Chevrolet Cobalt Service & Repair Manual: Engine Valve Seat Grind

Chevrolet Cobalt Service & Repair Manual / Engine Service / Engine Valve Seat Grind

It is usually recommended to send head to machine shop for valve seat grinding.
A suitable valve grinding tool kit is required to grind valve seats.
Valve guides must be in good condition before grinding because a pilot is used in guide to center stones. If guides are worn out of limit, pilot will be off-center and in turn, valve seats will not center. Replace valve guides if worn.
When grinding valve seats, object of this procedure is to grind to a specific width completely around and positioned at an exact location on valve. A too narrow of a seat doesn't shed heat away from valve enough and causes valve and/or seat to burn. A too wide of a seat traps carbon under seat and valve which also causes burning. A good general guideline for seat width is .06 to .09 inch for two valve per cylinder heads. In three to four valves per cylinder heads, seats are closer and .04 inch works well for us here. A well matched seat to valve position is for valve to contact seat slightly above center on valve face.
Valve guides require very careful cleaning. Carbon left in guide will deflect pilot, resulting in inaccurate work.
Every new stone should be dressed for angle accuracy before being used, and should be dressed whenever it is mounted on a stone sleeve. To dress stone accurately for minimum stone runout, stand must be clamped to a workbench.
To facilitate seat grinding use two stone sleeves. One sleeve may carry a reconditioning valve seat grinding stone, another a 15 degree narrowing stone. This procedure eliminates delay involved in changing stones on one sleeve for once pilot is placed in a valve guide both finishing and narrowing cuts are usually made to complete that particular valve seat before removing pilot.
1. Remove valves from head as outlined in "Engine Valves, Replace."
2. Remove carbon from engine block and cylinder heads using a suitable wire brush. A bright, burnished surface will retard carbon accumulation in completed job. Kerosene will help to remove very hard carbon. Use proper brushes on aluminum heads to avoid scoring. After carbon cleaning thoroughly remove all loose carbon dust.
3. Clean valve guides using a suitable drill motor as follows:
  a. Insert cleaner in guide before starting drill.
  b. For better cleaning, move cleaner up and down in guide several times, ensure portion of bristles is kept within valves guides.
  c. Wipe bristles with clean cloth after cleaning each guide.
  d. Bristles will not be adversely affected by oil or gasoline.
4. After proper size pilot for valve guide has been selected, tapered sleeve on this pilot should be run out on its thread as far as possible. Insert this pilot until its upper taper stops on top of valve guide. Turn pilot clockwise with a pilot pin or pilot wrench to draw split sleeve against pilots lower taper, thus expanding split sleeve and locking pilot tightly in guide. Do not overtighten, a firm fit is desirable.
5. Apply thin film of oil to pilot, slide stone sleeve down on pilot shank until stone rests on valve seat.
6. Dress grinding stone as follows:
  a. Select stone in correct size. Screw stone tightly against collar of sleeve. Adjust arm of dressing stand so angle dressed on stone corresponds to angle ground on valve face.
  b. Wipe dressing stand mandrel free of dust and grit, and put one or two drops of light oil in stone sleeve bushing.
  c. Mount stand must be clamped to a workbench.
  d. Place stone and sleeve over mandrel and adjust diamond to contact stone by means of knurled head on outer end of diamond nib. Lock diamond nib in place with handle while dressing stones. Use unit to drive stone and sleeve.
  e. Put just enough pressure on driving unit to over- come its vibrating action. Too much pressure causes unwanted friction.
  f. With stone revolving, take a light cut by moving diamond slowly across stone by means of feed lever.
  g. Turn knurled head on diamond slightly to take additional cut. Repeat until diamond cuts entire face of stone, producing a true angle and a fresh cutting surface.
7. Insert hex ball spindle into stone sleeve socket and grind for a few seconds. "Stellite" seats require several minutes of grinding depending on eccentricity and wear.
8. While grinding support driver lightly to allow its vibrating mechanism to operate freely. Vibrating principle is important and pressure on driver would overcome action, causing stone to load rapidly.
9. Narrow with a 15° stone any badly worn seat made too wide by grinding. This prevents valve seat surface from becoming wider than valve face.
10. Valve seat grinding stone gives an accurate, mirror-smooth face on every job. Lapping valves in their seats is not required if precision equipment is correctly used and valve seat and valve face test true with Prussian Blue® or a suitable valve seat indicator.
11. To remove pilot, turn it once or twice counterclockwise and pull out.
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