1) Overview — what’s wrong and what we’re fixing (theory)
- The pinion bearings support the pinion gear and control its axial position (pinion depth) and rotational smoothness. Worn bearings allow endplay or radial runout, which changes gear mesh (backlash and contact pattern), produces gear noise (whine, chirp), vibration, increased wear and possible oil leakage at the pinion seal.
- Replacing the pinion bearings restores the correct axial position and smooth rotation of the pinion, allowing you to set the correct preload and ring-and-pinion tooth contact pattern. Proper preload and backlash give quiet operation and long life.

2) Tools, parts and measurements you’ll need (theory + practical)
- Tools: floor jack, stands, torque wrench, breaker bar, socket set, bearing puller/press or hydraulic press, driver/sleeves for races, dial indicator with magnetic base, gear marking compound, pinion nut socket, punch/soft mallet, RTV/sealant, pry bars, shop press, cleaning solvent.
- Parts: new pinion bearings (inner + outer), races (if required), new pinion seal, crush sleeve or shim(s) depending on axle design, new pinion nut if one-time-use, and gasket or RTV. Always replace any one-time-use crush sleeve and pinion nut.
- Measurements/targets: pinion preload (measured as rotational torque), ring gear backlash, carrier bearing torque/adjustment. These specs are factory-specific — consult the Toyota service manual for the exact 4Runner year/model. Theory: preload is what holds bearings under the nut; backlash is the lateral clearance between ring gear and pinion teeth.

3) Preliminary diagnosis and confirmation (theory)
- Confirm source of noise: Pinion bearings typically cause a whine that varies with vehicle speed, not engine speed. Check gear oil for metal particles (indicative of bearing/gear wear). Tooth pattern check (after disassembly and reassembly) is the final proof of correct position.
- Theory: distinguishing pinion bearing noise from carrier or axle bearings matters because replacing pinion bearings is only effective if the pinion bearings are the failing component.

4) Removal — ordered disassembly (in order with why)
1. Safety: support vehicle on stands and chock wheels. Remove driveshaft and plug the transfer/diff to avoid contamination.
- Why: gives access and prevents rotation/contamination.
2. Drain differential gear oil.
- Why: prevents spills and allows visual inspection of metal debris.
3. Remove axle shafts or disconnect as required to remove carrier (depends on 4Runner axle type).
- Why: carrier must be removed to access pinion nut on many designs.
4. Mark and record any shim positions or cap locations so reassembly references are available.
- Why: initial settings tell you how far things have moved and are reference for re-installation.
5. Remove differential carrier assembly from housing; support it.
- Why: frees pinion assembly for extraction.
6. Loosen and remove the pinion nut and extract pinion, seal, and bearings (use puller if needed).
- Why: getting the pinion out lets you press off old bearings and install new ones. If a crush sleeve is used, tapping off the pinion is usually required; if shims are used, you’ll change pinion depth with shims.

5) Bearing removal and inspection (theory + steps)
- Press or drive off the old bearings and races. Inspect:
- Bearing rollers/rollers for metallurgical damage (pitting, brinelling).
- Races for matching damage.
- Pinion gear and ring gear tooth surfaces for scoring, chipped teeth, or directional wear.
- Theory: if gears are damaged or tooth contact pattern is poor, simply replacing bearings isn’t enough — you must restore tooth pattern. Severe gear wear typically requires ring and pinion replacement or carrier replacement.

6) Installing new bearings — method and theory
- Clean pinion journal and housing thoroughly.
- Install new races and bearings using a press and correct drivers so you don’t load bearings improperly.
- If your 4Runner uses a crush sleeve:
- Install new crush sleeve and bearings; the pinion nut is torqued to crush the sleeve to achieve the factory-specified rotational preload (measured as a breakaway torque on the pinion).
- Theory: crush sleeve compresses axially as the nut is tightened; compression sets a fixed bearing preload. The rotational torque (measured with a torque wrench upon initial rotation) is the practical way to confirm preload.
- Important: crush sleeves are one-time-use — never reuse.
- If your 4Runner uses shims:
- Install the correct shim stack on the pinion or carrier to set pinion depth. Pinion depth determines where the tooth faces contact and is adjusted by changing shim thickness.
- Theory: shims change the axial position of the pinion relative to the ring gear; bearings must preload correctly but are not compressed by the nut as with a crush sleeve. Preload is set by tightening the nut against bearing race while shim depth sets gear mesh.
- Always replace the pinion seal.

7) Setting pinion preload (in order with explanation)
1. With new bearings and crush sleeve/shim in place, torque the pinion nut while measuring rotational torque (breakaway torque).
- Theory: rotational torque correlates to bearing preload. There’s a specified torque range for the bearing preload; too low = loose bearings and movement; too high = overheated bearings and premature failure.
2. If the design uses crush sleeve, tighten to the factory torque (or until collapse criterion met) and verify rotational preload falls within the specified range. If the design uses shims, adjust shim thickness and nut torque so the measured preload is in spec.
- Theory: preload is independent of the nut torque number because friction factors vary; the measured rotational torque is what matters.

8) Re-install the carrier and set backlash and tooth contact (ordered)
1. Install the carrier assembly with new or reused carrier bearings as required.
2. Set initial backlash using shims or adjusters on the carrier bearings until the measured backlash (dial indicator on ring gear tooth) is within factory spec.
- Theory: backlash is the lateral clearance between mating gears. Changing backlash moves the contact pattern across the tooth surface.
3. Apply gear marking compound to a few gear teeth, rotate the ring-and-pinion through several turns under load (spin the pinion), and inspect the contact pattern.
- Theory: the contact pattern shows where the teeth meet; it must sit in the correct zone (typically centered on face and flank to some percent). If pattern is too deep, shallow, toe/high/low, adjust pinion depth (shims) or backlash until the pattern matches factory guidance.
4. Iterate: adjust pinion depth (if necessary) and backlash, re-check pattern until correct.
- Why: correct preload + correct pinion depth + correct backlash = correct tooth contact pattern, which is what actually eliminates noise and premature wear.

9) Final assembly and checks (ordered)
1. Reinstall bearing caps and torque to spec.
2. Reinstall axles, driveshaft, and brake components, torquing to spec.
3. Refill differential with correct gear oil and check for leaks.
4. Road test and re-check for noise and for leaks. After a short break-in (couple hundred miles), re-check the pinion torque/preload and backlash if specified by manual.

10) How this repair fixes the fault (succinct theory)
- Worn pinion bearings allow axial movement and radial runout; that changes pinion depth and creates incorrect backlash and poor tooth contact. That manifests as whining noise, vibration, accelerated gear wear, and often pinion seal leaks.
- New bearings remove play and allow you to set proper preload. Setting proper pinion depth and backlash restores the correct tooth contact pattern. A correct contact pattern distributes load properly across tooth surfaces, eliminates edge loading, reduces noise and heat, and prevents premature failure.
- If gears were already damaged by bearing wear, bearing replacement plus correct setup will stop further damage but will not erase existing pitting/scoring — replacing the ring-and-pinion may also be required.

11) Common pitfalls (brief)
- Reusing crush sleeves or pinion nuts when one-time-use — leads to wrong preload.
- Not measuring actual rotational preload (torque) — tightening to a torque spec alone can produce wrong preload due to friction variability.
- Ignoring gear tooth pattern checking — correct backlash/preload without pattern verification can still produce noisy gears.
- Reusing damaged races or contaminating bearings during install — kills bearings quickly.

12) Final note (safety & reference)
- Always follow Toyota factory service manual specs for preload torque ranges, backlash, and contact pattern location for your specific 4Runner year/axle.
- Use proper safety procedures and presses/pullers to avoid personal injury.

That’s the ordered procedure with the theory behind each major decision and an explanation of how replacing pinion bearings fixes the fault.
rteeqp73

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