 |  | | | Building a high-performance engine with various speed parts is tricky business. If you follow the basic engine building rules (measuring every part to ensure they meet specs, trial-assembling the engine to check clearances), you shouldn't have any problems. One of the most critical clearances that should be physically checked is valve-to-piston clearance. Many first-time engine builders think that they can calculate the amount of valve clearance by knowing how high the piston is in the bore at top dead center and the total lift at the valve. Well, it just doesn't work that way. You won't know if the valve notches in the pistons are in the right place and exactly how everything will mesh unless you go through the process. Put clay on the top of the pistons, install the cylinder heads and valvetrain, and rotate the engine through a complete cycle (two complete revolutions).
Clay-Mation
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When I assembled my Buick 455, I got lazy and just assumed that there was enough valve-to-piston clearance. How much is enough? Most professional engine builders recommend .100-inch for stick-shift cars and as close as .070-inch for automatic transmission vehicles. Unless you're building the world-champion-whatever race engine and need every bit of compression you can get, use the .100-inch number to be safe (even more if you can get it). One missed shift at 7,000 rpm and valve float could instantly turn an expensive engine into a pile of junk. I was riddled with guilt and decided to pull off a cylinder head and check it out. Besides, doing the work only takes a couple of hours of garage time, about $1.00 of modeler's clay and an inexpensive measuring device. (We used our trusty $20 plastic dial caliper.) The first step is to clean the piston top with lacquer thinner to provide a clean, oil-free surface for the clay to stick to. Then cut a couple of strips of clay about 3/16-inch thick and place them where you think the valves could hit the piston.
Engine Rotation
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Next, spray the valves with a light coat of oil (to prevent them from sticking to the clay) and put the cylinder head back on the engine with a head gasket, then install the valvetrain and adjust the valve lash. Here's the tricky part: If your engine has hydraulic lifters, use a mechanical lifter or a piece of wood dowel or an aluminum slug that matches the pushrod seat height in the lifter. Why? A hydraulic lifter in a non-running engine will deflect the plunger and not give you true valve lift. Another possibility is to take an old hydraulic lifter apart and fill it with something solid so that the pushrod seat remains up against the retaining clip. Once you figure out this little issue, the rest is easy. You simply rotate the engine through two complete rotations. Do this slowly and with feeling. If the engine doesn't want to rotate completely through, don't force it. Remove the cylinder head and see what's hitting.
Squish Check
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Once you've successfully run the crankshaft through two complete revolutions, remove the cylinder head and inspect the clay. Hopefully, the clay was placed in the right location and stayed put for the valve opening and closings. (By cleaning the piston, the clay has plenty of stick, and by spraying the valves with light oil, they won't stick to the clay at all.) If you used thick enough clay, the valves should leave fairly large impressions that are easy to identify. Notice that there are two sets of impressions in this photo. The outside one was made by the edge of the combustion chamber. Only one valve (the intake valve) touched the clay on our engine; the exhaust valve didn't even leave a dent.
Measurements
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To determine how much valve-to-piston clearance you actually have, measure the deepest part of the groove made by the valve in the clay. Also, make sure that you measure the deepest groove at the highest portion of the piston dome. Our Buick pistons had a big dish in the center, so the clearance there was more than adequate. However, out on the raised edge of the piston, we measured the clay thickness with the wire-rod depth gauge of our dial caliper and measured only .075-inch clearance—just enough to squeak by. This big Buick redlines at 6,000 rpm, and the automatic trans should give us the needed safety factor.
Do yourself a favor and do this valve-to-piston inspection when you trial-assemble your engine. Inadequate clearance means disassembling the engine, having the pistons notched at a machine shop and starting the assembly process all over—a real pain!
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