1) Theory — what you’re adjusting and why
- Valves control intake and exhaust airflow; the valve train (camshaft lobes → rocker or bucket → valve stem → valve seat) converts cam profile to valve motion.
- “Valve clearance” (lash) is the small gap between the moving cam/rocker and the valve (or its bucket/adjuster). Clearance compensates for thermal expansion and ensures valves fully close at operating temperature.
- If clearance is too large: noisy ticking, slow valve opening, reduced effective lift, rough idle/power loss.
- If clearance is too small: valves can be held open at temperature, causing loss of compression, hot valves/seat damage, misfire and burned valves.
- There are three common adjustment systems: screw-and-locknut, shim-over-bucket (or under-bucket), and hydraulic lifters. The D21 family uses mechanical adjusters or shim/bucket depending on engine variant — use the correct method for your head.

2) Preparations (why each matters)
- Work with a cold engine so metal is at room clearance spec; hot checks give wrong results.
- Gather tools: correct feeler gauges, spanner for locknut or correct shims, torque wrench, socket set, breaker bar to turn crank, clean rags, service manual for specs and adjustment sequence.
- Safety: battery negative disconnect recommended if you’ll be rotating or working near ignition; support vehicle/accessory parts removed so you can remove the rocker cover cleanly.

3) Access (what to remove and why)
- Remove air cleaner, hoses, ignition leads/coils if they block, and any wiring over the rocker cover.
- Remove rocker cover to expose rocker arms/cams. Keep parts and fasteners organized and clean to prevent contamination.

4) Find TDC on compression stroke (why this position)
- Valve clearance is measured when the cam lobe for that valve is on its base circle (cam not lifting the valve). That happens when the cylinder is at Top Dead Center (TDC) on its compression stroke.
- Rotate the crankshaft clockwise until timing marks align for TDC on cylinder 1 compression, or follow the engine’s firing order and cam position method in the manual. Using TDC ensures both valves of that cylinder are fully closed and on the cam base circle.

5) Measuring clearances (procedure + theory)
- Slip the correct feeler gauge between the valve tappet/rocker and valve stem/cup or bucket while the cam lobe is on the base circle. There should be a slight drag on the gauge.
- The feeler ensures the gap equals the specified clearance so the valve will be closed at operating temperature. If it slides too loosely, the gap is too big; if it won’t fit, the gap is too small.

6) Adjustment methods (how and why)
- Screw-and-locknut: loosen the locknut, turn adjuster screw until you get the specified drag on the feeler gauge, then hold the screw and tighten the locknut while re-checking. This sets the exact gap mechanically.
- Shim-over/under-bucket: measure current clearance, calculate required shim thickness change (actual shim ± difference to reach spec), remove cam/rocker or lift bucket as required, replace shim and re-measure. Shims change the valve tip-to-cam geometry to obtain the spec gap.
- Hydraulic lifters: these are self-adjusting; if lash is out of spec, lifters are likely collapsed/clogged and must be replaced — they cannot be adjusted.
- After each adjustment, re-measure to verify; tightening the locknut can move the screw slightly so recheck is essential.

7) Sequence and rechecking (why order matters)
- Use the factory sequence (TDC cylinder 1, then follow firing order or specified cam rotations) so you always measure each valve when its cam lobe is on the base circle. Incorrect sequence gives false readings.
- After adjusting all valves, rotate the engine two full revolutions and re-check all clearances. This confirms repeatability and that nothing shifted during assembly.

8) Reassembly and test (what to expect)
- Reinstall rocker cover with a clean gasket and correct torque to prevent oil leaks. Reconnect any components removed.
- Start engine and listen: a correctly adjusted valve train will be quieter and the idle smoother. Do a road test: power and throttle response should improve, and overheating or misfire symptoms linked to valve seating should be reduced. For persistent compression/roughness issues, perform a compression or leak-down test — if compression is low on a cylinder despite correct lash, check for burned valves, badly seated valves, or head damage.

9) How the repair fixes common faults (direct cause → effect)
- Excess lash (too big) → noisy tick and loss of effective valve lift/duration → adjusting to spec restores proper lift/timing, reduces noise, and recovers power/idle quality.
- Insufficient lash (too small) → valves not fully seating at operating temp → loss of compression, overheating of valve/seat, misfire. Increasing clearance to spec allows full closure and restores compression and sealing.
- Worn lifters/shims/rockers → inconsistent clearances and intermittent noise/poor sealing. Replacing worn components restores correct geometry and reliable clearances.
- Worn valve stems/guides or burned seats → oil burning, poor sealing; adjusting lash won’t fix sealing problems — machining or valve replacement restores sealing by re-establishing correct valve-to-seat contact and combustion compression.

10) Final notes
- Always use the exact clearance specs and adjustment sequence from the Nissan service manual for your engine variant.
- If clearances are wildly out or you find metal fatigue/worn components, replace affected parts and retest compression.
- Properly set valve clearance maintains timing, compression and valve/seat longevity; incorrect clearance causes the faults described.

End.
rteeqp73

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