Summary (one line)
- Replacing the planetary gear set in an automatic transmission on a Mitsubishi 6G72-powered car is a medium‑to‑advanced job: it requires a lift or strong jack and stands, a transmission jack, the correct service manual (for exact specs), careful labeling, cleanliness, special tools, and patience.

Why this repair is needed — theory and symptoms
- Purpose of planetary gears: In an automatic transmission the planetary gear set(s) create the different forward/reverse ratios by changing which element (sun, planets/carrier, ring) is held, driven, or allowed to coast. Analogy: think of a solar system — the sun (sun gear) in the center, planets (planet gears) on a carrier, and an outer orbit (ring gear). By holding one “celestial body” still and driving another, you change the resulting motion/ratio of the remaining part.
- Why they fail: metal fatigue, chipped or worn gear teeth, failed needle bearings, broken planet pins, worn thrust washers, or damage from contaminated fluid/overheating. Symptoms that point to planetary set damage: slipping under load, harsh or jumped gears, grinding or whining noises that track to one gear range, loss of particular gears, debris in the pan/magnet, or immediate catastrophic failure (metal shavings, complete loss of forward/reverse).
- Why replace rather than repair: planetary sets are precision assemblies; once bearings/washers or teeth are damaged, other parts (clutches, drum bores, shafts) can be affected; replacement ensures proper gear ratio accuracy and avoids repeated failures.

Core components — what each part is and does
- Sun gear: central gear. Drives/receives motion depending on which element is driven/held. Often splined to input/shaft components.
- Planet gears (pinions): small gears that orbit the sun gear on pins. They share the load and change ratio. Each has needle/roller bearings on the pin.
- Planet carrier (planet carrier/arm): holds the planet pin shafts and transfers torque to the output shaft or drum.
- Ring gear (annulus): large outer gear with internal teeth. Often pressed into a drum or housing.
- Planet pins / pin bore: pins the planet gears rotate on; they ride in the carrier and have needle bearings or bushings.
- Needle/roller bearings: allow planet gears to spin freely on their pins. Failure here leads to heat and heavy wear.
- Thrust washers/shims: thin plates controlling axial clearance between gears and carrier; critical for preload and endplay control.
- Snap rings / circlips: locate and retain the planet carrier, ring, and other assemblies axially.
- Sun shell/hub / sun splines: interface parts that transfer torque; worn splines cause slip/noise.
- One‑way clutch (sprag/roller): often integrated near planetary sets; allows freewheeling in one direction and locks in the other.
- Clutch packs & drums adjacent to planetary sets: friction plates that lock a component to the input or casing to achieve held condition. Damaged clutches can mimic planetary failure.
- Transmission case, input shaft, output shaft, pump: surrounding hardware that must be removed to access planetary gears.

Tools & supplies (typical)
- Full metric socket set, torque wrench, breaker bar, ratchets
- Transmission jack or sturdy floor jack and transmission support
- Engine support or engine hoist if required
- Snap ring pliers, circlip set
- Seal driver / bearing driver, drift punches
- Gear puller, bearing puller, press (for bearings/pins if needed)
- Dial indicator and magnetic base (to check backlash/endplay)
- Feeler gauges, micrometer/calipers
- Parts trays, labels, marker, camera (for documentation)
- Clean rags, solvent, assembly lube, new transmission fluid (manufacturer spec)
- Replacement planetary gear set or rebuild kit (gears, bearings, thrusts, snap rings), new seals/gaskets, torque converter gasket if applicable
- Service manual for the exact transmission (pressures, disassembly order, shim thicknesses, torque specs)

Step-by-step overview (big-picture)
Note: every automatic transmission model differs in detail. The sequence below is a generic, bench‑level procedure that applies to most Mitsubishi automatic units used with the 6G72. Always verify with the factory service manual for your exact vehicle/transmission.

A. Preparation and safety
- Work on level ground. Use wheel chocks. Disconnect negative battery terminal.
- Raise vehicle safely (2-post lift preferred) or jack and support on stands; ensure engine is supported if removing transmission.
- Have a large drain pan and shop towels ready. Wear gloves and eye protection.

B. Remove transmission from vehicle
- Drain ATF from pan; remove transmission cooler lines (catch fluid), electrical connectors, speedo cable/sensor, shifter linkage, starter, and exhaust components as needed for clearance.
- Support the transmission with a transmission jack, remove transmission mount(s), and unbolt bellhousing bolts to the engine. If 4WD, disconnect transfer case/drive axles as required.
- Carefully separate transmission from engine and lower out of the vehicle with the jack. Keep torque converter in place if unsure — but note the transmission must be pulled far enough to access bolts to the flexplate if removing converter.

C. Bench disassembly to access planetary set
- Clean the exterior before opening to avoid contamination.
- Remove the torque converter from the input shaft (drain any fluid, unbolt if attached to flexplate and removed).
- Remove the transmission pan and filter. Note debris on magnet — metal flakes indicate internal damage.
- Remove valve body (label each bolt and location); set aside on clean rack. Be careful with check balls and springs — document positions.
- Remove servo covers, bands if blocking access, and valve shafts if necessary per manual.
- Remove front pump, then remove retaining rings/snap rings and any external drum pieces to expose planetary drums.
- Remove the drum or ring gear assembly to reveal planet carrier. Snap ring(s) and bolt(s) usually retain the carrier.

D. Remove planetary gear assembly
- Carefully remove snap rings/circlips holding planet carrier/ring. Keep track of orientation and order.
- Withdraw the planet carrier assembly. If the planet pins are pressed in, use a press or puller per manual.
- Remove planet gears and inspect rollers/needle bearings and planet pins.
- Remove the sun gear and ring gear assemblies as called for by the model. Remove thrust washers/shims and note their thickness/order.

E. Inspection (critical)
- Inspect all gear teeth for pitting, chipped/rounded teeth, or discoloration from heat.
- Check needle bearings for flats, discoloration, or seizure. Replace if any wear.
- Measure backlash between ring and planet/sun gear with a dial indicator — compare to spec. Also measure endplay and compare to service manual.
- Check thrust washer wear (look for scoring/grooves). Replace damaged washers and adjust with shims as per specification to achieve correct clearance.
- Inspect mating drums and bores for scoring. Replace or machine as required.
- Check the one‑way clutch (sprag) operation; replace if sticking.

F. Replacement and reassembly (bench)
- Clean all parts thoroughly with solvent and dry. Use lint‑free towels.
- Install new bearings/needles where required (use press if needed), install new thrust washers and shims per manual to achieve specified clearances. Use assembly lube where applicable.
- Install the sun gear, planet gears (with correct bearings/rollers), and carrier. Make sure planet pins are secured (bolts, retaining rings) properly and torqued to spec.
- Reinstall snap rings and verify they are fully seated in their grooves (use feel and visual check).
- Verify rotational play/backlash and axial endplay with a dial indicator and compare to spec. If out of tolerance, adjust shim stack or replace worn parts — do not force tight fits.
- Reinstall ring/drums and clutch packs, replacing friction plates if worn. Replace seals and gaskets with new.
- Reinstall pump and torque to spec, making sure the pump seal/pump alignment is correct (misalignment destroys pump).
- Reinstall valve body carefully, ensuring check balls/springs are in the correct locations. Torque bolts in the proper pattern.
- Reinstall torque converter (apply fresh ATF inside, ensure proper seating into pump — rotate and push until it engages fully) and bolt to flexplate if removed.

G. Reinstall transmission into vehicle
- Lift transmission to align with engine and slide into bellhousing; ensure torque converter engages the pump and input shaft properly. Torque bellhousing bolts to spec.
- Reconnect mounts, crossmember, driveshafts/cv axles, cooler lines, electrical connectors, shift linkage, starter, exhaust, etc.
- Refill transmission with the specified ATF to the proper level (start with manufacturer cold fill amount then check level with engine running and warm per manual).

H. Testing and break-in
- With the car on stands, start engine and cycle through gears (P–R–N–D–2–1) letting each engage for a few seconds — check for leaks and smooth engagement.
- Check ATF level at operating temperature and top off as needed.
- Road test gently at first: check shift timing, firmness, and listen for noises. Recheck fluid level and inspect for leaks after a brief drive.
- Change fluid/filter again after recommended break-in interval if specified.

Common things that go wrong (and how to avoid)
- Not using the service manual: leads to wrong shims/tolerances — always follow manual for clearance specs and torque.
- Improper snap ring seating: causes catastrophic shift problems or parts coming loose — always double‑check rings are fully engaged.
- Fail to replace bearings/thrust washers: new gear will wear quickly on old, worn washers; always replace wear items included in rebuild kit.
- Contamination: dirt/grit left in assembly causes rapid failure — work in a clean area and wash parts thoroughly.
- Misaligned pump/incorrect torque converter seating: pumps get starved and fail quickly — ensure proper seating and fluid filling procedure.
- Wrong seal orientation or reused brittle seals: leads to leaks — replace all seals/gaskets and use correct installation tools.
- Reinstalling with damaged drum bores/clutch surfaces: clutch slippage or binding — inspect and replace drums or clutch packs as needed.
- Incorrect fluid: use only the specified ATF for the transmission; wrong fluid causes bad shifting and clutch wear.

Tips for a beginner mechanic
- Photograph everything before and during disassembly so you can replicate orientation.
- Bag and label every bolt and small part — butt a bolt hole and bag the bolt that belongs there.
- Buy a rebuild kit rather than piecemeal parts — this will include snap rings, bearings, thrusts and is designed to work together.
- If you don’t have a press or specialty tools, consider having bearings/pins pressed by a machine shop.
- If you’re unsure about measuring or adjusting shims and endplay, have a transmission shop set final clearances — mistakes are expensive.
- Consider replacing torque converter (or at minimum inspect) when the planetary set has failed; converter can be contaminated with metal debris.
- After test drive check pan magnet — heavy new debris indicates further unseen damage.

When to call a pro
- If you don’t have a transmission jack and a lift, or you’re not comfortable indexing the torque converter to the pump and checking internal clearances.
- If dial indicator measurements and shim calculations are unfamiliar. Proper endplay/backlash setup is critical.
- If you find extensive damage to drums, shafts, or housings — machining or professional diagnosis may be required.

Final note (must‑do)
- Get the exact factory service manual for your vehicle/transmission. This guide gives the conceptual flow and practical warnings; the manual provides the exact order, bolt torques, shim charts, and tolerances you need to do the job correctly and safely.

No further questions asked as requested.
rteeqp73

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