GC4VETTE,
As long as you are going to change spark plugs, you can loosen them to relieve the compression. Leave the plugs in the cylinders loose, so that nothing can fall into the cylinders. This will make it easier to turn the engine over with a 19 milimeter socket on a 1/2 inch breaker bar (or wratchet) on the crankshaft pulley bolt. Trans
in neutral or park.
You turn the engine over to line up the cam lobes so that the heel of the lobe, not the pointed end, is over the valve you are checking. The pointed end of the cam lobe should point upward, and be perpendicular to the valve disk.
The disk, or "shim", is on top of the valve. Using a feeler guage, you measure the clearance, or "gap" between the heel of the cam lobe and the shim.
The intake valve clearance specified is .006 to .010, with .008 (in the middle) being the target gap. The exaust valve clearance specified is .010 to .014, with .012 being the target gap.
If the clearances are out of specification, it is adjusted by removing and replacing the shim with a shim that will bring it into specification.
If you decide to make adjustments to your valve clearances you will need two "special tools" from OTC, and a source of shims, such as SHO NUT.
OTC tool 1) compressor, tappet 303-344 $19.31
2) holder, tappet 303-345 $14.78
Plus shipping and taxes (if applicable). Phone 866-640-7305
You may want to get the tools first. Then make a list of the clearances, and the disks that are in the car. If, for example you have a clearance of .013 on an exaust valve, and the disk is a 2550, the next disk up (in size) is a 2575. The disks are very near .001 differnt in size. The 2575 is .001 thicker than the 2550.
In this example, a new 2550 may bring the clearance into specification, if only the disk is worn. If the cam is also worn, then the 2575 may be necessary to get the proper clearance. The numbers on the back side of the disk are the thickness of the disk in milimeters.
I suggest that you go into SHO Nuts web site and read about the shims, so that this will make more sence, that is, knowing what you have, and what you may need to correct it. Hope this helps, rangerj