GM Caprice 1990-1993 Repair Guide

Valve Seats

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The valve seats are not removable, they are an integral part of the cylinder head. Any cutting or grinding operation performed on the valve guides, should be done by a qualified machine shop.

LAPPING THE VALVES



See Figure 1



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Fig. Fig. 1: Lapping the valves using a hand valve lapping stick

When valve faces and seats have been refaced and recut, or if they are determined to be in good condition, the valves must be lapped-in to ensure efficient sealing when the valve closes against the seat.

  1. Invert the cylinder head so that the combustion chambers are facing up.
  2.  
  3. Lightly lubricate the valve stems with clean oil, and coat the valve seats with valve grinding compound. Install the valves in the head as numbered.
  4.  
  5. Attach the suction cup of a valve lapping tool to a valve head. You'll probably have to moisten the cup to securely attach the tool to the valve.
  6.  
  7. Rotate the tool between the palms, changing position and lifting the tool often to prevent grooving. Lap the valve until a smooth, polished seat is evident (you may have to add a bit more compound after some lapping is done).
  8.  
  9. Remove the valve and tool, and remove ALL traces of grinding compound with solvent-soaked rag, or rinse the head with solvent.
  10.  

Valve lapping can also be done by fastening a suction cup to a piece of drill rod in a hand eggbeater type drill. Proceed as above, using the drill as a lapping tool. Due to the higher speeds involved when using the hand drill, care must be exercised to avoid grooving the seat. Lift the tool and change direction of rotation often.

 
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