posted December 02, 2001 10:55 AM
Exactly What Do I Do To Test The Piston-To-Valve Clearance?
The process is really quite simple, but if you have not already figured out what you need to do, this will be a step-by-step procedure on how to do it.
First, you need to have your parts you plan on using for this engine ready to go. These include the block, crank, connecting rods (just need one or two for this test) with a piston hung on it, camshaft, cam bearings installed, rod and main bearings, lifters, pushrods, cylinder heads (with parts), timing chain set and a used head gasket.
You first assemble your short block with the crank, one or two piston rod combinations (remember that the big block Chevy as well as many other engines have "handed" cylinders. If this is the case you need to test each "hand"), the camshaft, and timing set.
With the short block pieces assembled you need to degree in and set up your camshaft where you plan on running the engine. If you plan on making future cam or cam timing changes, you should repeat the entire testing procedure for each variation you plan on using. Each change you make can cause an interference problem even if the first time you checked your clearances you were fine.
With that done, you will place a small amount of the modeling clay on the piston(s). (see image at right) Lightly oiling the piston top, valve faces, and chamber on the cylinder heads will save you a ton of grief when removing the clay. You need to place the clay over and around the valve reliefs on the piston top. Be careful not to place too much clay or making it too thick around the edges of the pistons. The clay only needs to be about 1/4" - 3/8" thick.
You now have the clay on the piston top, so you are ready to install the cylinder head. Use your used head gasket and place it on the deck and install the cylinder head. Use normal torquing procedures for your cylinder head and engine requirements. Once the head is on you can install the remaining valvetrain (lifters, pushrods, rocker arms) on the cylinder(s) that are to be checked. Install both the intake and exhaust lifters for each cylinder tested, and also the pushrods and rocker arms. Be aware that you must test using the rocker arm ratio you are going to use on the completed engine. If you plan on installing a high ratio rocker arm at a later date, test clearance for that now. This will save you problems in the future. When you set the lash, set it at "zero". It is preferred that you test hydraulic cams with a solid lifter installed (a used lifter is OK too), just to make sure there is nothing that can cause a variance of the valve lift in your measurement.
With all this complete, you will now be rotating the engine. Place your crank socket and wrench on the crank and rotate the engine two complete revolutions. Your engine operates over 720°, not 360° as many people believe. With your 4-cycle engine this mandates rotating the engine two complete revolutions. If when turning the engine you feel resistance beyond normal, be careful, you may have contacted the valve against the piston.
You now will take the cylinder head off carefully so as to not disturb the clay. When you remove the head you will see the clay with the valve indentations in it. Look at the clay and you will see thinner and thicker areas of the clay. The main areas you need to look at are the depth and edges of the eyebrows (valve reliefs), and the edges of the piston against the block (especially on big block Chevrolets).
Carefully take your razor knife at various locations on the clay and slice it. You can cut a section out if you wish and then use your caliper to measure the thickness of the clay that is still sitting on the piston top. You should not try to measure the clay you removed, because in most cases you will stretch it and change it's dimensions upon removal and handling. Check the clay in various locations to get a detailed view at what those valves need for clearance. If you find areas that have less than admirable clearances you will need to go to that next step, do some machining.
Use modeling clay and not Play-Doh, the modeling clay won't "spring back" on you.