Specifications, Service and Repair of Farm and Lawn Tractors


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.


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.


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.