Assumptions: Hino 500-series diesel engine (refer to the exact engine workshop manual for fasteners, torque, clearances and temperatures). Safety: disconnect battery, drain coolant/oil as needed, use PPE, work on a clean bench. Steps below are ordered and each step includes the theory and how the repair corrects the fault.

1) Diagnose and confirm symptom
- Action: Note symptoms (blue smoke on start/acceleration, high oil consumption, misfire, low compression, valve train clatter). Measure compression and do a leak‑down test; observe valve stem movement and blow-by.
- Theory: Worn guides increase valve stem-to-guide clearance causing oil to pass into the combustion chamber and allowing valve wobble that prevents proper sealing.
- Repair effect: Confirms guide wear as root cause so replacing guides addresses the source rather than symptoms.

2) Remove cylinder head and accessories
- Action: Label and remove intake/exhaust manifolds, rocker assemblies, injectors where required, and follow head removal sequence in manual. Keep parts organized.
- Theory: Full access is required to remove springs, retainers and extract valves/guides without damage.
- Repair effect: Enables accurate measurement and replacement of guides and valve seats.

3) Remove valves, springs, retainers and seals
- Action: Use a valve spring compressor, remove keepers, retainers, springs, then extract valves. Remove old valve stem seals.
- Theory: Removing these components prevents damage during guide removal and allows inspection of valve stems/ seats.
- Repair effect: Frees valves for measurement and replacement; seals will be renewed to stop oil migration after guide work.

4) Inspect valves, seats and stems
- Action: Measure valve stem diameter with micrometer at several points; inspect seats and faces for pitting/burn; inspect for bent valves.
- Theory: Valve stem wear pattern and seat condition determine whether only guides need replacement or also valves/seats.
- Repair effect: Ensures replacement parts and machining decisions are correct; replacing guides alone won't fix burned seats or bent valves.

5) Measure valve guide wear
- Action: Measure guide inner diameter (ID) with small bore gauge or go/no‑go gauge; measure valve stem OD. Compute stem-to-guide clearance. Also check axial play and stem runout.
- Theory: Clearance = guide ID − stem OD. Excess clearance indicates wear; uneven clearance indicates ovalization or seized deposits.
- Repair effect: Quantifies wear to decide OEM-size replacement, oversize valve or reamed/honed new guide.

6) Decide repair method
- Options:
a) Replace guides with OEM-fit new guides (press-in).
b) Re-bore/ream heads and install oversize bronze guides with oversize valves.
c) Line‑bore or sleeve depending on damage.
- Theory: Choice depends on material (aluminium head with interference steel guides vs cast iron head), guide wall condition, and extent of wear.
- Repair effect: Selecting appropriate method restores correct interference, concentricity, and clearance for long life.

7) Prepare head for guide removal
- Action: Clean head around guides. If press-fit, heat head evenly (OEM guidance) to expand the bore; clamp head in fixture. Use a guide removal tool or arbor press to drive guides out from the combustion side, supporting the head to avoid distortion.
- Theory: Heating expands head material, reducing interference and allowing safe extraction; correct support prevents seat/guide bore damage.
- Repair effect: Removes the old, worn guide without further damage to the head.

8) Clean and inspect guide bores
- Action: Clean carbon and corrosion. Inspect bore for ovality, cracks, or wall thinning. Measure final bore diameter.
- Theory: Detects whether bore is still within limits for new guide installation or requires rework (sleeving or welding/boring).
- Repair effect: Ensures new guide will sit concentric and at correct interference; prevents premature failure.

9) Install new guides (press-in) or fit bronze guides
- Action: For press-fit: heat head as recommended and press new guides into correct depth, using a driver that contacts the guide body only. For bronze inserts: machine bore to size, install insert, then ream to final ID.
- Theory: Proper interference and concentric seating ensures thermal expansion characteristics and alignment of guide to seat/valve axis.
- Repair effect: Restores correct radial support for valve stems, reducing wobble and oil leakage.

10) Ream/hone guides to final ID
- Action: Use a fixed reamer to bring guide to nominal ID, then hone to final finish and size. Check diameter and roundness frequently. Target valve stem-to-guide clearance within OEM or typical ranges (approx. intake 0.02–0.05 mm, exhaust 0.03–0.08 mm — confirm OEM).
- Theory: Reaming gives accurate bore; honing achieves final clearance and proper surface finish for oil film formation.
- Repair effect: Precisely restores the clearance that controls oil flow and stabilizes the valve stem.

11) Fit valve stem seals
- Action: Install new valve stem seals compatible with guide diameter; ensure proper seating depth.
- Theory: Seals control oil wicking down the stem; combined with correct guide clearance they limit oil consumption.
- Repair effect: Stops oil from reaching combustion chamber past the guide.

12) Recondition or replace valves and seats if required
- Action: Lap valves or machine valve seats as necessary to restore proper contact and sealing angles. Replace valves if stem wear exceeds limits.
- Theory: Even with new guides, poorly sealing valves/seats will cause compression loss and combustion effects.
- Repair effect: Ensures restored compression and prevents combustion gas leakage and seat burning that could damage new guides.

13) Reassemble valves/springs/retainers
- Action: Install valves with new seals, springs, retainers and keepers. Use correct spring dimensions and orientation.
- Theory: Correct spring tension and assembly ensure valve follows cam profile and closes positively.
- Repair effect: Restores valve timing and sealing consistency.

14) Check valve stem runout and side clearance after assembly
- Action: Use dial indicator to check valve wobble and concentricity. Verify clearance against spec.
- Theory: Confirms guide and valve are concentric and clearance is within limits.
- Repair effect: Validates repair before head reinstallation.

15) Refit cylinder head and torque to spec
- Action: Clean surfaces, fit new head gasket, torque head bolts in correct sequence and stages to OEM torque and angle specs. Reconnect manifolds, rockers, injectors, sensors.
- Theory: Proper torque ensures gasket sealing and correct cylinder sealing/compression.
- Repair effect: Prevents head lift, leaks and helps keep valve train geometry correct.

16) Adjust valve clearances / lash
- Action: Set tappet clearance or preload hydraulic lifters per spec. If bucket/shim system, measure and fit shims to spec.
- Theory: Correct clearance compensates for thermal expansion and ensures timing/valve lift as designed.
- Repair effect: Prevents valve seating issues, noise and premature wear.

17) Break-in and verification run
- Action: Start engine, warm up, check for oil leaks, abnormal noises, excessive smoke. Re-check tappet settings and torque on critical fasteners after initial run-in. Perform compression or leak-down test to confirm sealing.
- Theory: Thermal cycles seat components, and rechecks detect any settling or distortion.
- Repair effect: Confirms repair success and early detection of faults before returning to service.

18) Final checks and monitoring
- Action: Monitor oil consumption and smoke over first few hundred km. Re-inspect valve train for loosening or excessive wear.
- Theory: Early monitoring ensures no missed faults (seating issues, improper clearances).
- Repair effect: Confirms durable repair and prevents repeat failures.

How the repair fixes the fault (summary)
- Worn guides increase stem clearance, allowing valve wobble, oil passage to combustion chamber, reduced sealing and erratic valve timing. Replacing or re-boring and reaming guides restores concentric support and correct radial clearance. New seals prevent oil wicking. Re-lapping seats (if needed) restores compression sealing. Together these actions stop oil burning (blue smoke), reduce oil consumption, restore compression and valve timing stability, and reduce valve train noise.

Important notes (concise)
- Always follow the Hino workshop manual for torque, sequence, and clearances. Use calibrated measuring tools. Excessive heat, improper driving tools, or lack of head support will damage the head. If guide bores are cracked or extensively thinned, sleeve or replace the head.

End.
rteeqp73

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