It may not be pleasant, but setting the valve lash on a high-performance engine is a skill that must be learned. The reality is that even the most savvy engine masters had to learn about lash at one time or another. Whilehands-on experience is invaluable, with a little guidance you can easily learn to lash valves without breaking a sweat. Follow along as we run through the valve lash procedure on a typical small-block Chevy.
It all starts beneath the valve covers, where 16 rockers (two per cylinder) are responsible for taking up the valvetrain slack during engine operation. Whether lashing valves is performed in a vehicle or on an engine stand, it's best to remove all of the rockers and spark plugs first. Depending on where the engine stops its rotation, a minimum of two cylinders will be left attempting to open or close their valves. This places those rockers and valvesprings in a compressed state, and the rocker nut should not be removed until the spring is relaxed to its resting installed height. Removing the rocker nut while under valvespring pressure will cause the rocker stud to strip its threads. Before removing a pressurized rocker nut, rotate the engine until the valvespring comes to a complete rest against its valve lock.
From here we moved on to the installation of our 16 rockers. Our hydraulic lifter-equipped engine was rotated until the No. 1 exhaust lifter had risen and began to fall. Several degrees into the exhaust lifter's falling cycle we stopped the engine and began work on the No. 1 cylinder's intake valve. We placed the new rocker over its stud with the flat part of the rocker's center facing upward. Next we used one hand to move the pushrod up and down while the other hand tightened the nut until the rocker's pushrod cup lightly touched the pushrod. With minimal vertical pushrod movement being set by hand, there are three methods of setting various valve lash.
Intake Open/ Exhaust Close
The operation of any four-cycle valvetrain-equipped engine will follow the intake open/exhaust closed (IO/EC) formula, regardless of the engine manufacturer. The four-cycle camshaft design dictates that each lobe of the camshaft within the same cylinder be positioned roughly 180 engine degrees apart. With reference to the first-generation small-block Chevy camshaft in the captions, it's easy to see that the first group of lobes operates the intake and exhaust valves of the No. 1 and No. 2 cylinders. With this visual understanding it's easier to see how the resting valve can move across the backside of the camshaft lobe while its neighboring valve reaches its peak lift and begins to fall. The backside of the cam lobe places minimal pressure on the individual valvetrain piece, making it possible to adjust hydraulic or solid lash.
Hydraulic lifters are made of a hardened shell case that is partially hollow in the middle. The inside of a standard hydraulic lifter encompasses an open cavity with a pushrod, a plunger seat, and a lightweight plunger spring. The engine's oil pump transfers the fluid through the lifter valley, where it's then directed into the lifters to create a hydraulic plunger. With this type of design the lifters can safely compensate for valvetrain harmonics. Solid-lifter designs require the use a feeler gauge to create a camshaft manufacturer-predetermined lash gap between the rocker and the valve tip.
Hydraulic Lifter With Stock
Locking Rocker Nut
This type of lifter and rocker-nut combination requires the engine builder to turn the rocker nut 31/44-turn once the rocker and pushrod meet. This places a bit of load on the lifter's internal plunger and creates a smaller lifter cavity to make it easier for the oil pressure to provide a cushion for valvetrain lash.
Hydraulic Lifter With Two-Piece Locking Nut
The same theory holds true here, the only difference being the type of rocker nut and the method by which it is turned. In this case the rocker and pushrod meet just before the outer rocker nut is turned 11/42-turn. This places 21/43 the amount of lash on the hydraulic lifter's internal plunger. From there the internal rocker nut should be run down until it bottoms out. At this point the internal rocker nut needs to be held tight as the outer rocker nut is rotated an additional 11/44-turn. The internal lock nut will then secure itself against the external rocker nut to hold the proper rocker-arm load.
Solid-lifter applications require a feeler gauge to set a manufacturer's predetermined amount of lash. As the rocker nut places the rocker against the pushrod, the feeler gauge is pushed in between the valve tip and the rocker. This gap is the amount of lash the valvetrain is allowed. At this point either rocker-nut assembly mentioned earlier can be fastened to hold the proper amount of gap by allowing the feeler gauge to snuggly slip in and out of the gap between the rocker and valve tip.
We set the lash on our solid lifter-equipped engine using the two-piece locking nut design. The No. 1 cylinder led us off as we worked a cylinder at a time through all eight holes in a matter of minutes. Then we reinstalled the valve covers, fired up the engine, and listened for any rattling or snapping. It's important to note that a poor idle can also be the result of improperly lashed valves. Don't get discouraged if you don't get it right on your first attempt-just get under the hood and try again.