Chevrolet Full-size Cars 1968-1978 Repair Guide

Valve guide


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Fig. Fig. 1 Cutaway view of a knurled valve guide

The engines covered in this guide use integral valve guide; that is, they are a part of the cylinder head and cannot be replaced. The guide can, however, be reamed oversize if they are found to be worn past an acceptable limit. Occasionally, a valve guide bore will be oversize as manufactured. These are marked on the inboard side of the cylinder heads on the machined surface just above the intake manifold.

If the guide must be reamed (this service is available at most machine shops) then valves with oversize stems must be fitted. Valves are usually available in 0.001 in., 0.003 in. and 0.005 in. stem oversizes. Valve guide which are not excessively worn or distorted may, in some cases, be knurled rather than reamed. Knurling is a process which the metal on the valve guide bore is displaced and raised, thereby reducing clearance. Knurling also provides excellent oil control. The option of knurling rather than reaming valve guide should be discussed with a reputable machinist or engine specialist.


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Fig. Fig. 2 Using a special tool to check the valve seat for concentricity

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Fig. Fig. 3 Check the cylinder head for flatness across the head surface

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Fig. Fig. 4 Checks should be made both straight across the cylinder head and at both diagonals

Once the complete valve train has been removed from the cylinder head(s), the head itself can be inspected, cleaned and machined (if necessary). Set the head(s) on a clean work space, so the combustion chambers are facing up. Begin cleaning the chambers and ports with a hardwood chisel or other non-metallic tool (to avoid nicking or gouging the chamber, ports, and especially the valve seats). Chip away the major carbon deposits, then remove the remainder of carbon with a wire brush fitted to an electric drill.

Be sure that the carbon is actually removed, rather than just burnished. After decarbonizing is completed, take the head(s) to a machine shop and have the head hot tanked. In this process, the head is lowered into a hot chemical bath that very effectively cleans all grease, corrosion, and scale from all internal and external head surfaces. Also have the machinist check the valve seats and re-cut them if necessary. When you bring the clean head(s) home, place them on a clean surface. Completely clean the entire valve train with solvent.

Checking for Head Warpage

Lay the head down with the combustion chambers facing up. Place a straightedge across the gasket surface of the head, both diagonally and straight across the center. Using a flat feeler gauge, determine the clearance at the center of the straightedge. If warpage exceeds 0.003 in. in a 6 in. span, or 0.006 in. over the total length, the cylinder head must be resurfaced (which is akin to planing a piece of wood). Resurfacing can be performed at most machine shops.

When resurfacing the cylinder head(s) of V8 engines, the intake manifold mounting position is altered, and must be corrected by machining a proportionate amount from the intake manifold flange.

Lapping the Valves

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Fig. Fig. 5 Lapping the valves by hand

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Fig. Fig. 6 An example of a homemade valve lapping tool

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 seal.

  1. Invert the cylinder head so that the combustion chambers are facing up.
  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.
  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.
  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).
  9. Remove the valve and tool, and remove ALL traces of grinding compound with solvent-soaked rag, or rinse the head with solvent.

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.