Short version first: synchronizers (synchros) let gears engage smoothly by matching shaft speeds. Replacing them requires removing the transaxle, disassembling the gearbox to the main/lay shafts, swapping the worn synchro rings/hub/sleeve (and usually bearings/seals), reassembling with correct shims/preload/backlash and torques, then testing. Below is a complete, beginner-friendly (but thorough) walk-through: theory, components, tools, step-by-step procedure, inspection criteria, what can go wrong, and safety notes. Read it through before you start and have the factory service manual for the YD22DDTi transmission handy for all torque and clearance specs.

1) Theory — why synchronizers exist and how they work (analogy)
- Purpose: A manual gearbox has several fixed-speed gear pairs. To move a gear into mesh without grinding, that gear’s rotating speed must be matched to the shaft you’re about to connect it to. The synchronizer matches rotational speeds before the dog teeth make the final positive engagement.
- How: The synchronizer assembly sits between two gears on the main (output) shaft. It consists of a hub (splined to the shaft), a sliding sleeve (also called the collar) that moves on the hub, and a synchronizer (baulk) ring — a friction ring with a tapered (conical) friction surface. When you shift, the sleeve moves and first presses the synchro ring onto a cone on the gear, using friction to equalize speeds. Once speeds match, the sleeve continues and its internal dogs slide over the gear dogs and locks the gear to the shaft.
- Analogy: Think of two spinning bicycle wheels you want to clip together: the synchro is like a temporary hand that first brings the second wheel up to the exact speed of the first, and only after they match does a clamp lock them together. If that “hand” is worn, you try to clamp mismatched speeds and the teeth grind.

2) Symptoms of worn synchros
- Grinding when selecting a gear (especially 2nd or 3rd) or while shifting into first or reverse.
- Need to blip the throttle to downshift (you used to be able to downshift without rev-matching).
- Hard or notchy shifting, gear popping out, or inability to engage a gear without rocking the car.
- Metal particles in gearbox oil or heavily contaminated fluid.

3) Major components you will see/handle (detailed descriptions)
- Gearbox/transaxle housing (case): cast housing that contains shafts, bearings, and gears.
- Input shaft: connected to the clutch/drive; it transfers engine torque into the gearbox.
- Main (output) shaft: the shaft that ultimately sends rotation to the driveshafts; has splines and dog teeth.
- Lay/Countershaft: carries the mating gears that mesh with the main shaft; gears are usually fixed to this shaft.
- Individual gears: machined toothed wheels. Each gear has:
- Gear teeth (drive teeth)
- A cone or friction surface (for the synchro ring to bite on)
- Dog teeth (for final mechanical engagement)
- Synchronizer assembly:
- Hub: splined to the main shaft; the sleeve slides on it but the hub remains fixed axially.
- Sleeve (sliding collar): moves left/right to engage gears; contains dogs that mate with gear dogs.
- Synchronizer (baulk) ring: a friction ring with internal taper that mates with the gear’s cone. Often replaceable; commonly made of brass, steel, or a composite friction material.
- Return springs / small keys: some synchros have spring pieces to help alignment; be careful—small parts can fall out.
- Circlips / thrust washers: retain components axially.
- Bearings & races: support shafts; often needle or tapered roller bearings.
- Selector forks and rails: move the sleeves; forks can bend/wear.
- Selector mechanism/detents: ensures the shift lever snaps into gear positions.
- Seals and gaskets: front/rear seals, output seals, case gaskets.
- Fasteners, shims, spacers: set endplay and gear mesh.

4) Tools, supplies, safety equipment
- Tools: metric socket set, combinations, extensions, breaker bar, torque wrench, snap ring/circlip pliers, bearing puller, gear puller, press or arbor press, soft mallet, drift punches, screwdriver set, picks, seal driver, pick, clean rags, parts cleaner, bench vise (or gear press), dial indicator, micrometer/caliper, feeler gauges.
- Lifting/support: jack, sturdy jack stands, transmission jack or engine support bar where needed.
- Supplies: new synchro kit(s) for the affected gears (buy OEM or high‑quality), full new bearings/seals (recommended), new gearbox oil (spec per manual), gasket maker or new gaskets, clean trays for parts, assembly lube.
- PPE and safety: safety glasses, gloves, avoid loose clothing. Disconnect battery. Support engine/transmission weight—never rely on a jack alone.

5) High-level procedure (step-by-step)
Note: This is comprehensive but not a substitute for the factory manual. Follow the manual for model-specific fastener torques and clearances.

A) Preparation and removal
- Park safely on level ground. Chock wheels, disconnect the negative battery cable.
- Lift vehicle and support securely on jack stands.
- Remove components that block gearbox removal: undertray, airbox, battery, possibly engine mounts, exhaust parts, driveshafts (CV axles), starter, axles, clutch slave cylinder (or remove hydraulic line), shift linkage, speedometer cable/sensor, wiring harnesses, and any indicator brackets.
- Drain gearbox oil into a clean pan.
- Support engine/transmission. Remove transmission mount bolts and any engine supports preventing transaxle removal.
- Unbolt gearbox from engine (bellhousing bolts) and carefully lower the gearbox using a transmission jack. Keep clutch/pressure plate assembly orientation in mind. For a YD22DDTi FWD transaxle you will typically separate gearbox from the engine, taking care with the input shaft and the clutch housing.

B) Clean bench and initial inspection
- Clean external dirt before opening to avoid contamination.
- Photograph and label linkages/shims as you go for correct reassembly.
- Remove external covers (inspection/side covers) and release selector rails so you can take the gearbox apart methodically.

C) Disassembly to expose synchros
- Remove output shafts, countershaft retainers, and bearing caps to remove shafts as assemblies.
- Carefully mark the orientation of every gear and washer/spacer as you remove them (order matters).
- Remove snap rings/circlips that hold hubs/sleeves in place—note their orientation.
- Extract the hub/sleeve assemblies and the synchro rings. Take care: synchro rings often have a specific left/right or direction; note orientation and condition.
- Keep all small parts (springs, keys, detents) in labeled trays.

D) Inspection (this is critical)
- Synchro rings: check friction surface for glazing, grooves, or excessive wear; replace if any wear or if thickness below spec.
- Dog teeth: look for mushrooming, chipping, rounding; damaged dog teeth require gear or hub replacement.
- Gear cones: inspect the cone surface where the synchro ring contacts; scoring or hard spots indicate replacement or regrinding (usually replacement).
- Bearings: check for play or roughness—replace suspect bearings.
- Shafts: check splines and journals for wear or damage.
- Shift forks: inspect for wear where they contact the hub; bent forks cause misalignment.
- Circlips/shims: check for deformation; replace those that are not perfect.
- Case: inspect mating surfaces and threaded holes for damage.

E) Decide repair scope & parts
- Replace synchro rings, and commonly the hub/sleeve if heavily worn. In practice, many shops replace a full synchro kit per gear pair and often bearings/seals.
- If dog teeth or gear cones are damaged, you must replace the gears or the entire shaft assembly.
- Replace bearings and seals as needed—do not reuse bearings if you plan to rebuild.

F) Reassembly (critical points)
- Clean every part in parts washer; dry thoroughly.
- Install new bearings/seals and press shafts back into the case. Use correct tool to avoid bearing preload errors.
- Reassemble lay and main shafts in the exact reverse order: gears, spacers, washers, synchronizer hub, ring and sleeve—follow original orientation.
- Install new synchronizer rings with their tapered face toward the mating cone. Pay attention to timing notches or asymmetric features—some rings only fit one direction.
- Replace any spring pieces and circlips with new if recommended.
- Set endplay and gearbox backlash: install bearing caps and tighten to the specified torque pattern. Use a dial indicator to measure gear backlash between the mating gear pair. Adjust shims per factory spec until backlash and gear mesh are within limits.
- Check mainshaft endplay with a dial indicator and adjust using correct shims or spacers per manual.
- Reinstall selector forks and confirm the sleeve moves freely and aligns with selector rails. Confirm no binding.
- Torque all bolts to the factory values specified in the service manual.

G) Final assembly and testing
- Reinstall gearbox onto engine, aligning input shaft into clutch splines. Torque bellhousing bolts to spec.
- Reattach mounts, linkage, driveshafts, exhaust, wiring, and sensors.
- Refill gearbox with the correct oil type and capacity specified for the YD22DDTi transaxle.
- Start engine and test gear engagement (on stands) using the clutch; ensure smooth selecting through all gears.
- Road test with care; listen for noises, check for leaks, and verify shift quality.

6) What can go wrong (and how to avoid it)
- Contamination during rebuild: dirt is fatal to synchro fit. Work in a clean area and keep parts covered between steps.
- Missing orientation/parts: losing or flipping a synchro ring or spacer will cause poor mesh or immediate failure. Mark and photograph every step to avoid mistakes.
- Incorrect shims/preload/backlash: improper bearing preload or gear backlash causes noisy gears, early failure, and potential catastrophic damage. Use the manual’s shim chart and measure with a dial indicator and micrometer.
- Reusing worn parts: reusing worn rings, bearings, or seals shortens life. Replace what's clearly worn; if in doubt, replace.
- Bend or damage shift forks: forks must be straight; a bent fork causes misalignment and extra wear—replace if worn.
- Over/under-torque fasteners: either can cause thread damage or parts to come loose. Use a calibrated torque wrench and factory specs.
- Dog tooth damage: if gear teeth are damaged and you simply replace rings, the next shift will fail. Inspect dogs/grooves carefully.
- Seals leaking after reassembly: ensure seals are installed square and that shaft surfaces are undamaged. Replace seals rather than reusing.

7) Recommended best practices
- Replace all synchro rings for a gearset at once (e.g., 1–2–3 or all gears as needed) to avoid mixed wear rates.
- Replace bearings/seals at the same time—cheap insurance.
- Use OEM or quality aftermarket parts; cheap rings wear fast.
- Keep a clean, organized bench and bag/label every part.
- Work with the factory service manual specifying torque values, shim sizes, bearing preload, endplay, and backlash.
- If you don’t have a press or skill to measure backlash/endplay, consider sending shafts to a machine shop or paying a shop to press bearings and set clearances.
- Consider replacing clutch components while the gearbox is out (clutch disc, pressure plate, release bearing) since the labor is already done.

8) Final checks and break-in
- After reassembly and refilling, run the car through a break-in: avoid hard shifts for the first few hundred miles, drive gently and avoid heavy loads. Check the fluid level and for metal particles after initial run hours.

9) Time and difficulty estimate
- For a beginner, expect this to be a multi-day job. If you’re doing it for the first time, plan 2–5 full days including cleaning and measuring time. If you lack tools for pressing bearings or measuring backlash, budget for shop assistance.

Summary checklist before starting:
- Factory service manual for YD22DDTi to hand (torques & clearances)
- Full tool set + transmission jack and press or access to a shop press
- New synchro kit(s), bearings, seals, gaskets, and gearbox oil
- Clean workspace, trays, labels, camera for photos
- Safety gear and an extra person for handling heavy components

This is a complete, practical overview. Follow the factory manual for all numerical specs and use care—synchro replacement is repairable by a careful DIYer, but precise measurement and cleanliness are crucial to prevent repeat failure.
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