Valve Lash Adjustment
There are two kinds of valve lifters. Hydraulic and solid (mechanical). The simplest kind, solid, are just a slug of polished metal that follows the cam contour. Valve lash is the measurement of the slack in the lifter, pushrod, rocker arm (valve train) path. Solid lifters have a specific setting for exhaust and intakes and differ in setting between when hot and cold. Hydraulic lifters are similar in size and shape but have a hollow interior. A check valve allows oil under pressure to flow into the hollow. When the cam rotates and starts up on a lobe, the lifter closes the check valve and the oil becomes trapped inside. This makes the hydraulic lifter "solid" on it's upstroke. Why bother with the oil? The beauty of a hydraulic lifter is when the check valve is open, oil pressure causes the upper part of the lifter to completely close any gap that might exist between the top/bottom of the lifter and hence take up any slack that might exist as the cam, lifter, pushrods, and rocker arm wear. This means the hydraulic lifter will always have zero lash when correctly set in the center of it's operating range. Solid lifters are used only in high performance modern engines used in racing since they constantly require re-adjustment (about every 5000 miles use). Almost all modern cars even in the 1960's had hydraulic lifters in most engines. The only '65 not having hydraulic lifters was the HiPo engine. Setting mechanical lifters requires removal of the valve covers to gain access to the rocker arm adjusting nuts. There is a special tool (clip) required that snaps over the rocker to prevent oil from squirting everywhere when the engine is run. The engine is run at idle (hot) with the clips in place, and a feeler gauge of the correct thickness is placed between the rocker arm and the valve. The adjusting nut is loosened until the rocker starts to clatter. The feeler gauge is slipped between the valve stem and the rocker. The clatter level will go down when the feeler gauge is inserted. The adjusting nut is tightened until the clatter just disappears and no more. By putting one's thumb on the rocker while it's in motion, a distinct shock wave can be felt when the rocker is too loose (feeler guage in place). Correct lash is obtained when the shock wave just disappears. This is the ideal setting and should be consistent with the point where the clatter just disappears. Yes, you can set them "cold" by rotating the engine as suggested by another reply, but this method will result in the ideal valve lash. Repeat every 5000 miles. A hydraulic lifter is much easier to adjust. Everything is done the same way as above but no feeler guage is required. The adjusting nut is loosened until clatter is heard and a shock wave is present. The adjusting nut is then turned down one full turn from the point where the shock is first felt and clatter is heard. This adjustment centers the hydraulic lifter in it's operating range. No further adjustment is required until the next valve job or until the lifter gets sticky from aged oil. If you have sticking lifters, simply remove them one at a time, being absolutely certain to put the same lifter back in the same relative position. Take the assembly ring carefully out of the center of the lifter and clean all internal parts in solvent. Be absolutely to put the parts back in the same way they come out. The check valve part is usually hockey puck looking but is curved on one side. Turning it over by accident then reassembling the engine will leave you with a very noisy lifter that will not adjust. Caution: make damn sure you put it back together like it came apart. If you don't like this procedure, you can do the job with the engine cold/off by rotating the engine through 720 degrees to make all the adjustments. Problem with this method is it does not take into account the growth of the valve train with heat expansion. It works but is somewhat trial and error and depends on more first hand experience with a particular engine.
