Ordered, theory-first procedure for “crankshaft” work on Toyota H41/H42/H50/H55F transmissions

Note: many people say “crankshaft” when they mean the transmission’s layshaft/countershaft or the input/mainshaft assembly. Below I treat this as work on the transmission internals (input shaft / layshaft / main/output shaft) — diagnosis, removal, inspection, repair and how each repair corrects the fault. Steps are ordered; each step includes the theory and what the repair action fixes.

1) Define the symptom and likely failure modes (theory)
- Common transmission faults: gear noise (whine/grind), sloppy or impossible shifts, oil leaks, bearing rumble, backlash/gear tooth wear.
- Theory: noise/whine = worn bearings, incorrect backlash, or gear tooth wear; grinding = damaged/ worn synchronizers or misadjusted linkages; leaks = seals/gasket failure; excessive play = worn bearings or worn splines.
- Decide target: bearings, synchros, seals, countershaft (layshaft), input/mainshaft, or shims.

2) Preparation and safety (theory + why)
- Drain gear oil, disconnect driveshaft/clutch, label/linkage. Why: removing oil prevents mess and allows removal; labeling preserves geometry and avoids incorrect reassembly which causes misalignment/shifting faults.
- Remove transmission from vehicle or mount in a bench press. Why: full internal access is required to measure and correct clearances.

3) External teardown in order (theory)
- Remove bellhousing cover, shift linkage and selector forks, top cover/inspection plates. Why: you must free selector components so internals come apart without bending forks or misindexing detents.
- Split the case halves (if applicable) and support both halves. Why: allows access to shafts; note orientation marks — reassembly must restore original indexing to maintain shift geometry.

4) Remove shafts and gears in order (theory)
- Remove snap rings, circlips, retainer plates in sequence, then withdraw input/ layshaft (countershaft)/mainshaft assembly. Pull in the order that releases axial preload gradually to avoid damaging bearings.
- Theory: shafts are retained by circlips and thrust faces; abrupt removal can let gears drop and damage bearings or synchronizer keys. Keep parts in order and mark them.

5) Inspect components and measure (theory + pass/fail criteria)
- Bearings: spin test, feel roughness, check radial/axial play with dial indicator. Replace if rough, noisy, excessive play or pitted rollers.
- Gear teeth: inspect for pitting, chipped edges, polished scallops (indicating misalignment), or heat discoloration. Replace if chipped, broken, or severely worn.
- Synchronizers (sleeves and rings): inspect friction material thickness, broken keys, worn ring teeth. Worn rings produce grinding or missed engagement.
- Splines and shafts: check for burrs or wear at clutch spline and gear splines; worn splines produce slip and axial movement.
- Seals and bushings: inspect for leaks or scoring.
- Measure end-play/backlash: use dial indicator to measure lateral endfloat of shafts and tooth-to-tooth backlash between mating gears. Compare to service limits; excessive values indicate worn bearings/shims or incorrect assembly.
- Theory: accurate measurement determines which corrective action (bearing replacement, shim change, gear replacement) is required to restore correct geometry and contact pattern.

6) Decide repairs and parts to replace (theory)
- Replace bearings if noisy, have excessive play, or roller pitting. Why: bearings control shaft location and preload; worn bearings allow axial/radial movement that changes gear mesh and creates noise/accelerated gear wear.
- Replace synchronizer rings/sleeves if sliding engagement is rough or teeth are damaged. Why: synchros provide speed equalization and smooth engagement; worn friction or dog teeth cause grinding or missed shifts.
- Replace seals and gaskets to stop leaks.
- Replace gears or shafts if teeth are damaged beyond spec. Why: damaged teeth change load paths causing noise and further failure.
- Replace thrust washers/shims where worn; replace if spacers are scored. Why: shims set endfloat and backlash; incorrect shims create wrong gear mesh and premature wear.

7) Rebuild theory: bearing preload, shimming, backlash and contact pattern (ordered actions)
- Clean all parts; press-fit new bearings as needed with correct tools (drive on inner/outer race as specified). Theory: incorrect mounting can preload bearings incorrectly or damage rollers.
- Reassemble shafts loosely in order. Install thrust washers and retainer plates.
- Set bearing preload or endfloat: adjust shims or nut preload to specified endplay. Use a dial indicator to measure axial movement. Theory: correct preload prevents axial play yet avoids binding; it centers gears and bearings for correct mesh.
- Set gear backlash/contact pattern: using a marking compound on gear teeth, mesh gears under their operating position and check contact patch. Adjust shims to move the contact patch to the correct zone. Theory: proper backlash and contact patch distributes load across the gear tooth face; wrong backlash causes noise, wear, and tooth breakage.
- Reinstall synchronizers ensuring correct orientation and that keys/struts are intact. Theory: correct assembly restores synchronizer function and prevents grinding.
- Torque all fasteners and fit circlips in the order and style specified by OEM to lock axial locations.

8) Bench-testing before vehicle installation (theory)
- Manually rotate input vs output; shift through all gears and feel for smooth engagement, free rotation without binding, and absence of abnormal noise.
- Re-measure endfloat and backlash after final torque. Theory: final checks confirm that the assembly holds within tolerance and that preload/settings didn’t shift during tightening.

9) Reinstall, break-in and verification (theory)
- Reinstall transmission, fill with correct grade and amount of gear oil, reconnect linkage and driveshaft, and test drive. Theory: correct oil viscosity ensures bearings and synchros are lubricated and that shift feel is as intended.
- Initial break-in: avoid heavy loads/rapid shifts for a short period so mating surfaces wear into proper contact. Theory: controlled wear smooths microscopically rough surfaces and stabilizes friction materials.

How each repair action fixes faults (concise mapping)
- Replace bearings → removes rumble/whine and eliminates shaft movement that causes incorrect backlash and accelerated gear wear.
- Replace worn synchronizers → stops grinding/missed shifts by restoring friction surfaces and proper engagement geometry for speed equalization.
- Replace seals/gaskets → stops leaks and prevents oil starvation of bearings/synchros.
- Replace damaged gears/shafts → eliminates noise from broken/chipped teeth and prevents propagation of damage to mating gears.
- Adjust shims/preload/backlash → corrects gear mesh, restores proper contact pattern, removes whine and uneven wear.
- Correct assembly of forks/selector components → restores true gear engagement positions so shift linkage matches internal indexing, preventing false neutrals or difficult selection.

Key theory points to remember (no steps, just principles)
- Transmission function depends on precise shaft location and tooth contact pattern; bearings and shims set those locations.
- Synchronizers are friction devices that equalize speeds; if friction surfaces or keys are damaged the engagement will be harsh.
- Noise and wear are symptoms of either wrong geometry (backlash/preload), damaged rolling elements, or damaged gear teeth.
- Replacing parts without resetting shims/preload/backlash will often leave the underlying cause uncorrected.

End.
rteeqp73

Early H55F Transmission Rebuild: PT2- Detailed Reassembly After getting the bearings pulled I was able to finally dive into the H55f rebuild. I tried to detail every part of what I did. I found it very ...

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