1) Purpose & basic theory (very short)
- Rocker arms transfer camshaft lobe motion into valve motion, using a pivot or shaft and a contact surface on the valve stem (or tappet/pushrod).
- Valve lash (clearance) exists so valves fully seat when closed: correct lash compensates thermal expansion and prevents valve or cam damage. Too tight = valves don’t seat (loss of compression, burned valve faces). Too loose = noisy valve train, reduced valve opening effective duration, wear.

2) Fault symptoms that indicate rocker/valve-train work
- Loud valve-clatter increasing at idle, poor throttle response, hard starting, misfire, loss of power, smoke, increased fuel use.
- Visual signs: scored/pitted rocker tips, excessive play on rocker shaft/bushings, bent pushrods, collapsed valve springs, cam-lobe wear.

3) Preparatory checks & tools (theory: safe, repeatable measurements)
- Work on a cold engine for specified cold lash values. If adjusting hot is specified by the manual, follow that procedure.
- Tools: service manual (specs), feeler gauges, screwdriver/adjuster wrench, socket set, torque wrench, breaker bar for turning crank, marker or TDC indicator, clean rags, container for parts, penetrating oil. Gloves/eye protection.

4) Identify valve/TP positions (theory)
- You must adjust when the cam lobe for that valve is on its base circle (valve closed). For each cylinder set the crank to the point where both inlet and exhaust valves are closed — compression stroke TDC — or when the rocker shows maximum clearance. That ensures you are measuring against the cam base circle, not when the lobe is lifting.

5) Basic adjustment procedure (order of actions)
1. Disconnect battery earth for safety.
2. Remove air cleaner as required and unbolt rocker-cover/manifold components; clean area to keep dirt out.
3. Inspect rockers, shafts, pushrods, springs: look for pitting, scuffing, looseness. Replace obviously damaged parts. (Theory: replacing restores correct contact geometry and reduces friction/wear.)
4. Rotate engine by turning crank in normal direction until target cylinder is on compression stroke TDC (valves closed and cam lobe base circle under rocker). Confirm by noting both rockers for that cylinder are loose/at max clearance.
5. Place correct feeler gauge between rocker pad and valve stem/tappet. The gauge should slide with slight drag. (Theory: the gauge simulates the allowed thermal clearance.)
6. Loosen adjuster locknut, turn screw to obtain the slight drag on the feeler, hold screw, tighten locknut while keeping screw from moving. Recheck clearance; repeat until stable.
7. Move to next cylinder and repeat, using the correct sequence recommended by the engine manual (or ensure each cylinder is set when its valves are on base circle).
8. After all valves are set, rotate engine two full turns and recheck clearances to ensure no binding or shifting.
9. Refit rocker-cover with new gasket as needed, torque bolts to spec, reconnect battery.

6) If replacing rocker arms or shaft assembly (order + theory)
1. Remove rocker-cover and mark the position/sequence of rockers or mark pushrods so they return to original bores. (Theory: components wear-in as an assembly; keeping original pairing avoids seating/fit issues.)
2. Loosen rocker-shaft mounting bolts evenly, gradually, in sequence to avoid bending the shaft. Remove shaft assembly or individual rockers. Inspect the shaft for scoring and the bores/bushings for wear; measure for runout if possible.
3. Replace worn rockers/shaft/bushings/pushrods (bent pushrods must be replaced). Reassemble using the reverse procedure, pre-lubricating contact points with clean engine oil. Torque shaft bolts to spec in sequence.
4. Set valve clearances as above. (Theory: new surfaces restore correct geometry and contact radii so valve opens/closes at intended lift and timing; worn parts change effective valve lift and clearance leading to noise and compression loss.)

7) How the repair fixes specific faults (direct mapping)
- Noisy valve train: adjusting lash removes excessive clearance; replacing worn tips/shafts removes play and restores proper contact geometry so noise stops.
- Loss of power/poor starting/low compression: overly tight adjustment or bent/warped components prevent valves seating; correcting lash and replacing damaged components restores valve seating and compression.
- Excessive wear or oil burning: worn rockers can damage valve stems and guides; replacement stops progressive wear and restores sealing.
- Intermittent misfire/uneven idle: incorrect valve timing/effective lift from worn rockers or incorrect clearance changes combustion events; correction restores timing and lift, improving combustion.

8) Checks after repair (theory)
- Start engine, listen for steady, quiet valve train at idle.
- Re-warm engine to operating temperature if the manual requires a hot recheck; some engines need a hot re-torque or recheck of lash.
- Road/test under load and verify return to normal power, economy, and exhaust smoke. Re-check clearances after a short run-in period.

9) Warnings / common mistakes (brief)
- Don’t adjust when cam lobe is lifting; you’ll set the wrong clearance.
- Don’t overtighten locknuts or rocker-cover bolts beyond spec.
- Always use manufacturer clearance specs — not “typical” numbers — for final setting.
- Replace severely worn cam lobes, shafts, or pushrods rather than repeatedly adjusting; adjustment is a corrective measure only within wear limits.

End — follow the Perkins 3.152 service manual for exact clearance values, bolt torques, and the engine-specific sequence for cylinder TDC identification.
rteeqp73

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