Short overview — why this repair is needed (theory)
- Purpose: A gearbox synchronizer (synchro) equalizes the speeds of the gear you want and the shaft before the dog teeth engage, so changes of gear are smooth and without grinding.
- How it works (quick): When you select a gear, the shift fork moves the synchro sleeve onto a hub. The sleeve first presses a conical blocking (baulk) ring against the gear’s cone. Friction between cone surfaces slows or speeds the gear to match the shaft. When speeds match, the sleeve slides further and locks the gear with dog teeth. Think of it as two people walking together before grabbing hands — the ring holds one person until they match pace, then the sleeve connects them.
- Why it fails: Synchro components wear from heat, contamination, wrong oil, aggressive shifting, or broken parts. Typical symptoms: grinding on upshift/downshift, difficult or “notchy� shifts, gear popping out, or needing rev-matching to shift.

What a synchronizer assembly is — every component (plain-language descriptions)
- Mainshaft (output shaft): The shaft that carries gears to the gearbox output. Synchros mount here.
- Gear (gearwheel): Splined on the shaft or on a hub; has a cone-shaped friction surface (synchro cone) on the gear or hub.
- Synchro hub (hub): A discrete piece splined to the mainshaft; it’s the stationary central part that the sleeve rides on.
- Synchro sleeve (sliding collar / blocker sleeve / selector sleeve): Steel sleeve with internal dogs that slides laterally over the hub to engage the gear’s dog teeth when speeds match.
- Blocking (baulk) ring / blocker ring / friction ring: A ring with a tapered internal cone that mates with the cone on the gear/hub. Often made of brass/bronze or composite; provides friction to match speeds and prevents sleeve engagement until speeds are synchronized.
- Spring / spring clips (synchro springs / circlips): Small springs or wave-retainers that hold the blocking ring to the hub and keep correct axial position and pre-load.
- Dog teeth: The actual teeth on the gear hub and sleeve that lock the gear to the shaft when engaged.
- Keys / splines: The splines that allow torque transfer and the keys that locate parts.
- Shift fork & selector rod: Fork moves the sleeve; selector rails guide the fork.
- Bearings, thrust washers, circlips and spacers: Keep axial clearance and support loads.
- Gaskets and seals: Keep oil contained and protect internals.

Tools & consumables you will need
- Full metric hand tool set (sockets, wrenches, screwdrivers).
- Torque wrench (capable to the gearbox’s spec).
- Gearbox jack or transmission jack; engine support if needed.
- Screwdrivers, pry bars, hammer, soft-faced mallet.
- Circlip pliers, snap ring pliers.
- Slide hammer / puller set for hubs/gears.
- Bearing puller and/or hydraulic press (for bearing removal/fit).
- Dial indicator and magnetic base (measure endfloat/runout).
- Feeler gauges / calipers / micrometer (measure wear limits).
- Bench vise with soft jaws.
- Clean workspace, trays for fasteners, labels/marker to mark parts.
- Service manual for the exact gearbox (for torque specs, exploded diagrams, clearances).
- New synchronizer kit (blocking rings, sleeves, springs, hub if required) for the specific gearbox model.
- New bearings/seals (recommended to replace any open bearings or oil seals while in).
- Manufacturer-specified gearbox oil (consult manual).
- Solvent / degreaser, lint-free rags, assembly lubricant (light gear oil or moly paste if specified).

Safety first
- Work on level ground with the car stable on axle stands. Chock wheels.
- Support the gearbox with a rated jack when removing/installing.
- Drain gearbox oil before opening and dispose of oil properly.
- Use eye protection and gloves. Heavy parts require two people or a hoist.
- If you’re unsure about press-fitting bearings or measuring preload, seek help — incorrect bearing preload or endfloat can destroy a gearbox quickly.

High-level procedure (stages)
1) Preparation & information gathering
2) Remove gearbox from vehicle
3) Disassemble gearbox to reach the synchronizer assembly
4) Inspect and measure components, decide parts to replace
5) Replace synchronizer parts and any worn bearings/seals
6) Reassemble gearbox and measure clearances/preloads
7) Reinstall gearbox in vehicle, refit fluid, test drive

Detailed step-by-step (beginner-friendly, but thorough)

Stage 1 — preparation
- Get the factory service manual or a reliable gearbox exploded view for the F9Q-transmission combination. This will show which synchromesh kit fits the specific gearbox.
- Order a synchronizer kit (blocking rings, sleeve(s), springs, hub if necessary) and bearings/seals if you plan to replace them.
- Set up a clean bench and lay out labeled trays for bolts as you remove them.

Stage 2 — removing the gearbox from the car (summary)
- Disconnect battery.
- Remove shift linkage from gearbox (mark positions).
- Remove starter motor and any cables blocking gearbox removal.
- Support engine if necessary (depends on car layout).
- Support gearbox with a transmission jack.
- Remove drive shafts (axles) or disconnect universal joints depending on layout — mark the orientation if needed.
- Disconnect bellhousing bolts from engine and lower gearbox carefully.
- Remove clutch assembly? You will likely need to remove clutch/flywheel if gearbox removal requires it — again, follow manual for clutch removal and inspection.

Stage 3 — gearbox strip-down to synchromesh
- Clean exterior to avoid contamination.
- Drain gearbox oil and remove end covers and selector mechanisms.
- Remove shift forks/selector rails so the hubs/sleeves can slide off the shaft.
- Remove circlips/snaprings that retain bearings or gear sets (keep orientation).
- Remove mainshaft and layshaft if required — this gives access to the synchro hubs and rings.
- Note: Some designs allow you to remove only the mainshaft to access synchros; others require significant disassembly. Follow the gearbox-specific exploded view closely.

Stage 4 — inspect the synchronizer & related parts
- Inspect blocking rings (bronze rings) for scalloped wear, broken teeth, or glazing (smooth shiny surface). If cone surface is rounded or worn into grooves, replace ring and possibly the mating gear cone.
- Inspect the sleeve internal dogs for wear, rounded corners, broken teeth. Replace sleeve if dogs are worn.
- Inspect hub dogs and gear dogs for rounded faces (dog teeth engagement surfaces must be sharp).
- Check the spring tension and any small spring clips holding the blocking ring. Replace if weak or broken.
- Inspect splines on hub and shaft for galling.
- Measure wear: check endfloat of mainshaft, runout of gears, bearing play. Use dial indicator and micrometer to compare to manual limits.
- Check bearings and seals — replace if any play, roughness, corrosion, or runout.
- Inspect selector fork tips — if worn or mushroomed, they stick and cause late engagement.

Stage 5 — replacing the synchronizer
- Replace blocking ring(s) first. Note correct orientation — most blocker rings have chamfers and must face the sleeve/gear in a specific direction (manufacturer marks or tapered face). If unsure, compare to old part marking.
- Install new sleeve onto hub (or hub+ sleeve if kit includes replacement hub). Some kits require heating or light press fit; do NOT overheat (follow instructions).
- Replace springs and retainers correctly; ensure spring seats are clean and not worn.
- If replacing hub or sleeve, check fit and axial clearance per manual.
- Replace any worn bearings / seals while gearbox is open — it’s far simpler now than later.
- Clean everything with solvent, then let dry. Lightly coat friction faces with clean gear oil before assembly (do not use assembly grease on friction surfaces; some manuals specify light oil only).
- Reassemble in reverse order. When re-fitting dogs and rings, ensure gates and ramps line up; don’t force the sleeve — it must slide smoothly but not freely loose.

Stage 6 — reassembly tolerances, checks and tips
- Endfloat (axial play) on the mainshaft is critical. Too little = binding; too much = clutching problems. Adjust thrust washers or spacers to specified clearance.
- Bearing preloads: if you replaced tapered roller bearings, set preload per manual. Incorrect preload shortens bearing life.
- Torque all bolts to manufacturer specs (bellhousing bolts, bearing caps, output flange nut). Do not guess. Use the torque wrench.
- Check shift fork alignment: the fork should contact the sleeve on its full width, not the dog teeth or ring.
- Test by hand: with the gearbox assembled on the bench, try moving the selector rods and ensure the sleeve engages and disengages gears smoothly. Rotate the mainshaft and confirm that when the sleeve moves onto the cone it initially resists until the ring torques the gear, then it slides into dogs (you’ll feel resistance then a click).

Stage 7 — reinstalling gearbox and testing
- Refit gearbox to vehicle with transmission jack. Tighten bellhousing bolts to spec.
- Refit clutch, inlet/outlet shafts, driveshafts, starter, etc.
- Refill gearbox with manufacturer specified oil.
- Before a road test: start engine, operate clutch and shift through gears with vehicle stationary (engine at idle and in revs where safe) to confirm gears engage. If safe, do a slow test drive to check behavior.
- After short test drive, re-check gearbox oil level and bolt torques.

Common failure modes, diagnostics and fixes (what can go wrong)
- Worn blocking rings (baulk rings): cause grinding on shifts and can be seen as scallops or glazed surface. Replace ring; if gear cone is scored, replace/refurbish gear or hub.
- Worn sleeve or hub dog teeth: cause slipping or popping out of gear. Replace worn part.
- Broken selector fork or bent fork: causes misalignment and inability to engage gear — replace fork and inspect selector rails.
- Worn splines on mainshaft/hub: may allow hub to move under load; replace hub or shaft if splines are badly damaged.
- Contaminated or wrong oil (e.g., EP additives/GL-5 in a gearbox needing GL-4): causes clutching problems or accelerated wear. Flush/fill with correct oil.
- Weak or broken springs in synchro: cause late engagement, requiring replacement.
- Bearing failure or worn thrust washers: cause axial movement that prevents proper dog engagement — replace bearings and re-establish correct endfloat.
- Incorrect assembly (wrong orientation of blocker ring, omitted circlip): can cause catastrophic failure. Mark everything and follow manual.
- Not replacing seals/bearings when gearbox is open: can lead to repeat work — replace these now.

Testing & verification after repair
- Cold test: With vehicle stationary, cycle through gears while running engine at low revs. Shift into each gear and hold it briefly.
- Road test: Drive under varied loads and speeds; watch for noise, grinding, slipping, or popping out. Test both upshifts and downshifts.
- Recheck oil level and for leaks after the first few miles.

Analogy reminders (simple ways to remember how synchromesh works)
- Handshake analogy: blocking ring makes both “people� (gear and shaft) walk at the same pace; only then does the sleeve let them hold hands (engage).
- Speed camera: the blocking ring is the speed camera that prevents engagement until speeds match — if the camera (ring) is worn or missing, you’ll get fines (grinding).
- Zipper analogy: the sleeve is like the slider on a zipper — it must meet the two parts before locking them. If the slider is damaged, the zipper won’t close.

Practical tips for a beginner mechanic
- Take pictures at every step and label parts and bolt locations.
- Keep small parts in labeled containers to avoid mixing up different circlips or spacers.
- Replace wear-prone parts while you have the gearbox open (bearings, seals, selector forks).
- Don’t force parts — if hubs or bearings don’t come off, use a proper puller or heat and a press; forcing damages parts.
- If unsure about measuring tolerances (endfloat/preload), get help from someone experienced or a machine shop.
- Always consult the factory service manual for exploded views, torque specs, and clearance values for the exact gearbox mated to the F9Q1/F9Q2 in your vehicle.

Final note
- This job is intermediate-to-advanced: removing and disassembling a gearbox, then setting bearing preload and endfloat to specification, requires correct tools and careful measurement. If you are comfortable with disassembly, careful inspection, and have access to the manual and a torque wrench / dial indicator, you can replace synchros yourself. If not, consider a specialist gearbox shop.

No questions asked — good luck, and follow the manual for exact specs and orientation marks.
rteeqp73

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