Tools/parts/consumables (brief)
- New rear main crankshaft seal (OE or high‑quality equivalent), retainer gasket if applicable
- Service/repair manual for torque values and special tools for YD22DDTi
- Engine support (load leveler) or engine hoist, transmission jack
- Socket/ratchet set, breaker bar, torque wrench
- Flywheel/driveplate and clutch alignment tools, seal driver or suitable drift, small hammer
- Pry bars, screwdrivers, pick, gasket scraper, brake cleaner/solvent, lint‑free rags
- Threadlocker, anti‑seize, assembly lube, new fasteners if one‑time use bolts are specified
- Safety equipment (gloves, eye protection)

Overview — theory first
- What the rear main seal does: it’s a lip seal pressed into the rear crankshaft housing or retainer and runs on the rotating crankshaft flange to keep engine oil inside the crankcase. The sealing action depends on a clean, smooth shaft surface, correct radial interference of the lip, a spring in the lip to maintain contact, correct seating depth, and even compression around the bore.
- Why it leaks: lip wear, hardened/dried rubber, nicks/grooves on the crank flange, improper seating, or a warped/bad retainer/gasket. Excessive crankshaft end‑float, blown engine gasket, or overfilled oil can also cause or accelerate leaks.
- How the repair fixes it: replacing the seal restores the elastic sealing lip and preload spring; cleaning and machining (or replacing) the flange or retainer removes grooves and provides a correct running surface; correct installation ensures even preload and concentricity so the lip maintains continuous contact, stopping oil escape.

Step‑by‑step procedure with theory explained (in order)
1) Confirm diagnosis and prepare
- Theory: ensure the leak source is truly the rear main (oil at the bellhousing, gearbox input, or between engine and transmission). Other leaks (valve cover, oil pan, timing cover) can run back to the same area.
- Action: clean area, run engine briefly to observe leak path. If confirmed, disconnect battery and drain oil only if necessary for safe working.

2) Support engine and remove transmission/transfer case
- Theory: the rear seal sits behind the crank flange, so the gearbox must be removed to access the seal. The engine should be supported independently so removing the transmission doesn’t let the engine drop or change alignment.
- Action: support engine with hoist or engine support bar, unbolt drive shafts/clutch hydraulics, remove starter, disconnect harnesses, remove gearbox and set on a jack.

3) Remove clutch/flywheel or flexplate as applicable
- Theory: the flywheel or flexplate covers the seal area; removal exposes the crank flange and seal retainer. Inspect these components as they can hide damage contributing to leaks.
- Action: remove clutch assembly and then the flywheel or flexplate. Use correct holding tool to prevent rotation. Keep track of bolt torque pattern and orientation.

4) Inspect crank flange and retainer
- Theory: the lip seals against the flange. Grooves, wear, corrosion or burrs will compromise the new seal immediately.
- Action: clean the flange with solvent and lint‑free rags. Use a fine polish/stone only to remove very light scores; deep grooves require machining or replacement of the flange/engine part. Check crankshaft axial play — excessive endplay can cause premature seal failure.

5) Remove seal retainer (if fitted) and old seal
- Theory: some engines have a two‑piece retainer or one that carries a gasket. Removing allows full inspection and correct seating of the new seal.
- Action: unbolt the retainer and pry the old seal out carefully to avoid damaging the bore. Note orientation of the seal lip (typically the open lip faces the oil).

6) Prepare mating surfaces and new seal
- Theory: a smooth, clean bore and correct seal orientation are critical for even preload and radial sealing. Lubrication prevents dry start scuffing.
- Action: clean bore of old gasket material and oil. Lightly lubricate inner lip of new seal with clean engine oil or assembly grease. Confirm seal orientation (spring side toward oil, usually facing into the engine).

7) Install new seal squarely
- Theory: driving the seal in unevenly or too deep will distort the lip and cause leaks. Correct seating depth and even compression ensure uniform contact pressure around shaft.
- Action: use a seal driver sized to the outer diameter or a socket of appropriate size and tap gently until the seal is flush with the retainer or to specified depth. Do not cock the seal; work evenly around circumference.

8) Refit retainer with new gasket or sealant per manual
- Theory: the retainer must seal to the block and hold the new seal concentric. A damaged retainer or bad gasket undermines the seal.
- Action: install new gasket or use recommended sealant, torque retainer bolts to spec in a crisscross pattern.

9) Reinstall flywheel/flexplate and clutch
- Theory: correct flywheel torque and position ensure concentricity with crankshaft and proper clutch alignment. Loose or distorted flywheel can create eccentric shaft surface at the seal and cause leaks.
- Action: clean mating surfaces, fit flywheel, torque bolts to manufacturer spec in sequence. Replace single‑use bolts if specified. Refit clutch assembly using alignment tool.

10) Refit transmission and reconnect everything
- Theory: correct alignment during reassembly avoids loads that could misalign the crank/transmission and stress the seal area.
- Action: use a transmission jack, align input shaft with clutch, torque gearbox bolts to spec, reconnect electrical connections, hoses, exhaust and driveshafts.

11) Refill engine oil, prime system, and check
- Theory: a dry run with no oil hides leaks; pressure and temperature stresses can reveal poor installation. Also, if the engine was drained, proper oil level and priming are required.
- Action: refill oil to correct level, reconnect battery, start engine and inspect bellhousing area for leaks while idling and under light revs. Reinspect after a short test drive.

12) Post‑repair checks and prevention
- Theory: verify there are no sources of excessive crank end play, oil overpressure, or oil leakage elsewhere. New seals need to run in; minor seepage immediately after install may show but should stop quickly if installed correctly.
- Action: recheck torque on bolts after heat cycles per manual, monitor oil level over next few hundred kilometers.

How this repair fixes the fault (concise)
- The worn/broken lip and/or damaged running surface was allowing oil to pass between the rotating crankshaft and stationary housing. Replacing the seal restores the elastic lip and spring preload that keep the lip pressed against the rotating flange. Cleaning or repairing the flange/retainer provides a proper smooth running surface so the new lip can maintain a continuous oil‑tight interface. Proper seating, concentricity and torque prevent geometric causes of leakage (eccentricity, uneven preload, or gas pressure forcing oil past the lip).

Key pitfalls to avoid
- Installing the seal backwards or cocked.
- Driving the seal too deep or damaging the lip during install.
- Reusing a severely damaged flywheel/retainer or sealing gasket.
- Ignoring crankshaft endplay or flange grooves.
- Skipping correct torque sequences or using wrong bolts.

Note: Use the YD22DDTi factory service manual for exact torque values, any special tools (pilot/bearing alignment), and model‑specific procedures (some models may have two‑piece seals or require transmission removal sequences).
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