1) Quick overview of how reverse works (theory)
- Most D21-era Nissan manuals use an idler (reverse) gear that is moved into mesh between the counter/output gear and the layshaft to reverse rotation. Reverse typically is not synchromeshed: engagement is by aligning gears or dogs; proper axial positioning and shaft/bearing alignments are critical.
- Failure modes: linkage/misalignment, clutch not disengaging (transmission input still turning), worn/broken idler or mating gear teeth, worn/damaged shift fork or rail, excessive bearing/shaft play (mis-mesh), contaminated/low fluid causing wear, or internal broken snap rings/shims allowing axial movement.

2) Symptom-driven diagnosis (do this before teardown)
1. Verify symptom precisely: no engagement, grinding, pops out, or noisy only in reverse.
2. Check clutch operation: engine off, push car in neutral; with engine running and clutch depressed, can you engage other gears cleanly? If clutch slips or won’t fully disengage, reverse may not engage. Theory: reverse requires shafts to be at rest relative to each other; a slipping or dragging clutch keeps input shaft turning and prevents dog/gear engagement.
3. Check linkage and shifter free-play: shift into reverse with engine off; observe movement at the gearbox lever. Theory: misadjusted/loose linkage means the reverse shift rail/fork never fully selects the idler gear.
4. Check fluid level/condition and for metal debris: low fluid or metallic swarf indicates internal wear/broken parts.
5. Road/bench test (if safe): note noises and whether grinding occurs only when moving or when stationary. Theory: grinding while moving often means no synchro/dog engagement for reverse or that shafts are still turning.

If diagnosis points to external problems (linkage/clutch/fluid), fix those first; if internal, proceed to gearbox removal.

3) Prepare and remove transmission (order + why)
1. Safety: park on level, chock wheels, disconnect battery. (prevents injury)
2. Drain gearbox oil to reduce weight and mess.
3. Support transmission with a jack, remove driveshafts/CV axles or propshaft, starter, cross-member/mounts, shift linkage, speedometer cable/sensors, any interfering exhaust/covers. Unbolt bellhousing from engine and lower gearbox. Theory: you must separate gearbox from engine to access reverse idler/gear assembly and internal forks/rails.

4) Strip gearbox to access reverse components (order + what to inspect)
1. Secure gearbox in a clean bench vise on correct mounting points.
2. Remove outer covers (top cover/shift tower), undo snap rings, remove shift forks and selector rails in their order so you can track positions. Mark parts and orientation. Theory: selector rail and forks directly move idler; incorrect reassembly changes shift geometry.
3. Remove countershaft/layshaft cluster and output shaft as needed to access reverse idler gear. Inspect shafts for wear on splines and shoulders.
4. Inspect reverse idler gear, reverse gear teeth, mating faces, dogs (if present), bearings, synchros (if present in other gears), snap rings and shims, shift fork pockets, and fork tips for wear or deformation. Check bearing play on shafts and radial runout. Theory: gear teeth and dogs must be intact and concentric; shafts must be held to correct axial/radial tolerances for proper gear meshing. Worn bearings or missing shims allow axial movement that prevents gear dogs or idler mesh from fully engaging.

5) Typical faults and repairs (in order), with theory of why each repair fixes the fault
1. Linkage/selector damaged or misadjusted:
- Repair: Replace worn bushings, levers, or rebuild linkage; adjust travel so rail fully selects reverse.
- Theory: Restores full mechanical travel so the idler/rail moves to correct position; without this you never fully mesh gears.

2. Clutch not disengaging:
- Repair: Service clutch (adjust cable/hydraulics), bleed hydraulics, replace worn clutch/pressure plate/releases as needed.
- Theory: A free-floating or slipping input shaft prevents gear dogs/idler from meshing; a good clutch lets shaft stop so reverse can engage without grinding.

3. Worn/broken idler gear or mating teeth:
- Repair: Replace reverse idler gear and the mating gear(s) (output/counter gear) if teeth are chipped/flattened. Always replace both mating faces when wear is significant.
- Theory: Correct tooth profile and pitch are needed for positive mesh. Damaged teeth cause jumping, grinding, inability to hold gear engagement.

4. Damaged shift fork or worn fork pocket:
- Repair: Replace or rebuild shift fork; if pocket wear present, replace selector mechanism or use repair sleeve. Check fork alignment and tip condition.
- Theory: A bent or worn fork cannot move the idler into full engagement or lets it retract under load, causing disengagement or partial mesh.

5. Worn bearings/shafts or wrong endplay:
- Repair: Replace worn bearings, measure and reset endplay with correct shims, replace worn shafts if out of tolerance.
- Theory: Bearings and shims set axial position of gears and ensure concentric mesh. Excess endplay lets gears move out of mesh under load; replacing bearings and correcting shims restores geometry and preloads.

6. Broken snap rings/circlips/shims:
- Repair: Replace rings/shims and re-establish axial locations to factory spec.
- Theory: Snap rings retain gear positions; missing/damaged rings let components shift so reverse selection fails.

7. Seals/gaskets and fluid change:
- Repair: Replace seals and gaskets removed during disassembly; refill with correct spec gear oil.
- Theory: Proper lubrication prevents future wear and ensures correct bearing operation; seals prevent contamination and oil loss that lead to premature failure.

6) Reassembly (order + critical checks)
1. Clean all parts, degrease, inspect again.
2. Replace all bearings, seals, worn gears, forks, and snap rings as identified.
3. Reassemble shafts and gears in correct order, using new gaskets/seals. Install shims to set correct endplay per factory spec. Theory: correct endplay and torque on bearings restore axial and radial alignment so gears mesh at intended contact pattern.
4. Check gear mesh/contact pattern where possible (use gear marking compound) and correct shims if pattern is off.
5. Reinstall shift forks and rails, verify full travel to reverse stop and that forks move smoothly without binding.
6. Bench test shift through gears; verify reverse engages and holds under hand-rotated load (rotate input and output to check engagement).
7. Reinstall transmission in vehicle, reconnect linkage, clutch, driveshafts; refill with correct oil; torque mounts per spec.
8. Final road test: check for engagement, noise, and retention of reverse under torque.

7) How each repair fixes the fault — summary
- Replacing damaged gears restores correct tooth geometry and prevents grinding/jumping.
- Replacing forks/rails and correcting linkage ensures the mechanical motion fully engages the idler.
- Repairing clutch issues ensures shafts can be stationary relative to each other for engagement.
- Replacing bearings and resetting shims restores correct axial/radial alignment so gears mesh and remain engaged.
- Fresh fluid and seals prevent future wear; snap-rings/shims restore retention of components.

8) Key measurements to respect (critical tolerances)
- Endplay on output/countershaft (use feeler/specs from manual).
- Bearing preload where applicable, axial clearance of selector forks.
- Torque on critical nuts/bolts per factory data (don’t under/over-torque).
(If you need exact numbers, consult the Navara D21 workshop manual for the transmission model in your truck.)

9) Final notes (practical)
- If reverse damage is severe, replacing the damaged gearset and associated bearings and snap rings is usually required rather than patching.
- Always correct root cause (linkage/clutch/bearing) or the new parts will fail quickly.
- Use quality replacement parts and correct lubricant.

Done.
rteeqp73

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