Learning how to set valves on SBC engines is one of those rites of passage for anyone messing around with classic Chevys. Whether you just finished a fresh rebuild or you're trying to track down a mysterious ticking sound coming from under the hood, getting your valve lash or preload right is the difference between a motor that screams and one that just sounds like a sewing machine about to explode. It's not particularly difficult, but it is one of those jobs where being "mostly right" is actually "completely wrong."
If you've spent any time on the forums, you know there are about a dozen different ways guys swear by to get this done. Some people like to do it while the engine is running—which is messy as all get out—while others prefer the static method in the driveway. We're going to walk through the most reliable ways to get those rockers sitting pretty, focusing on the common hydraulic lifter setup most Small Block Chevys are running.
Getting your tools and mind right
Before you even crack a valve cover bolt, you need to have your gear ready. You don't need a massive rolling chest for this, but a few specific things will make your life a whole lot easier. You'll need a good socket set (especially a 5/8" for the rocker nuts), a long-handled ratchet or a breaker bar to turn the crank, and maybe some cardboard to catch the oil drips.
If you're running a solid flat tappet cam instead of the standard hydraulic setup, you'll absolutely need a set of feeler gauges. But for the vast majority of SBC owners, we're talking about setting "preload" on hydraulic lifters. This is basically just making sure the pushrod is sitting tight enough against the lifter plunger without bottoming it out.
Finding Top Dead Center (TDC)
The most important part of this whole process is making sure the piston is in the right spot before you start cranking on those nuts. You want to adjust the valves when the lifter is sitting on the "base circle" of the cam. If you try to adjust a valve while the cam lobe is already pushing it up, you're going to have a very bad day when you finally fire the engine up.
The easiest way to start is to get the number one cylinder to Top Dead Center on the compression stroke. You can do this by sticking a finger over the spark plug hole while a buddy turns the crank by hand. When you feel air pushing out, you're on the compression stroke. Line up the timing mark on your harmonic balancer with the "0" on your timing tab, and you're ready to roll.
The EOIC Method: The most reliable way
While some people like the "turn it once and do half the valves" shortcut, the EOIC method—which stands for Exhaust Opening, Intake Closing—is way more accurate, especially if you have a performance camshaft with a lot of duration. It takes a little longer, but it's much harder to screw up.
Here's the deal: you work cylinder by cylinder. To adjust the intake valve, you turn the engine over until the exhaust valve on that same cylinder just starts to open. Once that exhaust rocker moves up, you stop and adjust your intake.
To adjust the exhaust valve, you keep turning the engine until that intake valve opens all the way and has almost closed again. When it's about three-quarters of the way back up, that's when you set your exhaust lash. It sounds a bit fiddly, but once you get into a rhythm, it goes pretty fast.
Finding "Zero Lash"
This is where most beginners get tripped up. Zero lash is the point where all the slack is taken out of the valvetrain, but you haven't started pushing down on the lifter plunger yet.
The old-school way is to spin the pushrod between your thumb and forefinger while tightening the rocker nut. The second you feel a tiny bit of resistance, you're at zero lash. However, if you've got oily fingers or you're a bit heavy-handed, it's easy to blow right past zero.
A more foolproof way is the up-and-down method. Instead of spinning the pushrod, grab it and wiggle it up and down. Keep tightening the nut until you can't feel any more vertical "clunk" in the pushrod. That's your true zero lash. From that point, you'll usually want to turn the nut another half-turn to a full turn to set the preload. Most guys find that a 1/2 turn is the sweet spot for a street motor.
The messy way: Adjusting while running
If you're old school, or you just don't trust your hands to find zero lash while the engine is cold, you can set them while the engine is actually running. Be warned: oil is going to go everywhere. You can buy little clips that snap onto the rockers to help deflect the oil, or you can take an old pair of valve covers and cut the tops out of them to create a "tray."
With the engine idling, you back off one rocker nut at a time until you hear it start to clatter (the "clack-clack-clack" sound). Then, you slowly tighten it back down just until the clattering stops. That's your zero lash. From there, you slowly—and I mean slowly—turn it another half turn. The engine might stumble for a second as the lifter adjusts, but it should smooth right back out.
Why you shouldn't over-tighten
It's tempting to think that "tight is right," but over-tightening your valves is a recipe for disaster. If you put too much preload on a hydraulic lifter, it can't do its job of compensating for heat expansion. Worse, you might actually hold the valve slightly open all the time.
When a valve doesn't seat fully, it can't dissipate heat into the cylinder head. This leads to burnt valves, a rough idle, and a massive loss in compression. If you're ever in doubt, it's usually better to be a tiny bit on the loose side than too tight. A little noise is annoying, but a burnt valve is expensive.
Solid lifters are a different beast
If you happen to be running a solid cam, forget everything I said about "half a turn past zero." Solid lifters need a specific amount of "lash" or a physical gap between the rocker and the valve stem. You'll need the spec sheet from your camshaft manufacturer for this.
You use a feeler gauge to measure the gap. If the spec says .020", you slide that gauge in there and tighten the rocker until there's a slight drag on the metal strip. Solid cams sound like a sewing machine no matter how well you set them, so don't freak out when you hear that rhythmic clicking—that's just the sound of mechanical precision.
Wrapping it all up
Once you've gone through all sixteen valves, it's a good idea to rotate the engine by hand a few times just to make sure nothing is binding up. Double-check your work, throw the valve covers back on with some fresh gaskets, and fire it up.
If you did it right, the engine should sound crisp and responsive. If you still hear a loud "clack," you might have missed one or didn't quite find zero lash on a specific cylinder. Don't sweat it; just go back in and tweak that one. Learning how to set valves on SBC units takes a bit of patience, but once you get the "feel" for it, you'll be able to do it in your sleep. It's one of those basic maintenance tasks that keeps a Small Block Chevy running for decades.