Overview (what this is and why it’s done)
- The gearbox (transmission) transfers engine power to the wheels through different gear ratios. Replacing it is necessary when gears, bearings, synchros, seals, or shafts are worn or damaged — symptoms: grinding, gears popping out, slipping, whining noise, heavy vibration, oil leaks or no drive.
- Analogy: the gearbox is like the multi-gear hub and chainset on a bicycle. The clutch is the quick-release that lets the engine stop turning the drivetrain so you can change gears. If teeth are worn or bearings are shot, the drivetrain won’t drive smoothly or at all.

Which gearbox am I describing?
- The instructions below focus on a typical Toyota Hilux manual gearbox replacement (most generic manual-transmission Hilux steps). Automatic transmissions are similar in some stages (supporting the unit, disconnecting torque converter/lines), but automatics need extra steps (fluid lines, cooler, valve body, torque converter) and are more complex — consult a manual or a professional for automatics.

Major components and what each does (detailed)
- Engine flywheel (or flexplate on automatics): bolted to the crankshaft, gives the transmission something to spin against the clutch.
- Clutch disc (friction disc): sandwiched between flywheel and pressure plate, transmits torque when compressed.
- Pressure plate: clamps the clutch disc to the flywheel; springs provide clamping force.
- Release/throwout bearing (pilot/throwout): presses on the pressure plate fingers to disengage the clutch.
- Clutch fork / hydraulic slave: mechanically or hydraulically actuates the release bearing.
- Pilot bearing/bushing: supports transmission input shaft in the crankshaft.
- Transmission bellhousing: the casing that bolts to the engine and contains the clutch area.
- Input shaft: from clutch/pressure plate into the gearbox; connects to the gearbox’s gearsets.
- Countershaft (lay shaft) and gearsets: run parallel and engage with input/output gears to change ratios.
- Synchronizers (synchros): help match shaft speeds to engage gears smoothly.
- Output shaft: sends power out to the driveshaft(s).
- Reverse idler gear: reverses direction for reverse gear.
- Bearings and races: support rotating shafts.
- Shift forks/rail and selector mechanism: move gears/synchros to select gears.
- Output yoke / flange: where the driveshaft or prop shaft connects.
- Seals and gaskets: keep gear oil inside and contaminants out.
- Mounts/crossmember: hold gearbox to the vehicle body/frame.
- Speedo sensor/cable: measure output speed for speedometer/ECU inputs.

Parts you should replace while gearbox is out (recommended)
- Clutch kit (clutch disc, pressure plate, pilot bearing, release bearing) — if manual.
- Rear main seal (crank seal) — accessible while bellhousing off.
- Transmission input/output seals if leaking.
- Transmission mount(s) and crossmember bushings.
- Shift linkage bushings or cables if worn.
- Gear oil and any filters (if present).
- New gearbox or fully rebuilt unit with service-tested bearings/gears.

Required tools and supplies
- Vehicle-specific service manual with torque specs and procedures.
- Floor jack and two or more quality jack stands; wheel chocks.
- Transmission jack (strongly recommended) or second floor jack with good load support.
- Full socket set (deep and short), extensions, swivel sockets.
- Wrenches (metric), breaker bar, torque wrench.
- Screwdrivers, pry bars, punch/hammer, snap ring pliers.
- Clutch alignment tool (specific to disc splines).
- Pilot bearing puller, seal drivers.
- Penetrating oil, brake cleaner, rags, gasket scraper.
- Hydraulic line tools (flare nut wrench) if clutch is hydraulic.
- New gear oil, clutch fluid (if hydraulic), anti-seize, thread locker if specified.
- Protective gloves, eye protection.

Safety first (do these before starting)
- Park on a level surface, chock rear wheels. Disconnect negative battery terminal.
- Lift only with manufacturer-approved jacking points, support with jack stands — never rely on a jack alone.
- Wear gloves and eye protection; transmission fluid is hot and slippery.
- Have a fire extinguisher nearby, and be careful of sharp edges.

High-level procedure (detailed step-by-step for a manual gearbox)
1) Preparation
- Read the service manual for your Hilux model (year/engine/trans code). Note torque specs and any model-specific steps (4x4 transfer case, etc.).
- Gather parts, tools, and a transmission jack.

2) Drain fluids and prepare for removal
- Remove the gear oil drain plug on the gearbox and drain oil into a pan. Also drain transfer case if 4x4 and you’ll separate propshafts.
- Mark and/or photograph linkage, shift cables, and wiring for exact reassembly.
- Remove the center console and shifter boot (interior) to access shifter linkage/cable fasteners if necessary.

3) Disconnect driveline components
- Remove the prop shaft (rear driveshaft or front/rear shafts on 4x4). Mark orientation of flanges for reassembly and secure bolts.
- Disconnect front driveshaft/axles if they interfere (on solid front axle or older models you may remove U-joints).
- If 4x4 with transfer case, you may unbolt and move the transfer case or remove it with gearbox depending on design — consult service manual. Many Hilux 4x4s require separating the transfer case from the gearbox and supporting it.

4) Disconnect external gearbox items
- Remove starter motor (so you can access bellhousing bolts).
- Disconnect shift linkage/shift lever, speedometer cable or sensor harnesses, reverse light switch, and any electrical connectors.
- Disconnect clutch hydraulic line: open bleeder and cap line, or disconnect slave cylinder depending on location. If it’s a hydraulic concentric slave inside bellhousing, follow manual instructions — you may have to separate lines but do not drain entire system initially.
- Remove exhaust components if they block access (heat shields, mid-pipe, crossmember).

5) Support gearbox and engine if needed
- Support the engine with an engine support bar or a jack under an oil pan padding if you will remove motor mounts (some Hilux models require slight engine lift to separate bellhousing). Follow service manual to avoid stressing engine mounts.
- Place a transmission jack under the gearbox and secure it. At least two people recommended to steady heavy components.

6) Remove crossmember / gearbox mounts
- Unbolt gearbox mount and crossmember that hold the gearbox to chassis. Lower enough that the gearbox can be moved slightly to access bellhousing bolts.

7) Unbolt bellhousing from engine
- Remove all bellhousing bolts (there may be long bolts from cradle to bellhousing too). Keep bolts organized by location and length.
- Remove starter if not already done (often starter is removed so its bellhousing hole is free).
- Be careful: gearbox may hang slightly on the splines/pilot bearing.

8) Separate gearbox from engine
- Using the transmission jack, carefully pull gearbox back while supporting it. If stuck, wiggle gently; use a pry bar between bellhousing flange and engine block if necessary — pry from specified points only to avoid cracking housing; consult manual.
- Ensure the clutch disc clears the pressure plate and input shaft before gearbox exits. If the input shaft is stuck in the pilot bearing, use gentle axial force — never hammer on the input shaft.

9) Lower gearbox
- Once separated, lower gearbox on the transmission jack and angle it to clear the chassis. Watch for wiring harnesses or lines still connected.

10) Inspect clutch/flywheel and related
- Remove pressure plate bolts and remove clutch disc. Inspect flywheel for heat spotting, cracks, or glaze. Resurface or replace flywheel if necessary.
- Replace pilot bearing and rear main seal while you have access.
- Inspect release/throwout bearing and fork. Replace as needed.

11) Prepare new/rebuilt gearbox
- Compare input/output flanges and mounting points with old unit. Replace any output seals and apply a thin coat of gear oil on splines.
- If gearbox is used or rebuilt, install new seals and check for proper end play. Ensure shift selector is in neutral.

12) Install clutch (new components)
- Clean flywheel contact surface with brake cleaner, mount pressure plate and clutch disc using alignment tool so the disc is centered on the pilot bearing. Torque pressure plate bolts gradually and in star pattern to factory spec.
- Ensure release bearing is correctly oriented on the fork and that clutch fork pivot is greased per manual.

13) Reinstall gearbox
- Lift gearbox on transmission jack. Align input shaft splines with clutch disc and pilot bearing using the alignment tool. Guide gearbox forward until bellhousing mates flush with engine. Start bellhousing bolts by hand.
- Tighten bellhousing bolts to factory torque sequence and specs.

14) Reattach mounts, driveshafts, and components
- Reinstall crossmember and gearbox mount(s); torque to spec.
- Reattach shift linkage/cables, speedo sensor, wiring, reverse light switch.
- Reinstall starter.
- Reinstall driveshaft(s), marking orientation. Torque flange bolts to spec.
- Reconnect any transfer case bolts and drives.

15) Fill fluids and bleed clutch
- Refill gearbox with specified gear oil to correct level. Fill transfer case and differentials if drained.
- If clutch is hydraulic, bleed the system (bench or bleed on car) until no air remains and pedal feels firm. Check for leaks.

16) Final checks before starting
- Double-check all bolts, inspect for tools left in engine bay, reconnect battery negative.
- Slowly rotate driveshaft by hand (or crank engine in neutral with wheels blocked) to verify no odd binding.
- Start engine and test clutch engagement with vehicle elevated and parking brake on; shift through gears to check engagement.
- Lower vehicle and perform a careful test drive, listening for noises and checking shifts. Re-check torques after initial test drive.

What can go wrong (common failure modes and mistakes)
- Input shaft seized in pilot bearing: forcing can damage splines or crankshaft nose — use proper pullers or replace pilot bearing first.
- Clutch misalignment: if clutch not aligned, gearbox won’t slide into place — will feel stiff or impossible to mate. Use alignment tool.
- Forgetting to disconnect or remove wiring, lines, or the starter — causes damage when lowering gearbox.
- Incorrect torque or missed bolts: can lead to leaks, looseness, or cracked housings.
- Broken bellhousing bolts or stud threads: they can be hard to remove and may require helicoils/repairs.
- Damaged seals causing leaks later; old seals not replaced.
- Not supporting engine properly — can shift dangerously and break mounts.
- Air in hydraulic system after reconnecting lines: leads to soft clutch pedal and incomplete disengagement.
- Using wrong oil or wrong capacity — causes insufficient lubrication, gear noise, damage.
- Reusing a tired clutch disc or not resurfacing flywheel — premature clutch failure.
- Not marking drive flange orientation — can cause vibration if reassembled wrong.

Troubleshooting after installation
- Grinding when shifting: clutch not fully disengaging (bleed clutch, check release linkage), or damaged synchros in gearbox.
- Gear popping out: worn shift forks/synchros or wrong gear selector adjustment.
- Whine or growl: worn bearings or incorrect oil level/grade.
- Vibration: driveshaft out-of-balance or flange misaligned; check U-joints and balancing marks.
- Fluid leak: locate seal/gasket leak; check drain/fill plugs and replace seals if necessary.

Tips and tricks from practical experience
- Label everything and take photos as you remove parts — reassembly is much easier.
- Keep bolt locations organized (ziplock bags labeled by area).
- Replace all rubber mounts and bushings accessible while gearbox is out — cheap and saves later labor.
- Use anti-seize on bolts where service manual permits; use thread locker where specified.
- Always use a transmission jack or equivalent; gearboxes are heavy and awkward — one-person risky.
- If unsure about bearing play or endfloat on a used gearbox, compare to spec or have a shop inspect it.

Final notes
- Consult a factory workshop manual for your exact Hilux year/engine/transmission for bolt torques, special tool numbers, and any model-specific quirks (4x4 transfer case steps differ, concentric slave designs vary).
- If you are uncertain about any step or lack tools like a transmission jack, consider hiring a professional — transmission replacement is heavy, precise work where mistakes can be costly or dangerous.

No further questions per your instruction.
rteeqp73

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