Short version first: a transmission leak on a Toyota SGK6/SDK6/SDK8 skid steer usually means hydraulic fluid escaping from seals, fittings, hoses, drain/fill plugs, or cooler lines in the hydrostatic drive/transmission assembly. Fixing it requires: diagnosing the exact leak source, safely depressurizing and supporting the machine, removing/repairing the leaking component (seal, hose, fitting, or cover), cleaning and inspecting mating surfaces and shafts, installing new seals/gaskets/fasteners correctly, refilling and bleeding the hydraulic/transmission fluid, and testing for leaks and correct operation. Below is a detailed, beginner-friendly walkthrough including component descriptions, theory, tools, step-by-step repair, what can go wrong, and preventive tips.

Safety first
- Work on a level surface. Chock wheels and lower the boom and bucket to the ground to prevent movement. Use boom supports or blocks — do not rely on hydraulics only.
- Isolate and turn off the engine, remove key, disconnect battery negative to avoid accidental start.
- Relieve hydraulic pressure before opening any hydraulic lines: run the engine with controls centered, then shut down, or follow manufacturer pressure-relief procedure in the service manual.
- Wear eye protection, gloves, and steel-toe boots. Hydraulic fluid under pressure can penetrate skin and cause serious injury.
- Use jack stands or suitable supports if you lift the machine. Beware of heavy components; use a hoist if needed.
- Capture and properly dispose of used hydraulic fluid and contaminated rags per local regulations.

Basic theory — how the system works (simple analogy)
- The skid-steer “transmission” is a hydrostatic drive: the diesel engine drives a hydraulic pump (like a heart pumping blood) that pushes hydraulic fluid under pressure to hydraulic drive motors (the legs/feet) mounted at the wheel hubs or final drives. The drive motors turn planetary gears or final drive gears to move the wheels/tracks.
- Seals and gaskets are like rubber door seals that keep the fluid in the piping and housings. If a seal fails, the pressurized fluid finds a path out — like water leaking through a cracked pipe joint — reducing system pressure, causing loss of drive or overheating, and allowing dirt to enter.
- The fluid performs power transmission, cooling, and lubrication. Low/contaminated fluid reduces torque, causes overheating, wears bearings and gears, and can destroy the pump or motors.

Key components you’ll encounter (what each does and how it can fail)
- Engine-driven hydraulic pump: creates flow and pressure. Failure modes: internal wear, cavitation, contaminated fluid. Usually not the leak point unless external fittings fail.
- Control valve (joystick valves): routes fluid to travel and other circuits. Leaks: external ports or internal seals.
- Hydraulic lines and hoses: flexible or rigid conduits. Leaks from chafing, cracked hoses, loose fittings, or O-ring failure.
- Transmission / hydrostatic unit housing: contains pump/motor, gears, and fluid. Leaks from case seams, cover gaskets, or threaded plugs.
- Final drives / wheel motors (hydraulic motors): convert fluid pressure to rotation. Leaks from shaft seals (output shaft seals), case bolts, or external fittings.
- Shaft seals (lip seals): seal rotating shafts where they exit housings. Very common leak source when worn or shaft is scored.
- O-rings/gaskets: static seals between flanges or covers. They age and harden, then leak.
- Drain/fill plugs and sight glass: simple leak points if washer/gasket missing or threads damaged.
- Cooler lines and fittings: external cooler hoses can leak from fittings or corrosion.
- Breather cap: if blocked or missing, pressure can build and force fluid out weak points.

Tools and supplies you’ll need
- Basic hand tools: wrenches, sockets (metric), ratchets, breaker bar, torque wrench (for reassembly torque values from manual), screwdrivers, pliers.
- Seal puller / pick set, flat punch, hammer.
- Seal driver set or appropriately sized socket and soft mallet.
- Jack/crane or hoist, jack stands or machine supports.
- Drain pan(s), clean rags, parts cleaner/degreaser.
- New replacement seals, O-rings, gaskets, crush/copper washers, and any bolts specified by OEM if needed.
- Hydraulic fluid of correct spec (consult OEM manual for grade and volume), and funnel or pump for refilling.
- UV leak dye and UV lamp (optional) or fluorescent dye compatible with hydraulic fluid for leak tracing.
- Thread sealant/tape only if specified by manual (do not use Teflon tape on certain fittings — check manual).
- New fluid disposal containers.
- Personal protective equipment.

Diagnosis — find the leak source
1. Clean the area thoroughly: degrease the suspected area (transmission housing, hoses, motors) so you can see fresh leaks.
2. Inspect visually and by feel: look for wet spots, drips, or fluid trails. Trace up from wettest point to source.
3. Run the machine briefly to operating temperature and low idle with controls neutral (someone seated in operator station), watching for leaks. Keep clear of moving parts.
4. If leak is hard to find, apply UV dye to the fluid (follow dye instructions) and run briefly, then inspect with UV lamp. Or use brown paper towels around joints to see where fluid appears.
5. Check fluid level and condition. Low level indicates a substantial leak; milky or muddy fluid indicates water contamination; metal particles indicate internal wear.
6. Note whether leak appears only under load or constantly — intermittent leaks under pressure often mean a seal or fitting; constant slow seep may be a case gasket.

Common leak locations on hydrostatic drives
- Output shaft seal at final drive/wheel motor
- Motor-to-final-drive flange O-rings
- Case seam gasket between transmission halves or cover
- Drain or filler plug washer
- Hoses/fittings between pump and motor or cooler
- Cooler core or cooler hose fittings
- Breather/overflow cap area

Step-by-step repair procedure (typical shaft seal replacement and related repairs)
Note: exact disassembly order and torque specs vary by machine. Use the Toyota workshop manual for model-specific instructions. This is a general, detailed guide.

A. Preparation
- Get the exact replacement parts: transmission/oil seal(s), O-rings, gaskets, crush washers, and any bearings if needed. Using OEM or good aftermarket parts is important.
- Park machine on level ground, chock wheels, lower boom and bucket, shut engine off, remove key, disconnect battery negative.
- Support machine and any components you will remove. If removing a final drive or motor, support it with a hoist.

B. Depressurize and drain fluid
- Relieve hydraulic pressure per manual. Often involves moving controls with engine at idle and then shutting off.
- Position a drain pan under the transmission/final drive drain plug.
- Remove drain plug(s) and drain fluid. Keep it clean; inspect for metal flakes.
- Replace drain plug with new crush washer when reassembling.

C. Remove obstructing items for access
- Remove shields, skid plates, hoses, and lines that block access to the seal or component. Label hoses/lines for correct reinstall.
- If replacing an output shaft seal you may need to remove the wheel, sprocket, or final drive housing.

D. Remove the old seal
- Expose the seal. Capture the housing bolts and keep organized.
- Use a seal puller or small pry tool to carefully remove the old seal. Work around the circumference and avoid scratching the shaft or bore.
- Inspect the shaft and bore condition. Light scoring can sometimes be polished out; deep grooves or a worn shaft will require shaft replacement or machining. A scored shaft will cause repeated seal failures.

E. Inspect bearings and mating surfaces
- While the cover is off, check bearings for play and noise and inspect gear teeth surfaces for pitting or metal debris.
- If bearings or gears are damaged, replace them; continuing with a bad bearing will ruin a new seal quickly.

F. Prepare and install new seal
- Clean the housing bore thoroughly.
- Lightly coat the seal lip with fresh hydraulic fluid or assembly grease (do not use petroleum grease that can swell some seal materials; use manufacturer recommended lubricant).
- Make sure the seal orientation is correct: the sealing lip faces the fluid/pressure side (usually towards the inside of the housing). For lip seals, the garter spring, if visible, often faces the outside but check OEM guidance.
- Use a seal driver or a socket of appropriate outside diameter to press the seal squarely into the bore until it seats flush. Hit evenly; do not cock the seal.
- Do not hit the seal face directly with a hard hammer — use a driver.

G. Reassemble
- Replace any O-rings/gaskets on covers and fittings with new parts.
- Clean bolt threads and apply the correct torque procedure/sequence. Tighten bolts in a star or cross pattern to specified torque values from manual.
- Reinstall hoses/lines using new washers or O-rings where specified. Make sure fittings are clean and do not cross-thread. Use thread sealant only where specified.
- Reinstall shields and components removed earlier.

H. Refill and bleed system
- Refill with correct hydraulic/transmission fluid to the specified level. Do not overfill.
- Start the engine and run at low idle. Cycle travel controls slowly and operate the machine through movement ranges to warm fluid and expel air.
- Recheck fluid level when warm and top to the correct level.
- Inspect for leaks under light pressure first, then under load.

I. Test and final checks
- Operate the skid steer under low load and observe: unusual noises, overheating, sluggish travel, or leaks.
- Re-torque bolts after initial warm-up if manual specifies re-torque after heat cycle.
- Re-check fluid after several hours of operation and again after a day.

What can go wrong (and how to avoid it)
- Replacing only the seal but not fixing a damaged shaft: the new seal will fail quickly. Inspect and replace or machine the shaft if needed.
- Reusing old O-rings or gaskets: they may compress and leak; always use new ones.
- Incorrect seal orientation: seals must face the pressure side — installing backwards causes immediate failure.
- Over-tightening or under-tightening bolts: can warp covers or leak. Use torque wrench and OEM values.
- Contaminated fluid: dirt or water will destroy pump and motors. Keep everything clean during assembly.
- Air trapped in the system: causes cavitation, noise, and overheating. Proper bleeding is essential.
- Using wrong fluid type or grade: affects lubricity, viscosity, and seal compatibility; always use manufacturer-specified fluid.
- Not replacing crush washers on drain plugs: leads to slow leaks.
- Cross-threading fittings: causes leaks and damaged threads.
- Not supporting heavy components: causes injury or damage. Use proper lifting/support equipment.

When a leak means more than a seal
- Metal particles in the drained fluid → internal wear (pump or motor damage). Inspect magnetic plugs and filters. If heavy contamination, a full system flush and component replacement may be needed.
- Repeated seal failures → shaft misalignment, bent shaft, or bearing collapse. Investigate bearings and alignments.
- Overheating and degraded fluid → check cooler, fan, and filtration. Replace fluid and coolers if compromised.

Preventive maintenance tips
- Change hydraulic fluid and filters at intervals recommended by Toyota.
- Inspect hoses and fittings for chafing and soft spots; replace aged hoses.
- Keep breathers and vents clean to avoid pressure buildup.
- Check fluid level daily or pre-shift; address low level promptly.
- Keep machine clean to easily spot early leaks.

Final notes and cautions
- Always consult the Toyota SGK6/SDK6/SDK8 official workshop manual for model-specific procedures, part numbers, torque specs, and fluid type/volume. This guide gives a thorough process and the right mindset for the work, but exact specs and sequences matter.
- If the leak source is a major internal failure (pump or motor core failure, damaged gears), expect larger repairs and possibly sending components to a machine shop or ordering replacement final drives.
- If at any point you are unsure about bearing condition, machining, or press-fitting heavy components, get help from a professional technician or a machine shop.

You now have the complete theory, component descriptions, diagnostics, step-by-step repair flow, and common failure modes for fixing transmission/hydrostatic fluid leaks on the Toyota skid steer models listed. Follow safety, keep things clean, replace seals/O-rings/gaskets, and verify shaft condition — that’s the core to a lasting repair.
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Below is a practical, step‑by‑step workshop style guide to diagnosing and repairing a transmission/hydrostatic fluid leak on a Toyota skid steer loader (SGK6, SDK6, SDK8 family). It’s written for a beginner mechanic but thorough — theory, system overview, every relevant component described, tools and parts, detailed repair steps, testing, what can go wrong, and prevention. Read everything carefully before you start and follow the safety notes.

Important safety notes up front
- Work on a level surface with parking brake on and wheels chocked.
- Lower the lift arms/attachments to the ground.
- Relieve hydraulic pressure before disconnecting lines (see procedure).
- Support the machine securely with blocks/stands if you’ll be under it. Do not rely on hydraulic pressure to hold components.
- Wear safety glasses, gloves, protective clothing.
- Disconnect battery to avoid accidental engine start while you work on hydraulic/tranmission components.
- Collect and recycle waste hydraulic fluid; avoid spills.

What this repair is for (theory / why needed)
- The skid steer transmission/hydrostatic system uses hydraulic fluid as both the power medium (pumps and motors) and the lubricant. A leak reduces fluid level and pressure, allowing air and contaminants into the system, which causes poor drive performance, slipping, overheating, increased wear, internal damage (pump/motor bearings, gears, seals), and environmental contamination.
- A small persistent leak becomes a big failure risk: running low causes cavitation in the pump, heat, metal wear and eventual loss of drive. Fixing leaks restores system integrity and prevents catastrophic component failure.

How the hydrostatic transmission system works (simple analogy and overview)
- Analogy: think of the hydrostatic drive as a pair of water pumps connected by high‑pressure hoses — the engine drives a hydraulic pump (the “pressure generator”), which pressurizes fluid and sends it to wheel motors that turn the wheels. Flow controls the speed; pressure produces torque. Returned fluid goes to a reservoir (tank), passes a filter and cooler, and returns to the pump.
- Main functional flow: Engine → Pump(s) → High‑pressure lines → Hydrostatic motor(s)/transaxle → Return lines → Reservoir → Filter/Cooler → Pump suction.
- If fluid leaks out anywhere along that loop, the pump can’t maintain the pressure/flow necessary to drive the motors properly.

Detailed descriptions of every relevant component (what they are and how they can leak)
- Engine drive coupling / PTO shaft: connects engine output to pump — leaks typically from shaft seals at pump input.
- Hydraulic pump (charge pump / main pump): pressurizes flow — pump housings, shaft seals and mounting flanges can leak.
- Hydrostatic motors / transaxles (drive motors, final drive): convert fluid pressure into wheel rotation — leaks can come from motor shaft seals, case seams, or line fittings.
- High‑pressure hoses and return hoses: flexible lines between pump, valves and motors — leak at abrasion, end fittings, hose ends, and crimp failures.
- Hose fittings, banjo bolts, O‑rings / crush washers: common leak points at connections. O‑rings and crush washers degrade and leak.
- Reservoir (hydraulic tank): stores fluid; leaks at drain plugs, sight gauge, filler cap O‑ring, or if tank is cracked.
- Filter assembly: filter housing, seals and gaskets can leak if improperly installed or damaged.
- Cooler and cooler lines: heat exchanger often located front or near radiator; leaks in core or connecting lines.
- Valve block / control manifold: internal seals and external ports can leak if seals fail or bolts loosen.
- Breather cap: if clogged, tank can pressurize and force fluid out seals.
- Case gaskets and cover seals: flange/gasket joints between housings; gasket failure causes external seepage.
- Fasteners & drain plugs: loose bolts allow seepage.
- Sensors / switches with O‑rings (temp/pressure sensors): small but common drips.

Tools, supplies & parts you’ll need
- Personal protective equipment: gloves, safety glasses, shop rags.
- Vehicle support: wheel chocks, blocks, hydraulic stands or jack and stands rated for machine.
- Basic hand tools: socket set (metric), spanners, torque wrench, screwdrivers, pliers.
- Specialty tools: seal puller, O‑ring pick, snap ring pliers, soft mallet, bearing puller (if bearing replacement needed), line wrenches for hydraulic fittings, flare nut wrenches.
- Drain pan, fluid transfer pump, funnels, RTV gasket maker (if specified), threadlocker (as specified).
- Clean solvent / degreaser and wire brushes for cleaning leak area.
- Replacement seals, O‑rings, gaskets, crush washers, hoses or fittings as diagnosed. Use OEM or equivalent parts; seals sized for each location.
- Hydraulic fluid specified by Toyota for those models (consult the service manual or parts sticker for exact spec - do not guess).
- Replacement filter(s), and possibly a new cooler line or hose clamp if damaged.
- UV leak dye (optional) and UV light for locating stubborn leaks.
- Shop manual/service manual for SGK6/SDK6/SDK8 for torque specs, line routing, part numbers and fluid type.

Diagnosis — find exactly where the leak is
1. Clean the area first: use solvent and brushes to remove built‑up grime so you can see fresh fluid.
2. Start engine and operate controls to pressurize the system briefly (DO NOT run long with a leak). Watch the cleaned areas to see where fresh fluid appears. Use paper towels to trace the drip path downhill — fluid drips run along surfaces.
3. For small or intermittent leaks use UV dye and run briefly, then inspect with UV lamp.
4. Identify whether the leak is from a fitting, hose, shaft seal, housing seam, drain plug, filter, cooler or breather. Mark the exact leak point.

Decision tree — what repair to do based on leak source
- Hose or fitting leak: replace hose or replace fitting and O‑ring/crush washer. Often the quickest and cheapest.
- Fitting threads loose: remove, clean sealing surfaces, replace crush washer/O‑ring, torque to spec.
- Seal (shaft seal) leak at pump or motor: requires removing the component endcap or motor to access and replace the seal — moderate difficulty.
- Housing seam / gasket leak: remove cover, replace gasket or apply specified sealant, inspect mating surfaces.
- Filter or reservoir leak: tighten/replace gasket or cap, replace filter and O‑rings.
- Cooler/core leak: replace cooler or hoses.
- Extensive damage (scored shaft, cracked case, failed bearing): may need major rebuild or replacement of pump/motor/drive unit.

Step‑by‑step example repair: replacing a leaking drive motor (shaft) seal (typical mid‑level repair)
This example covers a common leak: the output shaft seal on a hydrostatic drive motor or transaxle. The exact layout differs model‑to‑model — always follow the machine-specific service manual for bolt patterns, torque values, and disassembly order.

Preparation
- Park machine on level ground, lower lift arms, chock wheels. Disconnect battery.
- Clean around suspected motor/hub area. Drain the hydraulic fluid to a level below the work area or fully drain into a waste container if instructed by manual. (You must avoid contamination and spills.)
- Place drip pan under the motor/housing.

Relieve system pressure
- Before disconnecting hydraulic lines, relieve any trapped pressure by cycling the controls with the engine off and then cracking a low‑pressure fitting slowly (wear eye protection) to confirm no pressure remains. Consult manual for recommended sequence.

Remove wheel/hub if applicable
- If the motor is behind a hub/sprocket, remove the wheel, hub, or sprocket to access the motor endplate. This typically requires removing retaining bolts and possibly loosening drive chain/sprocket. Keep parts in order.

Disconnect hydraulic lines
- Label each hydraulic line (use tape and marker) so you can reconnect correctly. Cap and plug lines and ports immediately to prevent contamination and fluid loss.

Unbolt motor or end cover
- Support the motor/assembly. Unbolt the motor endcap or remove the motor from its mount so you can access the shaft seal. Keep hardware organized for reassembly.

Access the seal
- Remove the endplate or bearing retainer as needed. The shaft seal usually sits in a bore at the end of the housing. Use a seal puller to remove the old seal carefully — avoid scratching the shaft or bore.

Inspect shaft and bore
- Rotate the shaft and inspect for grooves, pitting or scoring. Light polishing with very fine emery or crocus cloth may remove minor roughness, but deep damage requires shaft repair or replacement (or a sleeve). Also inspect bearings for play/noise — a leaking seal with bearing play often indicates both should be replaced.

Install new seal
- Lubricate new seal lip with clean hydraulic fluid or specified grease. Ensure correct orientation: the seal lip faces fluid pressure. Use a seal driver or socket of appropriate diameter and a mallet to seat the seal squarely and flush to the bore. Do not distort seal.

Reassemble bearings/retainer/housing
- Replace any O‑rings, gaskets or fasteners removed. Clean all mating surfaces and install gasket or use approved gasket maker per manual. Torque bolts to spec (consult service manual). Reinstall hub/wheel or sprocket.

Reconnect hydraulic lines and fill system
- Remove plugs and reconnect lines to their correct ports, replacing crush washers or O‑rings as required. Tighten fittings to specified torque.

Refill hydraulic/transmission fluid
- Refill to the correct level with the manufacturer‑specified hydraulic/transmission fluid. Use a clean funnel and avoid contamination. Replace filter(s).

Bleed air from system and check level
- Start engine and run at idle. Slowly cycle all drive controls back and forth to move fluid through kit and purge trapped air. Some systems require a specific bleed procedure — follow the manual. After bleeding, shut off engine and recheck fluid level; top up to the mark.

Check for leaks and test
- With machine on blocks (not under load), test controls and look for leaks at the repaired area and all fittings. If OK, do a low‑load test drive and recheck fluid level and temperature. After the first few hours of operation, recheck torque on fittings and bolts and inspect for leaks again.

Common things that can go wrong (and how to avoid them)
- Wrong seal orientation: leads to immediate leak. Avoid by verifying lip side toward fluid pressure.
- Damaged shaft/bore: if scored, new seal will fail quickly — inspect and repair shaft or install sleeve.
- Improperly seated seal or damaged seal during installation: use correct driver and don’t distort the seal.
- Reused or damaged O‑rings/crush washers: always replace these sealing elements.
- Cross‑contamination and dirty parts: dirt will damage pump/motor; keep everything clean.
- Air trapped in system: causes pump cavitation, heat, and poor drive — bleed properly.
- Wrong fluid or insufficient fluid: causes overheating and poor lubrication — use OEM spec fluid and correct levels.
- Loose bolts/fittings: torque to spec; use threadlocker where specified.
- Overpressurizing/forcing parts: don’t hammer bearings or bolts; follow service procedures.

Troubleshooting after repair
- If leak persists: double‑check origin (you might have misidentified the source), inspect the seal edge/pick‑up for sharp edges, ensure the groove and bore are clean, and verify part numbers.
- If system produces whining/cavitation: check fluid level, air ingress at suction line or breather, and the pump suction line for restrictions.
- If drive performance is poor after repair: confirm lines connected correctly (supply/return reversed will alter behavior), check for trapped air, and verify proper fluid type.

Preventive maintenance to avoid future leaks
- Replace hoses and O‑rings at scheduled intervals, inspect clamps and routing (hoses rubbing on frame cause wear).
- Keep breather clean—pressure build up can push seals out.
- Change fluid and filter at recommended service intervals to reduce internal wear and reduce seal degradation.
- Torque bolts to spec and inspect for vibration induced loosening.

Environmental and disposal considerations
- Collect used hydraulic oil in a sealed container and take to a recycling facility. Don’t dump on ground or into drains.
- Clean up spills with absorbent and dispose of contaminated rags per local regulations.

Where to get model‑specific data (must do this)
- Consult the Toyota SGK6/SDK6/SDK8 official service manual (workshop manual) for exact diagrams, torque specs, part numbers, and the correct hydraulic fluid specification. Dealer parts departments can supply genuine seals/O‑rings and part numbers. The general procedures above are correct in principle but the exact disassembly order and torques are model‑specific.

Quick checklist summary (at a glance)
- Clean and identify leak source.
- Gather correct replacement seals/hose/fittings.
- Relieve pressure, drain/contain fluid below work area.
- Replace faulty component(s) — O‑rings, hoses, shaft seals, gaskets.
- Reassemble with new seals, torque to spec, reconnect lines.
- Refill with correct fluid, replace filter, bleed air.
- Test under low load, recheck for leaks and top off level after run‑in.

Final note
- If you discover significant internal damage (scored shafts, failed bearings, cracked housings) the right solution may be a remanufactured pump/motor or professional rebuild. For first time or uncertain repairs, get the machine’s service manual and consider assistance from a trained technician to avoid costly mistakes.

That covers the practical workshop approach, component descriptions, theory, step‑by‑step seal replacement example, and troubleshooting. Follow the safety items and the Toyota service manual for model‑specific details (torques, fluid spec, sequence).
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1) Quick theory — what a planetary final drive does and what fails
- Components: sun gear (input), planet gears (three or more), planet carrier (holds planet pins), ring gear (internal), bearings, spacers/shims, seals, fasteners. The drive multiplies torque by distributing load across multiple planet teeth and bearings.
- Geometry matters: concentric alignment and correct tooth contact pattern, backlash and bearing preload determine smooth torque transfer. Wrong clearances → noise, heat, reduced life, teeth fatigue.
- Common faults and causes:
- Grinding/whine: worn teeth, incorrect backlash, misaligned gears, damaged bearings.
- Clunking/slippage: chipped teeth, excessive backlash, broken splines.
- Overheating/low efficiency: contaminated or low lubricant, high friction from incorrect preload.
- Leaks: failed seals → lubricant loss → accelerated wear.

2) Preparation and safety (order step 1)
- Safety: lock out machine, lower boom, block wheels, relieve hydraulic pressure, support machine securely.
- Tools: basic hand tools, pullers, soft mallet, press, dial indicator, torque wrench, micrometer/calipers, gear marking compound, feeler gauges, bearing race gauges, cleaning solvent, replacement parts, new seals, correct lubricant.
- Theory: removing power and supporting load prevents accidental motion; correct tools let you measure geometry precisely.

3) Initial diagnosis and baseline checks (order step 2)
- Run test (if safe): note noise character, rpm range, temperature, oil level, leaks.
- Check oil condition: metallic particles in fluid mean gear/bearing wear.
- Measure play at output shaft with dial indicator (axial/radial). Note approximate backlash/noise frequencies.
- Theory: symptoms + fluid condition help isolate planet vs input/hub vs hydraulic issues — metal in oil indicates internal wear; specific noises at certain load/rpm often indicate gear mesh or bearing defect.

4) Remove drive assembly from machine (order step 3)
- Drain fluid into a clean container for inspection.
- Disconnect hydraulic lines, remove guards, slacken and remove output coupling, remove final drive from wheel/swing assembly following the workshop layout.
- Theory: clean working environment reduces contamination on reassembly; draining preserves fluid sample and prevents contamination.

5) Disassemble planetary final drive (order step 4)
- Note or photograph assembly orientation and shim stack positions.
- Remove ring gear / housing cover, carrier, planet gears (press out planet pins), sun gear, bearings, seals, shims.
- Keep components organized and flagged (which side, which position).
- Theory: accurate reassembly requires preserving original shim/stack order to restore geometry; removing parts carefully prevents secondary damage.

6) Clean and inspect every component (order step 5)
- Clean parts with solvent; inspect gear teeth for pitting, spalling, scoring, chipped teeth.
- Inspect bearings: race discoloration, brinelling, rough rotation, play.
- Inspect splines and shafts for wear and fretting.
- Check seals, O-rings, housings for distortion or scoring.
- Measure:
- Tooth wear: depth of root wear, chipped count.
- Backlash: measure gear tooth-to-tooth clearance (bench with dial indicator).
- Bearing internal clearance and race diameters (micrometer).
- Runout/shaft straightness.
- Theory: replacement threshold is when wear exceeds service limits or when bearings show false brinelling/rounded rollers because continued use will propagate failure; geometry changes (worn teeth or worn races) shift contact patterns and exacerbate wear.

7) Decide repair vs replace (order step 6)
- Replace any planet or ring gear with broken/chipped teeth, significant pitting, or wear beyond limits.
- Always replace bearing sets and seals when disassembling — bearings commonly fail and are cheap insurance.
- If carrier or ring is cracked or heavily worn, replace the whole carrier/ring assembly.
- Theory: replacing only bearings while leaving worn gears means geometry remains wrong; full restoration requires geometry (shims/gears) + fresh bearings/seals.

8) Prepare components for reassembly — races, bearings, and shims (order step 7)
- Fit new bearings to correct races using a press and heating if required; install without damaging cages.
- Clean and inspect shims; replace any deformed shims.
- Theory: bearings must seat without press-induced stress; shims control tooth backlash and bearing preload — they are the geometric correction elements.

9) Set gear mesh and backlash (order step 8)
- Reassemble carrier with planets and sun gear loosely.
- Use dial indicator to measure ring-to-sun backlash. Adjust with shims (typically between carrier and housing or ring and housing depending on design) to reach nominal backlash per the workshop manual.
- Use gear marking compound to rotate and observe contact pattern on gear teeth:
- Ideal: contact centered across face width and middle of tooth height.
- If contact is toward face or root, adjust shim stack accordingly.
- Iterate: change shims, recheck backlash and contact until correct.
- Theory: backlash determines clearance between mating teeth. Too tight → excess heat and bearing overload. Too loose → impact loads, noise, and progressive wear. Contact pattern ensures load is distributed across tooth face, preventing localized pitting.

10) Set bearing preload (order step 9)
- After backlash and contact are correct, set bearing preload using the specified method for the unit (nut torque or shim stack). Measure:
- Axial endplay (if spec'd) or rotational torque at the carrier.
- Bearing preload should be within service spec: smooth rotation, specified torque/axial play.
- Theory: correct preload keeps gears in correct mesh under load and prevents fretting. Under-preload = endplay and shock loads; over-preload = heat, high friction, premature bearing failure.

11) Replace seals, assemble final housing (order step 10)
- Fit new seals and O-rings using correct assembly lube.
- Torque fasteners to spec (use manual values).
- Ensure breathers/plug threads are clean and sealed.
- Theory: seals prevent lubricant loss; proper torque ensures flange faces remain flat and shims remain seated — both critical to preserve alignment.

12) Bench test and verification (order step 11)
- Spin assembly by hand and with low-speed motor: check for binding, smooth rotation, no abnormal noise.
- Recheck backlash, contact pattern, and bearing preload after a short run-in.
- Inspect for leaks from seals.
- Theory: bench spin catches assembly errors (binding, missing shims, misfits) before reinstalling under load. Small adjustments here avoid catastrophic failure in service.

13) Reinstall to machine and fluid fill (order step 12)
- Refit final drive to axle/hub, reconnect couplings, torque bolts to spec.
- Refill with correct lubricant to correct level, using filtered fluid.
- Bleed hydraulic circuits if required.
- Theory: correct lubricant reduces friction, carries heat, and suspends wear particles. Tight bolts and correct fluid ensure final drive operates under intended conditions.

14) Functional test and break-in (order step 13)
- Run at idle and then progressively increase load, listening for noise, watching temp and leaks.
- After a short running period, stop and re-torque external fasteners and re-check oil level.
- Reinspect oil for metal shavings after initial run—if present, re-open and inspect.
- Theory: initial run reveals any residual misalignment or contamination; early re-check prevents accelerated wear.

15) How each repair action fixes the faults (concise)
- Replacing worn gears removes weakened tooth geometry that created impact loads and noise.
- Replacing bearings removes play, restores smooth rotation, and prevents uneven tooth loading.
- Correct shimming/backlash restores tooth contact pattern so load is shared across the tooth face, reducing stress concentrations and pitting.
- Correct bearing preload ensures gears remain in mesh under load and reduces fretting and endplay.
- New seals and fresh lubricant stop leaks and restore lubrication film preventing adhesive wear and overheating.
- Thorough cleaning removes abrasive contaminants that cause accelerated wear.

16) Common pitfalls to avoid (order step 14)
- Don’t mix used and new gears/bearings without correcting shims — geometry changes.
- Don’t reuse damaged seals or deformed shims.
- Don’t guess torque/preload/backlash — use measurements and manual specs.
- Avoid contamination during reassembly — even small metal particles cause rapid failure.

17) Final notes
- Use the Toyota SGK6/SDK6/SDK8 workshop manual for exact service limits (backlash, preload, torque, shim part numbers) and parts lists.
- If multiple planets show uneven wear but ring/sun are fine, replacing entire set or carrier is often best to maintain geometry.

End.
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