GM Corvette 1963-1982 Repair Guide

Valve Guide and Seats



See Figures 1, 2, 3 and 4

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Fig. Fig. 1: Critical valve dimensions

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Fig. Fig. 2: View of common valve stem wear patterns

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Fig. Fig. 3: Cutaway view of a knurled valve guide

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Fig. Fig. 4: Check the valve seat concentricity with a run-out gauge

The small-block engines (305, 327, 350) do not have separate valve guides and seats. The small-block head is basically a one-piece casting; the valves move inside guide bores and seal against seats which are machined into the cylinder head.

All big-block (396, 427, 454) cylinder heads use separately pressed-in valve guides. Chevrolet does not sell replacement valve guides nor do they publish a factory recommended procedure for replacing the guides. They (the guides) can be replaced, but this can be performed only by a professional automotive machine shop. Valve seats of cast-iron big-block heads are machined into the cylinder head; they cannot be replaced. These seats, as those of the small-block, can be machined during a valve job to provide optimum sealing between the valve and the seat. Aluminum big-block cylinder heads use hardened steel valve seat inserts pressed into the cylinder heads. As in the case with big-block valve guides, Chevrolet does not sell replacement valve seats nor have they published a factory recommended procedure for replacing the seats. Due to the extremely high cost of replacement aluminum heads be absolutely sure that the machine shop you chose has a noteworthy reputation for precision work.

Whether you have a small or big-block engine, valve guides are most accurately repaired using the bronze-wall rebuilding method. In this operation, "threads" are cut into the bore of the valve guide and bronze wire is turned into the threads. The bronze "wall" is then reamed to the proper diameter. This method is well received for a number of reasons: a) relatively inexpensive b) offers better valve lubrication (the wire retains oil) c) less valve friction d) preserves the original valve guide-to-seat relationship.

Another method of repairing worn guides is to have the guides "knurled." The knurling process entails cutting into the bore of the valve guide with a special tool. The cutting action "raises" metal off of the guide bore which actually narrows the I.D. of the bore, thereby reducing the clearance between the valve guide bore and the valve stem. This method offers the same advantages as the bronze wall method, but will generally wear faster.

Either of the above services must be performed by a professional machine shop which has the specialized knowledge and tools necessary to perform the service.


See Figures 5 and 6

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Fig. Fig. 5: Lapping the valves using the proper tool

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Fig. Fig. 6: A valve lapping tool can be fabricated as shown

When valve faces and seats have been re-faced and re-cut, 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.
  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. Moisten the suction 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.