1) Fault description and theory (what “windshield” likely means here)
- Typical gearbox complaints on H41/H42/H50/H55F that are solved by gearbox repair are: gear whine/“wind-up” noise, whining under load, gear engagement grinding, or heavy synchro wear. These all come from the same physical causes: worn/damaged bearings, incorrect gear mesh/backlash, worn synchronisers or selector components, or damaged gear teeth.
- How each cause produces a symptom:
- Bearings with radial play or pitting let shafts misalign under load so gear teeth don’t mesh on their designed contact pattern → whine, noise, accelerated tooth wear.
- Incorrect gear backlash or incorrect pinion/gear shim settings shift the gear contact pattern toward the edge of the tooth → noise, scuffing, scarring and rapid wear.
- Worn synchroniser rings or cones cannot equalize speeds between gear and shaft quickly enough → grinding when engaging and fast wear of dogs/sleeves.
- Broken/chipped teeth or contaminated oil create impact/noise and metal debris that accelerates further wear.

2) Diagnostic theory and confirmation (what you must determine before repair)
- Road and idle tests identify when and on which gear the issue occurs (e.g., whine only in 3rd under load = gear/shaft bearing on that ratio or lay/shaft mesh).
- Static checks: magnetic drain plug metal quantity, oil condition (metal fines, burnishing), endplay checks on shafts with dial gauge reveal excessive play (bearing wear).
- Listening and stethoscope pinpoint which shaft or side of case transmits the sound (input vs output/layshaft).
- Conclusion drives repair scope: if noise is gear-tooth pattern/mesh-related you’ll need to set shims/backlash and probably replace bearings; if synchroniser grinding, you’ll replace sync rings and possibly sleeves/dogs.

3) Preparation and theory behind teardown order
- Theory: accurate reassembly requires preserving reference relationships (shim stacks, bearing preload, orientation of shafts). So note/mark original shim stacks, housing orientation and clearances before removing.
- Steps: drain fluid, remove driveshafts/axles, linkage, bellhousing where applicable, and ancillary components. Support gearbox, separate from engine if required. Keep fasteners and tags in order. Photograph/label shim stacks and spacer positions — these determine shim/calibration baseline.

4) Disassembly with inspection points (and why)
- Remove gearbox cover and inspect gear teeth contact visually; then remove layshaft and mainshaft assemblies.
- Inspect bearings (roller/caged, tapered) for pitting, brinelling or axial/radial play. Theory: replacing bearings restores correct radial/axial location of shafts and eliminates play that causes misalignment.
- Inspect gears for pitting, spalling, chipped teeth or uneven tooth wear and scoring. Theory: damaged gears change contact geometry; worn surfaces increase noise and reduce load-carrying area.
- Inspect synchroniser rings, dogs, sleeves, and selector forks for wear; worn cones or dogs let gears clash and wear. Theory: new synchro cones restore frictional capacity to match speeds for smooth engagement.
- Measure endplay and clearances against factory specs; note any worn/mismatched shims.

5) Determine parts to replace and why
- Replace bearings that show wear/pitting or exceed play specs — because bearing wear changes shaft position and mesh.
- Replace worn synchroniser rings, sleeves or dogs — because their friction surfaces are sacrificial and needed for smooth engagement.
- Replace gear sets only if teeth are severely damaged. Minor wear may be fixed by correctly setting mesh/backlash; severe scoring needs new gear(s).
- Replace seals/gaskets and any worn selector components.

6) Gear mesh/backlash and bearing preload theory and how to correct it
- Theory: gearset performance depends on two geometric settings: tooth contact pattern (axial and circumferential placement on the tooth) and backlash (radial clearance between mating gear teeth). Both are set by shim stacks or spacer thickness and bearing preload. The correct contact pattern centers the load on the tooth and avoids edge loading; correct backlash provides thermal expansion clearance and avoids excessive noise.
- Procedure conceptually in order:
- Fit the new or inspected pinion/gear with the bearings and initial shims per baseline.
- Use gear-marking compound on a dressed tooth and rotate under load to see the contact pattern. The pattern tells you whether to move gear closer to or away from pinion (change shims) or adjust pinion depth.
- Adjust shims iteratively: adding or removing shims on one side moves the gear axially, changing axial contact; changing spacer thickness or bearing preload alters backlash.
- Measure backlash with a dial gauge between gear teeth and set to the specified range.
- Set bearing preload (tapered roller or thrust) per spec; correct preload eliminates play but avoids overheating from overtightening. Preload is critical because it fixes shaft axial position, which hugely affects contact pattern.
- How this fixes the fault: re-establishing correct pitch line and contact surface spreads load correctly and prevents whine and rapid wear; correct preload eliminates play that causes noise and gear misalignment.

7) Reassembly theory and critical checks
- Use new seals, bearings and shims as determined; torque fasteners to factory spec to avoid distortion of case halves or bearing caps which would change geometry.
- Reassemble synchroniser assemblies in correct orientation; ensure selector forks are not bent and operate freely. Properly installed synchronisers restore the speed-matching friction interface for smooth shifts.
- Fill with the recommended viscosity/type gear oil to ensure proper film thickness and lubrication at operating temperature — oil viscosity affects gear friction/noise and bearing lubrication.

8) Testing and verification (what to verify and why)
- Bench spin: rotate input and observe/noise and play; check for smooth rotation and no binding.
- Use gear-marking pattern and backlash final verification after running-in if required.
- Road test across all gears and loads; a correct repair will eliminate the original whining, grinding or engagement problems. Monitor oil for fresh metal particles after break-in — excessive metal indicates missed damage.
- Re-check for leaks and re-torque if necessary after initial run.

9) Specific repair outcomes and why they stop the fault (summary)
- Replacing worn bearings removes radial/axial shaft play -> restores correct shaft alignment -> tooth contact pattern and backlash return to design -> reduces whine/noise and wear.
- Correct shimning/backlash adjustments move the contact patch to the intended area of the tooth -> avoids edge loading and noise -> stops progressive scuffing/pitting.
- Replacing synchronisers and selector parts removes the source of engagement grinding -> smooth gear shifts and less wear of dog teeth/sleeves.
- Replacing damaged gears removes mechanical defects that produce noise and metal contamination; combined with correct mesh settings, the new gears run quietly and last.

10) Practical cautions (short)
- Always use the factory service manual specs for shim thickness, backlash and bearing preload — do not guess.
- Use proper tools (bearing pullers, press, dial gauges, torque wrench, gear marking compound).
- If gear teeth are damaged beyond light wear, replace the gear set or the affected gear — setting mesh on damaged teeth is a temporary fix at best.

That’s the repair theory sequence: diagnose which component or setting is wrong → disassemble preserving references → inspect and measure → replace worn bearings/synchros/gears as needed → set bearing preload and gear mesh/backlash with gear-marking and dial gauges → reassemble to spec and test. Each repair action corrects the physical parameter (shaft location, tooth contact, friction surface, clearance) that caused the original noise or engagement fault.
rteeqp73

Early H55F Transmission Rebuild: PT3- Install, Test drives in FJ60 & Performance Upgrade! Its been a hell of a journey and lots of learnings to get this 5 speed transmission built and now finally installed. The camera work in ...

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