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JD 6068 Engine - Cylinder head and valve checks and adjust
Make preliminary inspection of cylinder head and valve assembly during
disassembly.
Look for the following conditions:
1) Sticking Valves - Carbon deposits on valve stem. Worn valve guides.
Scored valve stems. Warped valve stems. Cocked or broken valve springs.
Worn or distorted valve seats. Insufficient lubrication.
2) Warped, Worn, or Distorted Valve Guides - Lack of lubrication.
Cylinder head distortion. Excessive heat. Unevenly tightened cylinder
head cap screws.
3) Distorted Cylinder Head and Gasket Leakage - Loss of cylinder head
cap screw torque. Broken cylinder head cap screw. Overheating from low
coolant level operation. Insufficient liner standout. Coolant leakage
into cylinder causing hydraulic failure of gasket. Leaking aftercooler.
Cracked cylinder head. Cracked cylinder liner. Damaged or incorrect
gasket. Overpowering or overfueling. Damaged cylinder head or block
surfaces. Improper surface finish on cylinder head. Improperly tightened
cylinder head cap screws. Faulty gasket installation (misaligned).
4) Worn or Broken Valve Seats - Misaligned valves. Distorted cylinder
head. Carbon deposits on seats due to incomplete combustion. Valve
spring tension too weak. Excessive heat. Improper valve clearance.
Improper valve timing. Incorrect valve or seat installed.
5) Burned, Pitted, Worn, or Broken Valves - Worn or distorted valve
seats. Loose Valve Seats Worn valve guides. Insufficient cooling. Cocked
or broken valve springs. Improper John Deere 6068 engine operation.
Improper valve train timing. Faulty valve rotators. Warped or distorted
valve stems. “Stretched” valves due to excessive spring tension. Warped
cylinder head. Bent push rods. Carbon build-up on valve seats. Rocker
arm failure. Incorrect valve or seat installed. Incorrect
piston-to-valve clearance.
6) Improper Valve Clearance - Inefficient use of fuel. Engine starts
harder. Maximum engine power will not be achieved. Shorter service life
of valve train. Greater chance for engine to overheat.
7) Excessive Recession - Worn valve guides. Bent valves. Debris passed
through valve train.
Check and adjust valve clearance
Little valve clearance throws valves out of time. Valves open too early
and close too late. This causes the valves to overheat due to hot
combustion gases rushing past valves when out of time. Overheating
lengthens valve stems which prevents proper seating of valves. The
valves seat so briefly or poorly that normal heat transfer into the
cooling system does not have time to take place, causing burned valves
and low power.
Too much valve clearance causes a lag in valve timing causing engine
valve train imbalance. The fuel-air mixture enters the cylinders late
during intake stroke. The exhaust valve closes early and prevents waste
gases from being completely removed from cylinders. Also, the valves
close with a great deal of impact, which may crack or break the valves
and scuff the camshaft and followers.
Valve clearance should be checked with engine cold. Remove rocker arm
cover and ventilator hose. Remove plastic plugs.
Visually inspect contact surfaces of valve tips or wear caps and rocker
arm wear pads. Check all parts for excessive wear, breakage, or cracks.
Replace parts that show visible damage.
Rotate John Deere 6068 engine with the Flywheel Turning Tool until
Timing Pin engages timing hole in flywheel. If the rocker arms for No. 1
cylinder are loose, the engine is at No. 1 “TDC-Compression.” If the
rocker arms for No. 6 cylinder are loose, the engine is at No. 6
“TDC-Compression.” Rotate the engine one full revolution to No. 1
“TDC-Compression.”
With engine lock-pinned at “TDC” of No. 1 piston’s compression stroke,
check and adjust (as needed) valve clearance on Nos. 1, 3 and 5 exhaust
valves and Nos. 1, 2 and 4 intake valves.
VALVE CLEARANCE SPECIFICATIONS
Intake Valves - 0.331-0.431 mm (0.013-0.017 in.)
Exhaust Valves - 0.457-0.559 mm (0.018-0.022 in.)
If valve clearance needs to be adjusted, loosen the locknut on rocker
arm adjusting screw. Turn adjusting screw until feeler gauge slips with
a slight drag. Hold the adjusting screw from turning with screwdriver
and tighten locknut to 27 Nm (20 lb-ft). Recheck clearance again after
tightening locknut. Readjust clearance as necessary. Rotate flywheel
360° until No. 6 piston is at “TDC” of its compression stroke. Rocker
arms for No. 6 piston should be loose.
Check and adjust (as needed) valve clearance to the same specifications
on Nos. 2, 4 and 6 exhaust and Nos. 3, 5, and 6 intake valves. Tighten
valve adjusting screw locknut to 27 Nm (20 lb-ft). Recheck clearance on
all valves again after locknut is tightened.
Check valve lift
Measuring valve lift can give an indication of wear on camshaft lobes
and cam followers or bent push rods. For a more accurate measurement, it
is recommended that valve lift be measured at 0.00 mm (in.) valve
clearance and with engine cold.
Remove turbocharger oil inlet line clamp and rocker arm cover. Loosen
locknut on rocker arm. Set valve clearance at 0.00 mm (in.). Tighten
locknut. Put dial indicator tip on valve rotator. Be sure that valve is
fully closed. Check preset on dial indicator. Set dial indicator pointer
at zero. Manually turn engine in running direction, using the JD 6068
Engine diesel rotation tools previously mentioned for checking valve
clearance. Observe dial indicator reading as valve is moved to fully
open position.
VALVE LIFT SPECIFICATION AT 0.00 MM (IN.) CLEARANCE
Intake - 13.53—13.71 mm (0.533—0.540 in.)
Wear Tolerance - 12.65 mm (0.498 in.)
Exhaust - 14.52—14.70 mm (0.572—0.579 in.)
Wear Tolerance - 13.64 mm (0.537 in.)
Adjust valve clearance to specification after measuring lift. Repeat
procedure on all remaining valves.
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