High level theory first (short):
- The input shaft is the engine-to-gearbox drive: spline engagement to the clutch disc, supported by pilot bearing and transmission bearings, and meshes with the countershaft via gears/splines. Proper concentricity, bearing preload/endplay and undamaged splines are required for smooth, quiet power transfer and correct synchronizer operation.
- Typical failure modes: worn/damaged input‑shaft bearings, worn splines, bent shaft, damaged gear faces or retained race wear. Symptoms: whining/growling at certain RPMs, metallic vibration, excessive end‑play, difficulty engaging gears or grinding, clutch chatter.
- Replacing the input shaft restores shaft straightness, bearing surfaces and spline integrity, which restores concentric rotation, eliminates endplay/noise, and allows synchronizers to match speeds correctly.

Ordered procedure with the applied theory for each major action (consult factory manual for torque/specs and sealing details):

1) Confirm diagnosis (theory: isolate source to shaft/bearings)
- Road/bench test symptoms, listen for noise changes with clutch engaged/disengaged and at different gears/RPMs. Check clutch disengagement and freeplay. Verify noise persists when input shaft is spinning (with appropriate checks) to separate engine/transmission.
- If noise/play localizes to transmission and inspection shows play at input spline/shaft or hard bearing vibration, proceed.

2) Prepare and remove the transaxle (theory: get access while protecting other components)
- Drain tranny fluid. Disconnect battery. Remove driveshafts/CV axles, shift linkage, starter, exhaust components or crossmember as required to allow transaxle removal. Support engine if needed. Undo bellhousing bolts and remove gearbox from engine using a transmission jack.
- Theory: removing the box intact preserves internal alignment until bench disassembly; proper support prevents misalignment/damage.

3) Remove clutch/flywheel components (theory: input shaft must be freed from clutch and pilot bearing)
- With gearbox off, remove transmission-side clutch pressure plate and disc if not already. Remove the pilot bearing/bushing from crankshaft (inspect; replace if worn).
- Theory: worn pilot bearing contributes to shaft misalignment and bearing wear, so replace any worn pilot bearing at the same time.

4) Bench disassembly to access input shaft (theory: preserve orientation and parts order for correct reassembly)
- Secure gearbox on bench. Reference and photograph each part location. Remove outer housings, bearing retainers, snap rings, shift forks/rails only as needed to free the input shaft.
- Use proper drivers/press to remove bearings/races. Keep track of shims, spacers, and thrust washers that control endplay.
- Theory: endplay and spacing shims set axial location of the input shaft and bearings; losing orientation causes improper preload or binding.

5) Extract the input shaft (theory: separate shaft without damaging gears/countershaft)
- Remove snap rings or retaining nuts holding the shaft in place. Slide the shaft out together with its gears/synchronizer hubs as required. If bearings are pressed, use a press or bearing puller carefully.
- Inspect mating countershaft gear faces, synchro rings, hub splines and thrust surfaces for damage.
- Theory: input shaft removal allows inspection of contacts that cause noise and wear. Damaged synchros or countershaft gears should be repaired/replaced or the new input shaft will fail faster.

6) Inspect and decide replacement scope (theory: replace all worn interacting parts)
- Replace input shaft if bent, splines are damaged, or bearing journals are worn. Replace associated bearings, races, snap rings, seals, and any worn synchros, shift forks or countershaft gears. Replace pilot bushing/bearing and clutch disc/pressure plate if worn.
- Theory: a single new shaft in a worn gearset or with worn countershaft bearings will not fix concentricity or gear mesh issues; replace or refurbish mating components as needed for durability.

7) Prepare new input shaft and components (theory: bearings and races must be properly fitted)
- Fit new bearing races into the case if removed. Press new bearings onto the shaft to correct depth and orientation. Fit any required oil seals. Clean and lightly grease splines/needle bearings and apply assembly lube on bearing surfaces where appropriate.
- Theory: correct seating of races and bearings and appropriate lubrication ensure correct preload and immediate protection at first rotation.

8) Reassemble with correct shimning/preload and measure endplay (theory: restore designed clearances)
- Reinstall shaft and mating components in the original order. Install and torque retaining hardware to factory spec. Reinstall any shims or thrust washers in the same locations, or adjust shims to achieve specified axial endplay/preload for the input shaft (measure with dial indicator/feeler gauges).
- Check bearing preload or endplay per manual. Replace or adjust shims until within factory spec.
- Theory: correct axial clearance and bearing preload keeps the shaft concentric under load, prevents bearing overloading or axial movement that damages synchronizers and causes noise.

9) Reassemble gearbox internals and external housings (theory: keep geometry and clearances)
- Reinstall synchros, gears, forks and rails as removed; ensure shift forks are not bent and engage hubs properly. Refit housing halves with new gaskets/sealant as required and torque fasteners to spec.
- Theory: assembly stresses and misalignment here will reintroduce the fault; clean mating surfaces and use correct fasteners/sequences.

10) Refit transmission to vehicle and final assembly (theory: alignment and clutch interface)
- Reinstall clutch disc/pressure plate (use alignment tool) and mate gearbox to engine ensuring dowels align. Torque bellhousing bolts, reconnect linkages, driveshafts/CV axles, starter, exhaust/crossmember, reconnect battery.
- Refill with correct grade and amount of transmission fluid.
- Theory: clutch alignment and correct fluid are necessary for smooth engagement and lubrication of new bearings/gears.

11) Break‑in and test (theory: verify repair and allow seating)
- Start engine, verify no leaks. Test engagement in all gears on the lift first, then road test. Listen for any residual noise and recheck fluid level after warm run. After a few hundred miles, re-torque fasteners and recheck endplay if recommended.
- Theory: initial run-in lets mating surfaces bed and reveals any clearance or alignment issues early.

How the repair fixes the fault (concise):
- Replacing the input shaft restores straightness and undamaged splines, so the clutch disc runs concentric and synchronizers can match speeds without forced grinding.
- New bearings and correctly set preload/endplay remove axial and radial play that caused whining, vibration and premature synchro/gear wear.
- Replacing damaged mating parts (pilot bearing, countershaft bearings/synchronizers) ensures correct gear mesh and load distribution, preventing recurrence.

Important practical notes (brief):
- Use the factory service manual for bolt torques, shims, endplay specs and reassembly sequences.
- Replace pilot bearing and any suspect synchros/bearings at the same time.
- Use proper presses, pullers, and a transmission jack. Ensure cleanliness and correct lubricants.
- Test thoroughly before regular use.

That is the ordered, theory‑oriented procedure and why it cures the symptoms.
rteeqp73

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