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Tools Required

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J 8520  Camshaft Lobe Lift Indicator Set

  1. Inspect the valves for the following conditions:
    • Burnt or eroded areas (1)
    • A worn margin (2)
    • A bent stem (3)
    • A worn or scored stem (4)
    • A worn key groove (5)
    • A worn stem tip (6)
    Fig 1: Inspecting Valve
    G02081644Courtesy of GENERAL MOTORS CORP.
  2. Inspect the valve face for the following conditions:
    • Worn margin (4)
    • No margin (1)
    • Pitted surfaces (2)
    • Burnt or eroded areas (3)
  3. Inspect the valve seats for a loose fit in the cylinder head.
    Fig 2: Inspecting Valve Face
    G02081645Courtesy of GENERAL MOTORS CORP.
    NOTE: Proper valve stem-to-valve guide clearance is necessary for proper engine operation and component life. Excessive clearance may cause a noisy valve train, premature valve stem oil seal failure, excessive oil consumption and related component damage. Insufficient clearance may cause a noisy valve train, rough engine operation or sticking valves that may lead to related component damage. Failure to inspect valve stem-to-valve guide clearance and repair or replace necessary components may lead to extensive engine damage.
    Fig 3: Inspecting Valve To Guide Clearance
    G02081646Courtesy of GENERAL MOTORS CORP.

    Important:  Excessive valve stem-to guide clearance may cause the following conditions:

    • A noisy valve train
    • Premature valve stem oil seal wear
    • Component damage
    • Excessive engine oil consumption
  4. Inspect for excessive valve stem (1) to guide (2) clearance.
  5. Use a dial indicator in order to measure the valve stem-to-guide clearance. Complete the following steps:
    1. 5.1. Install the valve into the guide.
    2. 5.2. Install the J 8520  onto the cylinder head.
    3. 5.3. Locate the dial indicator so that the movement of the valve stem from side to side, crosswise to the cylinder head, will cause a direct movement on the indicator stem.
    4. 5.4. Ensure that the indicator stem contacts the side of the valve stem just above the valve guide.
    5. 5.5. Drop the valve head about 2 mm (1/8 in) off the valve seat.
    6. 5.6. Use light pressure and move the valve stem side to side in order to obtain a clearance reading.
    7. 5.7. If the valve stem to guide clearance is not within specification, replace the valve and/or repair the guide in order to obtain the proper clearances.
    Fig 4: Inspecting Valve Stem Movement
    G02081647Courtesy of GENERAL MOTORS CORP.

    Important:  Replace a valve stem that has excessive scoring or wear. A valve guide that is worn and has excessive stem-to-guide clearance may need to be sleeved.

  6. Ream the valve guides for oversize valves if the clearance exceeds the specifications.
    Fig 5: Reaming Valve Guides For Oversize Valves
    G02081648Courtesy of GENERAL MOTORS CORP.

    Important:  Follow the manufacturers instructions when using valve guide reamers, sleeves, and installers.

  7. Ream the valve guide in order to obtain the proper specification.
    Fig 6: Reaming Valve Guide
    G02081649Courtesy of GENERAL MOTORS CORP.

    Important:  Valves that are pitted must be refaced to the proper angle. Replace valve stems that show excessive wear, or replace valves that are warped. Several different types of equipment are available for refacing valves and valve seats. Follow the manufacturers recommendations when performing these procedures.

  8. Inspect the valves for excessive scoring.
    Fig 7: Inspecting Valves
    G02081650Courtesy of GENERAL MOTORS CORP.
  9. Inspect the valve face.

    Replace the valve if the valve margin is not within specifications after grinding.

  10. Reface pitted valves on a valve refacing machine in order to ensure the correct relationship between the head and the stem.

    Important:  Reconditioning the valve seats is very important. Ensure perfect seating of the valves in the engine in order to deliver optimum power and performance. Clean the valve guides of all debris and dirt before grinding the valve seat.

  11. Recondition the valve seats after reaming the valve guide bores or installing the new valve guides.
  12. The valve seats should be concentric to within 0.05 mm (0.031 in) total indicator reading.
    Fig 8: Inspecting Valve Face
    G02081651Courtesy of GENERAL MOTORS CORP.
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