Applicable scope: this procedure describes replacing synchronizer rings (synchros) in a manual-transmission Toyota 4Runner (5-speed family found on many generations). If you have an automatic transmission this does not apply.

Tools and consumables
- Basic hand tools: socket/ratchet set (incl. 10–32mm), combination wrenches, torque wrench.
- Impact wrench and breaker bar (helpful).
- Transmission jack or floor jack + transmission support.
- Engine support bar or support jack under oil pan (to support engine when trans removed).
- Jack stands and wheel chocks.
- Drain pan, shop rags, parts tray/labeling tape.
- Snap‑ring (circlip) pliers (internal & external).
- Bearing puller / gear puller.
- Hydraulic press or arbor press.
- Soft‑face hammer, brass drift set, punches.
- Seal drivers and driver set.
- Dial indicator with magnetic base.
- Feeler gauges and a set of shims (may be needed).
- Torque angle gauge (if required by spec sheet).
- Cleaning solvent (parts washer or brake cleaner) and compressed air.
- RTV gasket maker or new case gasket (if applicable).
- New transmission fluid (Toyota spec).
- New replacement parts: synchronizer rings (all worn rings), synchronizer hubs/sleeves if scored or worn, mainshaft/input shaft bearings (recommended to replace), thrust washers, snap rings, shift forks or fork bushings if worn, output/input seals, case gasket or RTV. Always buy OEM or high-quality aftermarket rebuild kit for your exact transmission model.
- Safety PPE: gloves, eye protection, hearing protection.

Safety precautions (non-negotiable)
- Work on a flat, level surface. Chock wheels and set emergency brake.
- Disconnect negative battery terminal.
- Use proper jack stands — never rely on a jack alone.
- Support engine when transmission is removed; otherwise the engine can tilt.
- Use a transmission jack to lower/raise the gearbox.
- Keep fluids away from hot surfaces and dispose of old fluid legally.
- Use eye protection when pressing or removing snap rings and springs.

Preparation / initial steps
1. Drain transmission fluid while car is elevated.
2. Disconnect battery negative.
3. Remove driveshaft(s) and transfer case (if 4WD). Label and mark orientation of driveshaft and transfer case to transmission.
4. Disconnect linkage, speedometer cable/sensor, electrical connectors, shifter assembly, starter (if required), coolant lines (if any), and any crossmembers obstructing removal.
5. Support transmission with jack, remove transmission-to-engine bolts, separate gearbox from engine (may need gentle pry). Support engine with support bar or jack under oil pan during separation.

Transmission removal and teardown
6. Lower and remove the transmission from vehicle to bench. Clean exterior before opening to prevent contamination.
7. Place transmission on bench. Remove tail housing and top cover(s) to access shift forks and rails.
8. Note/photograph gear order, orientation, and positions. Keep all related parts grouped and labeled as removed.
9. Remove shift rails and shift forks carefully, noting which fork corresponds to which gear. Inspect fork ears and selector keys for wear.
10. Remove snap rings retaining gears and bearings. Use circlip pliers; keep rings organized.

Accessing synchros
11. Remove mainshaft and input shaft assembly from case. This usually requires removing output shaft parts and sliding gears off. Use a gear puller or press as needed.
12. With shafts out, remove synchronizer assemblies from the mainshaft: slide off the hub & sleeve, then remove the synchro ring (wear ring) and any keys/springs.
13. Inspect synchronizer components: brass (or composite) synchro rings should have mating friction surface intact and matching wear pattern. Rub any scoring, broken teeth, or missing teeth. Replace rings showing glazing, broken or rounded teeth, or severe wear. Also inspect hub/sleeve teeth for wear—if dog teeth are rounded replace hub/sleeve assembly.
14. Inspect mating gear cones (the conical surface on gear that the synchronizer rings mate to). If cones are deeply scored or rough, they must be replaced or reground per manufacturer; a new synchro ring on a damaged cone will wear immediately.

Parts replacement strategy
15. Replace:
- All worn or damaged syncro rings (recommended to replace all rings on that shaft when one is bad).
- Hubs/sleeves if dog teeth are worn or excessive play is present.
- Needle/roller bearings and races if pitted or noisy.
- Input/output seals and case gasket.
- Thrust washers and shims as required.
- Shift fork bushings if worn (wear here will cause misengagement and fast syncro wear).
16. Use a rebuild kit specific to your transmission model whenever possible; kits often include rings, springs/keys, snap rings, and bushings.

How to remove/install synchro rings and hubs (tool use and technique)
17. Removing:
- Use snap‑ring pliers to remove circlips retaining gears or hubs.
- Use a bearing/gear puller to coax gears off the shaft, or a press to push off hub assemblies.
- Keep shafts upright and supported to avoid bending or damaging splines.
18. Installing new synchro rings:
- Clean shaft and mating gear cone thoroughly; deburr light roughness.
- If rings are brass, many mechanics warm new rings slightly (in hot water or oven to 120–150°F / 50–65°C) to ease installation — do NOT exceed manufacturer guidance. Alternatively, use an arbor press to press rings onto the gear cone squarely. Use a soft sleeve or block to protect ring faces.
- Ensure ring is oriented correctly (some rings have orientation marks).
- Install hub and sleeve over the ring; slide on until engaging teeth. If press-fit, use arbor press with even force—do not cock the hub.
- Reinstall circlips/snap rings in their grooves; ensure they seat fully.
- Reinstall any keys/springs into the hub per orientation and tension per spec.

Reassembly and measurement
19. Reassemble mainshaft/input shaft and bearings into case. Replace bearing races and seals using drivers.
20. Check endplay (mainshaft axial endfloat) with dial indicator once shafts installed. Compare to factory spec and add/remove shims as required to obtain correct endplay. Common pitfall: incorrect endplay leads to premature wear and gear misalignment.
21. Check synchronizer engagement: sleeve should shift smoothly; no binding across gears. Check free play and fork alignment.
22. Torque all bolts to Toyota factory specifications during reassembly (engine/transmission bolts, case bolts, etc.). If you don’t have specs, obtain the factory service manual for your year/model — do not guess torque.

Final steps
23. Reinstall transmission into vehicle, reconnect bellhousing bolts and torque to spec.
24. Reattach transfer case/driveshafts, linkage, electrical connectors, and crossmembers. Refill with specified Toyota manual transmission fluid to correct level.
25. Test operation: with vehicle supported, cycle through gears while engine at idle to verify smooth engagement. Road test carefully — check for noise or grinding. Recheck fluid level after test.

Common pitfalls to avoid
- Not replacing damaged cones or hubs: new rings on damaged cones will fail quickly.
- Reusing worn bearings/seals/circlips — cheap parts that cause big issues when reused.
- Contaminating internals with dirt: keep everything immaculately clean.
- Not supporting engine or transmission properly — risk of injury and component damage.
- Forcing parts without press or heating—don’t hammer gear teeth; use proper press or heating methods and protect components.
- Skipping endplay/shim checks — incorrect endplay causes bearing overload and synchro failure.
- Reusing shift forks or bushings that are worn — misalignment leads to poor engagement and stripped dog teeth.
- Forgetting to torque bolts to spec: can cause leaks, misalignment, or case distortion.

Notes from an experienced tech
- Replace synchros in matched sets where practical. If gear engagement is poor on multiple gears, likely hubs, sleeves, or forks are worn.
- A full transmission rebuild kit and OEM service manual for your exact year/model will save time and mistakes.
- If you don’t have a press or gear puller, don’t improvise with heat/hammering on the vehicle — better to remove to a shop that has proper tooling.

Done. Follow the factory service manual for torque values, clearances, and any model‑specific procedures.
rteeqp73

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