rings can seat.
- Reboring fixes dimensional errors (taper/out-of-round) so piston/ring clearance and concentricity are restored.
- Sleeving replaces unusable metal with a new precision surface when reboring isn’t safe.
- Proper ring/piston replacement and break-in complete the restoration, restoring compression, stopping blow-by, and controlling oil.

Follow the Toyota factory service manual for exact measurements, torque specs and tolerances for your 4Runner engine.
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