Tools & PPE
- PPE: safety glasses, gloves, steel-toe boots, hearing protection, mechanic’s coveralls.
- Lifting/support: engine/transmission hoist or forklift, transmission jack, axle stands, wheel chocks.
- Hand tools: full metric socket/torque wrench set (3/8" & 1/2"), breaker bar, extensions, ratchets, hex/Allen/torx set, combination wrenches.
- Specialty tools: slide hammer, bearing puller, gear puller, hydraulic press (bench press) with adapters, drift/punch set, snap-ring pliers, seal driver set, pilot/bearing puller, clutch alignment tool, torque-angle gauge (if needed).
- Measurement: micrometer, Vernier caliper, dial indicator with magnetic base, Feeler gauges.
- Consumables & shop supplies: penetrating oil (PB Blaster), anti-seize, thread locker (medium), clean rags, brake cleaner, gasket sealant, new gearbox oil, oil drain pan.
- Safety aids: transmission straps, marking paint/marker, pry bars, drain plugs and fittings replacements.

Overview / Safety precautions (must-do)
1. Work on level ground; chock wheels and disconnect battery negative.
2. Support vehicle solidly on stands; never rely on jack alone.
3. Use a transmission jack or hoist rated above gearbox weight; transmissions are heavy and unbalanced.
4. Drain fluids before unbolting to avoid spills; collect and dispose used oil legally.
5. Mark all linkages, cables and electrical connectors before removal.
6. Keep the clutch assembly supported when separating transmission from engine — do not let it hang from the input shaft.
7. Use lifting straps through bellhousing bolt holes and secure gearbox to jack to prevent tipping during maneuvering.
8. Work methodically, lay out parts in order, and photograph positions for reassembly.

High-level procedure (step-by-step)
1. Preparation
- Park, chock wheels, disconnect battery.
- Raise truck, support on stands, remove driveline/prop shaft (mark orientation), and drain transmission oil (save drain plug hardware).
- Remove shift lever boot, linkage, speedo cable/electrical connections, and any frame brackets attached to transmission.

2. Separate transmission from driveline and engine
- Support gearbox with transmission jack; remove starter if it obstructs bellhousing bolts.
- Remove bellhousing bolts in a crisscross pattern; leave two bolts partially threaded at top to support until ready to separate.
- Slide transmission back off input shaft carefully; catch and support clutch/pressure plate assembly to avoid damage.
- Inspect input shaft splines and clutch disc for wear; note pilot bearing/bushing condition.

3. Remove clutch assembly (if part of job)
- Remove pressure plate bolts gradually and evenly in a star pattern.
- Replace clutch disc, pressure plate, throw-out bearing, and pilot bushing if wear is present. Always replace throw-out bearing and pilot bearing on gearbox out jobs.

4. External inspection & strip
- Clean external grime; remove all external covers, selector housings, and speedometer housing.
- Photograph or tag selector forks, rails, and positions prior to removal.
- Remove output shafts, flange nuts, and snap rings as needed to free shafts.

5. Internal disassembly (follow factory manual order)
- Remove end covers/cases. Use punch/drift to remove stubborn dowels carefully.
- Extract shafts: mainshaft, countershaft, and input shaft. Use bearing pullers and slide hammer as needed.
- Press off gears, synchros, and bearings in controlled fashion using press and appropriate drivers.
- Keep track of spacer/shim arrangement; mark orientation of gears and collars.

6. Inspection & measurement
- Inspect gears for pitting, chipped teeth, scoring, excessive wear.
- Inspect synchros (blocking rings) for groove wear and heat discoloration; replace if worn or burnt.
- Inspect bearings and races for brinelling or roughness — replace all worn bearings and races.
- Measure:
- Bearing internal/external diameters with micrometer.
- Gear tooth backlash with dial indicator.
- Mainshaft end play and input shaft end float.
- Synchro clearance/thickness against new parts’ specs.
- Replace seals, O-rings, bushings, and shift fork contact pads if worn.

7. Common replacement parts (typical)
- Mainshaft bearings, input shaft bearings, countershaft bearings and races.
- Synchro rings, synchro hubs (if scored), shift forks, fork pins, shift rails.
- Output seals, input seal, end cover gaskets, case gasket, speedo gear if needed.
- Clutch kit: disc, pressure plate, throw-out bearing, pilot bearing/bushing.
- Fasteners: replace any stretch/torque-to-yield bolts and damaged studs/dowels.
- All gear oil and any oil seals.

8. Reassembly (cleanliness & sequence)
- Clean all parts with solvent and blow dry; inspect surfaces before assembly.
- Install bearings and races using correct driver diameter; heat the case lightly if interference fits are tight.
- Assemble shafts and gears in reverse of disassembly; ensure snap rings and shims are correctly reinstalled in original order or per new spec.
- Set bearing preload and gearbox end play per factory method:
- Use shims to set mainshaft end float; measure with dial indicator.
- Set gear backlash per manual using shim thickness and test indicator on pinion/counter gear.
- Replace all seals using seal drivers; avoid damaging lips.
- Reinstall selector forks and rails, verify smooth operation and correct engagement of gears by shifting through all positions.
- Torque all fasteners to OEM specs (see note below).

9. Reinstall transmission
- Clean bellhousing flange surfaces; replace bellhousing gasket if present and ensure dowel pins are present and undamaged.
- Use clutch alignment tool to centralize clutch disc on pilot bearing; engage transmission input shaft into clutch carefully — ensure splines align.
- Bolt bellhousing in crisscross pattern to specified torque.
- Reconnect linkage, speedometer cable/sensor, and driveline (ensure prop shaft orientation marks match).
- Refill with OEM-specified gear oil to correct level.

10. Adjustment & testing
- Adjust clutch free play per manufacturer spec (pushrod travel or pedal height).
- Start engine, check for leaks, and test shift engagement with vehicle on stands before road testing.
- Road test under light load, check for noise, syncro grabbing, or gear slip; recheck oil level after short run.

How specific tools are used (short)
- Transmission jack & straps: cradle gearbox, secure with straps; lift/lower slowly and align bellhousing to input shaft.
- Hydraulic press: press bearings on/off shafts, using drivers that bear on race only; heat bearings ~100°C if interference fit required (use oven, not torch).
- Bearing puller/slide hammer: remove stubborn bearings and seals; use backing plate to avoid damaging shaft ends.
- Dial indicator: measure backlash between gear sets — magnetic base on housing, tip on tooth face; rotate gear and record max-min.
- Micrometer/calipers: measure shaft journals and gear bores for wear against new part dimensions.
- Snap ring pliers: remove/install internal/external circlips — ensure rings fully seated in groove.
- Clutch alignment tool: centers clutch disc while torquing pressure plate so input shaft slides in easy on assembly.
- Torque wrench: final torque of bellhousing bolts, mainshaft nut, and case fasteners—use specified torque values and patterns.

Common pitfalls & how to avoid them
- Not supporting clutch or engine when removing transmission — avoids damage to input shaft and clutch.
- Ignoring worn input shaft splines — worn splines will damage new clutch disc quickly; inspect and replace if grooved.
- Reusing worn synchros or bearings — causes poor shifting and premature failure; replace suspect parts.
- Incorrect bearing preload or backlash — leads to premature bearing/gear failure or noisy gearbox; always measure and shim per spec.
- Not replacing pilot bearing or throw-out bearing — cheap insurance; failure leads to noise and clutch issues.
- Missing or damaged dowel pins — can misalign bellhousing and cause input shaft runout; replace any damaged dowels.
- Using wrong gear oil or wrong fill level — results in poor lubrication, noise, and wear. Use OEM spec oil and correct volume.
- Over-torquing or under-torquing case bolts — may cause distortion or leakage. Use torque wrench and specified pattern.
- Reusing old seals or O-rings — always replace to prevent leaks.
- Not marking or indexing prop shaft orientation — can cause driveline vibration on reassembly.

Replacement parts summary (minimum recommended on overhaul)
- Full bearing set (input / main / countershaft)
- Synchro ring set and hubs as required
- All seals & gaskets
- Shift fork pins/bushings if worn
- Clutch kit (disc, pressure plate, throw-out bearing, pilot bearing)
- Fasteners/dowels/studs as needed
- Correct grade gear oil

Notes on specs, torques & fluids
- Exact torque values, shim pack thicknesses, preload, backlash and gear oil grade are model-specific. Before reassembly, consult the Hino N04C / vehicle factory workshop manual for the transmission model in your truck for:
- Bolt torques, torque sequence, endplay spec, and oil capacity/type.
- Service limit values for shaft diameters, gear teeth, and bearing tolerances.
- If you don’t have a factory manual, obtain the OEM workshop manual or full-length service data — these procedures must be validated against the manual.

Final check (before road test)
- All electrical connectors reconnected and secured.
- Clutch free play set, no binding on cables/rods.
- Fresh oil at correct level, no leaks.
- Shifter operates smoothly through all gates.
- No unusual noises during low-speed test; recheck bolts after initial road run.

Done correctly, gearbox overhaul is methodical: remove, clean, measure, replace wearing parts, set preloads/backlash to spec, reassemble carefully and test. Follow OEM specs for critical measurements and torques.
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Safety first
- Work on level ground with the parking brake set. Use rated jack stands under the frame — never rely on a jack alone. Wear gloves, eye protection, and use hearing protection when using presses or air tools. Have a clean workspace and a parts tray for fasteners.

Tools & materials (typical)
- Basic: socket set, breaker bar, torque wrench, pry bars, punch/hammer, screwdriver set, rubber mallet.
- Specialized: hydraulic press or bearing driver set, dial indicator with magnetic base (for backlash), depth/pinion setting tool or shim kit, bearing puller, gear-marking compound (Prussian blue or similar), torque-angle gauge (if required), seal driver.
- Consumables: correct grade gear oil (Hino manual recommended, often GL‑5 80W‑90 or 75W‑90 synthetic), new crush sleeve or pinion shims (if applicable), new pinion seal, new carrier and pinion bearings, axle seals, gasket or gasket maker for cover, threadlocker, clean rags, parts cleaner, anti-seize.
- Safety: shop manual for Hino N-series (for specs and special tools).

Overview — what the differential is and why service is needed
- What it does: The differential lets the two drive wheels turn at different speeds while delivering engine torque to both. Inside the housing, the pinion gear (driveshaft output) meshes with a large ring gear attached to the carrier that holds the spider/side gears that drive the axle shafts.
- Why service: Gear oil breaks down, picks up metal particles, and becomes contaminated. Bearings wear, seals leak, backlash or pinion preload can change over time. If left, this leads to noisy gears, accelerated wear, overheating, and eventual catastrophic failure (broken teeth, bearing destruction).
- Analogy: Think of the ring and pinion as two combs meshing. If the combs are misaligned, they wear fast or snap teeth. Bearings are like the comb’s handle supports — if loose or worn, the comb moves and ruins the mesh.

Key components and what each does (detailed)
- Differential housing / carrier: The casing that holds gears and bearings; mounts to the axle. Protects internal parts and retains oil.
- Ring gear: Large gear bolted to the carrier. Transmits torque from pinion to the carrier/axles.
- Pinion gear: Small drive gear on the end of the driveshaft; meshes with ring gear and reduces speed/increases torque.
- Pinion shaft (yoke) & nut: Holds pinion gear and secures preload.
- Pinion bearing(s): Support pinion rotation and set pinion depth into the ring gear. Two bearings: inner (on the pinion head) and outer (at the nut end) — sometimes a tapered pair.
- Crush sleeve or shims: If used, a crush sleeve is compressed to set pinion bearing preload; shims are an alternative method.
- Carrier bearings: Support the carrier within the housing and help set backlash (lateral position of the ring gear).
- Carrier bearing caps: Bolts that clamp carrier bearings and maintain alignment.
- Side (sun) gears & spider (pinion) gears (differential gears): Allow wheel speed difference when cornering.
- Thrust washers: Provide bearing surface for side gears.
- Axle shafts: Carry torque to wheels. Have splines that engage side gears.
- Axle seals: Prevent oil from leaking where axle shafts exit the housing.
- Cover / gasket / sealant: Keeps oil in; removable for service.
- Breather: Allows pressure equalization; if blocked can force oil out of seals.
- Magnetic drain or inspection plug: Captures and reveals metal debris.

Theory — what to check and why
- Oil condition: Dark, milky (water), or metallic-smelling oil indicates contamination or failure. Metal flakes on the magnet indicate wear — some fine particles are normal after break-in; large flakes/“chunks” indicate failure.
- Bearing play and noise: Bearings wear and create looseness or growling noise. Bearing failure changes pinion depth/backlash and ruins gear mesh.
- Backlash: Side-to-side gap between ring and pinion teeth. Too much backlash = clunking and chipped teeth; too little = overheating and tooth breakage.
- Pinion depth: The pinion must be positioned so ring and pinion tooth faces contact properly. Incorrect depth gives poor pattern and rapid wear.
- Contact pattern: A dye-painted pattern on teeth shows where teeth touch — ideal pattern area is specified by the manual. It tells if shim/pinion depth/backlash need adjustment.

Routine differential service — drain, inspect, refill (good for beginner)
1. Lift vehicle and support on stands. Remove wheel(s) and brake parts as needed to access differential.
2. Clean area around fill and drain plugs to avoid contamination.
3. Remove fill plug first, then remove drain plug to drain oil into a pan. Inspect oil: color, smell, and magnet for metal flakes.
4. Remove cover (if applicable) or loosen bolts. For gasketed covers, pry carefully to avoid damaging flange. Clean mating surfaces.
5. Visual inspection: Check ring gear teeth for chips, pitting, scoring; check bearings for discoloration (heat), play, or rough rotation; inspect seals and breather for clogging.
6. Clean interior with solvent and rags. Do not pressure-wash the bearings.
7. Replace drain plug crush washer/gasket if fitted, and reinstall.
8. Reassemble with a new cover gasket or RTV per spec. Torque cover bolts to manual values.
9. Refill with correct gear oil through the fill hole until level is even with fill hole. Install fill plug.
10. Test drive and recheck for leaks and unusual noise.

Full differential rebuild / service (step-by-step, detailed)
Note: This is a generalized rebuild flow. The Hino workshop manual provides exact specs, shims, and special tools. Follow it for torque and measurement values.
A. Preparation
- Obtain new bearings, seals, ring gear bolts (replace bolts once), pinion seal, crush sleeve/shims if required.
- Mark orientation of carrier and bearing caps before removal for reference.
B. Remove differential from axle (if needed)
- Remove driveshaft from pinion yoke.
- Remove axle shafts (remove brake drums/rotors, C‑clips if used, or unbolt axle flange).
- Unbolt carrier bearing caps and carefully remove the carrier assembly (ring gear stays bolted to carrier).
C. Disassemble carrier & drive pinion
- With carrier out, loosen and remove ring gear bolts and separate ring gear from carrier if replacing or truing.
- Remove pinion nut; extract pinion assembly. If a crush sleeve is used, the nut crushes sleeve — you must press out pinion and bearings with a press.
- Press bearings off pinion; remove races from housing with a driver.
- Remove carrier bearings from carrier (press).
D. Clean & inspect parts
- Clean all metal parts. Inspect ring/pinion teeth for pitting or chipped teeth — any sign of distress requires replacement of both ring and pinion as a matched set.
- Inspect bearing races for scoring or discoloration. Replace bearings and races as a set.
E. Install new races & bearings
- Heat the races or use a driver to seat them squarely in the housing. Install new pinion inner bearing.
F. Set pinion depth and preload
- If using shims: install shim, pinion bearing, pinion gear, and measure depth with a depth gauge or trial shims per manual.
- If using a crush sleeve: install new crush sleeve and tighten pinion nut to specification to achieve correct preload (measured as rotational torque).
- Verify pinion preload (rotational torque) with a torque wrench. This measures bearing preload and is critical. If using torque as measure, tighten gradually and check rotation torque until within manual spec.
- Example (for illustration only): light truck pinion preload might be ~8–15 in‑lb for tapered bearings — but consult Hino manual for exact numbers.
G. Install carrier & set backlash
- Place carrier with new carrier bearings into housing with initial shims per manual.
- Torque bearing cap bolts to spec.
- Use a dial indicator to measure backlash: rest indicator tip on ring gear tooth and turn pinion/back the ring gear and measure the amplitude of tooth movement. Adjust backlash by changing carrier shims or adjusters until within spec.
- Example: typical backlash for many axles is 0.10–0.20 mm (0.004–0.008 in), but Hino spec may differ — check manual.
H. Check gear contact pattern
- With carrier and pinion torqued to spec but before final torque on ring gear bolts (or after, depending on procedure), paint gear teeth with gear-marking compound and rotate pinion through several rotations.
- Examine contact pattern: you want a smooth contact area centered on the tooth face per manual. If pattern is too close to face edge, change pinion depth; if too close to heel/toe, change backlash.
- Repeat shim/pinion adjustments until the pattern is correct.
I. Final torques and sealing
- Once the pattern and backlash are correct, torque ring gear bolts to spec in a criss-cross pattern, recheck backlash and pattern.
- Install new pinion seal and axle seals.
- Reinstall cover with new gasket or RTV, torque fasteners to spec.
- Refill with correct oil, spin the axle to distribute oil, check for leaks.

How to measure backlash and pinion depth (practical)
- Backlash: mount dial indicator to housing and place tip on a single ring gear tooth. Rotate ring gear back and forth (while holding pinion steady in alternating directions) and record the total movement. That's backlash.
- Pinion depth: use a depth gauge or shim pack method per manual, or use pattern checking as verification. The pinion depth determines where the pattern sits on the tooth.
- Pinion preload: for tapered bearings the rotational torque method measures preload. With pinion installed and yoke off, use a torque wrench on the pinion nut to measure rotational torque while the gear is dry; target is per manual. If using crush sleeve, torque permanently and replace sleeve.

Common failure modes and what causes them
- Whine at certain speeds: often incorrect pinion depth or worn bearings. Think of teeth slipping instead of meshing cleanly.
- Clunk when getting on/off throttle: excessive backlash or worn splines/axle shafts.
- Grinding or growl: bearing failure or broken gear teeth.
- Oil leaks: failed axle seals, pinion seal, or compromised gasket/cover.
- Overheating oil/metal discoloration: insufficient oil, wrong oil type, or heavy drag from incorrect preload.
- Metal chunks on magnet: catastrophic failure — replace ring and pinion set and inspect housing for damage.

Troubleshooting quick guide
- Noise at all times, even when coasting: likely ring & pinion/ bearings. Inspect pattern and bearings.
- Noise only on acceleration: likely ring & pinion.
- Noise only on deceleration: often pinion bearings.
- Vibration: check driveshaft and universal joints first; if vibration persists, inspect carrier bearings and backlash.
- Leaks: replace seals and breather; torque cover and check axle retaining components.

Reassembly tips & best practices
- Cleanliness is critical. Dirt is abrasive and kills bearings/gears.
- Replace fasteners like ring gear bolts — they stretch and lose strength.
- Always replace bearings in matched sets and replace races if damaged.
- Use threadlocker where specified. Torque fasteners in the correct sequence.
- Double-check backlash and pattern after final torques.
- Break-in gently after rebuild: avoid heavy loads for first few hundred kilometers, then check oil and magnet for fresh metal.

What you must consult the Hino workshop manual for
- Exact torque specs for pinion nut, ring gear bolts, carrier cap bolts, cover bolts.
- Exact backlash and pinion preload specifications and whether the axle uses crush sleeve or shims and their dimensions.
- Gear oil grade and capacity, and any model-specific procedures or special tools.

Final check & test
- After reassembly and fill: rotate drive shaft by hand to ensure free movement and that gears are lubricated.
- Road test under gradual load, listen for noise, check for leaks.
- After a short run, re-torque cover bolts if required and recheck oil level.

Summary (short)
- The differential transfers power to wheels while allowing different wheel speeds. Service ranges from simple drain-and-fill and seal replacement to full teardown, bearing replacement and precise setting of pinion depth and backlash. Correct measurement and adjustment of pinion preload and backlash — and obtaining the proper contact pattern — are critical to long life. Always follow the Hino N-series workshop manual for the exact specs and special-tool requirements.
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