A properly tuned engine is an incredible mix of components all moving in harmony. Air, fuel, and metal all working together to produce that precious magic we gearheads call torque.
But engines aren’t black magic. They are good-old engineering and science. And that’s the good news because it means that anyone can build and maintain their own engine. All you need is the right knowledge, and that’s what we here at Chevy High Performance are all about—giving the info you need to make the most of your Bowtie life.
A critical factor for any engine is the proper clearances between parts. Valve lash is the available clearance (or gap) between the rocker arm and the tip of the valve stem when the lifter for that valve is sitting on the base circle of the cam lobe (when the valve is intended to be fully closed). Lash isn’t really an issue with a hydraulic cam and lifter setup because the plunger in the lifter moves to take up the lash. But on a solid cam and lifter setup, lash is a very big deal because it must be set manually.
If you are running solid lifters, it is important to check your lash at least semi-regularly because, if you are smart, it can be a tell to help you catch a developing engine issue before it becomes a full-blown problem. If the lash has increased since the last time you set it, there are only a few things that it can be: damaged valve tips, a bad rocker arm trunnion, or the lash adjuster mechanism has either moved or has something wrong with it.
Often, you can hear the clatter in the engine when the lash opens up too much. Other times you might not notice it until you physically check the lash. So, for most situations, check the lash on at least a few lifters every time you change the oil. If you find the lash has grown, you’ve got to determine if it is something normal like the adjusters loosening up or if you need to dig deeper into the engine to find the real source of the problem.
On the other hand, if you find you’ve lost lash, it normally it means the valve seat is eroding, allowing the valve to move higher in the cylinder head.
Understanding the Process
The actual process of lashing your solid valvetrain isn’t difficult or complex. Basically, you want to check each valve when it is on the heel of the cam (when the valve should be closed) using a feeler gauge between the valve stem tip and the end of the rocker arm to determine lash. If it is off, the lash can be changed with the adjuster nut on stud-mounted systems or a lash adjuster on shaft-mounted systems.
Really, the toughest part in the whole process is determining when the lifter for the valve you are checking is actually on the heel of the cam lobe and not starting to ride up the opening ramp or off the closing ramp, which would give you a false lash reading. But there’s a great trick to always make sure you are checking lash at the exact right time in the engine’s cycle, and that’s called the “EOIC” method.
EOIC stands for Exhaust Opening, Intake Closing. In other words, when working with a pair of valves for one cylinder (one intake and one exhaust) you can know when the valve is on the base circle of the cam by the action of its mate. When the exhaust valve is opening, you know that the intake valve will always be closed. And likewise, when the intake valve is closing, you know that the exhaust valve will always be closed. This is true no matter the number of cylinders or the cam’s specs, including lobe duration and separation. Heck, it even works with those overly complex overhead cam engines your Blue Oval friends and import fanatics spend their time with.
To use the EOIC method, pull the valve covers so you have access to all the rocker arms. You can do this with the engine either on a stand or in the car, but you must be able to rotate the crank either by hand or with a bump starter.
Start with the No. 1 cylinder. Rotate the engine until the exhaust valve is beginning to open—not until it is all the way open—and check the lash between the tip of the intake valve stem and the rocker arm. If it is off, use the adjuster on the rocker arm until the correct thickness feeler gauge will just fit between the valve stem and the rocker without forcing it and lock the adjuster down. Finally, always recheck your lash after you have everything tightened down just to make certain nothing has moved.
After the intake lash is set, continue spinning the engine until the intake valve is fully open and begins to close. Now you can set lash on the exhaust valve for that cylinder. Instead of skipping around, work your way down each bank of cylinder heads methodically so you will know for certain you haven’t missed a valve.
Hot vs. Cold
What we haven’t mentioned so far is how much lash you actually want on your solid lifter valvetrain. The good news is that your camshaft manufacturer will give you the spec for that right on the cam card. Trust your cam manufacturer on your lash. If you have too little, you stand the chance of the lifter riding the cam lobe all the way around and the valve never fully closing. Too much, however, becomes very damaging to the lifter and the rest of the valvetrain.
Of course, the lash figure the camshaft manufacturers give you is “hot lash.” Just like it says, hot lash is the amount of lash in the valvetrain when the engine is at operating temperature and all the components involved—including the block and the cylinder heads—have expanded. If you are checking lash on an already operating engine, that’s no big deal. Just run the engine for a few minutes until it is at operating temp, shut it off, pull the valve covers and (carefully!) check the lash.
But how do you set the hot lash on a new engine build, or after you have made upgrades to your existing engine that required pulling the rocker arms? Before you can fire the engine for the first time, you will need to set the cold lash, and that will require a few assumptions to get you into the ballpark.
The trick with setting the lash when the engine is cold is to predict as accurately as possible how much the lash will change when the engine heats up and all the metal components start expanding.
We spoke with several engine builders for this story and asked specifically about finding the right cold lash so that when the engine heats up, the hot lash will fall right into place. And we noticed that there is an unofficial rule of thumb in the industry that can be very helpful. It’s not perfect because there are lots of variables like tall-deck versus short-deck blocks and different cylinder head designs, but it should at least get you close.
Generally speaking, if you are running an engine with an iron block and heads, the cold lash is the same as the hot because the iron block and heads expand at approximately the same rate as the steel pushrods. However, if you run an iron block and aluminum 23-degree cylinder heads, run the cold lash 0.006 of an inch tighter than your hot number. And for an aluminum block and heads, tighten the cold lash 0.010 over the hot. Again, don’t expect this guideline to be perfect for every situation, but it should get you into the ballpark so the engine is at least safe to crank until you can dial in your hot lash.
Finally, you can also manipulate your lash settings while on the dyno to determine if you could potentially make more power with a different camshaft.
Closing up the lash on an engine makes the camshaft act as if it is “larger,” with more lift and duration. Likewise, adding lash reduces the effective lift and duration, since more of the cam lobe’s lift is used up before the gap in the valvetrain is fully closed, making the cam act smaller.
When testing on a dyno, you can perform lash loops, or tests, by making dyno runs with the lash progressively larger or smaller to see how it affects performance.
Running lash loops is a great idea whenever you have the chance. Normally, it’s recommend on lash loops to go in 0.004-inch steps, make a dyno pull and see what it does. If you went either smaller or larger and you found that it helped, change the lash in that same direction again and see what happens.
As you are changing the lash, don’t go beyond 0.012 of an inch either tighter or looser than the recommended lash. If you go beyond 0.012—especially if we are talking about 0.012 looser—you can cause damage to the engine. If you want to make a dyno run close to that mark, at least don’t make the pull all the way to peak power. That way you can still look at the curve and make an educated guess at what it will do while still protecting the valvetrain.
If you find that your engine makes more power with the lash either tighter or looser than your camshaft manufacturer’s recommended lash, you’ve now got a decision to make. Do you stick with the lash change? Go back to the manufacturer’s recommendations and give up the found power? Or maybe spend cash on a new camshaft grind?
We wish we had a simple answer, but unfortunately, every situation is different. Talk to your engine builder as well as your cam manufacturer to see what they recommend. Just remember, the goal isn’t an engine that simply runs strong, but one that runs both strong and long.
1. If you run a solid lifter valvetrain in your engine, setting and maintaining proper valve lash is critical to not only making maximum power, but also keeping your engine healthy. Here’s an easy method to make sure you are doing it right.
2. Lash is important because all the components in the valvetrain—the cam lobes, lifters, pushrods, rockers, and valve stem—require room to expand with heat. Too little lash and you run the risk of not allowing the valve to fully close. Too much and that extra gap can cause damage to the engine and valvetrain.
3. Unless you are running an engine with high-end shaft rockers, you likely will be working with stud-mount rocker arms that look something like this. These aluminum Scorpion rockers depend on an adjuster nut to help set the lash.
4. Here’s an example of a shaft-mount rocker arm system mentioned previously. This particular example is on an LS built for road racing. As you can see, the rockers are mounted solidly to the rocker stand so you can’t adjust lash by moving the entire rocker arm higher or lower on the stud. Instead, the rocker arm has an adjuster built into the pushrod side of the rocker.
5. This is a new Chevy small-block going together, but the valvetrain is similar to just about all other stud-mount systems. The first step is to make sure the pushrods are the correct length to center the tip of the rocker arm’s movement over the valve stem tip. If you aren’t sure how to do this, go to chevyhiperformance.com and search for “pushrod length.”
6. Using the EOIC method, rotate the crankshaft until you see the pushrod for the intake valve dropping down (the valve would be closing if the rocker arm was installed). Now you know the lifter for the exhaust side of that cylinder is on the base circle of the cam. Drop the rocker arm into place and lightly thread on the adjuster nut.
7. Use a feeler gauge that’s the same thickness as the lash target. Tighten the adjuster nut until you can just insert the feeler gauge between the rocker arm and the valve stem tip without having to force it.
8. Now here comes the part that’s a little like trying to pat your head and rub your stomach at the same time. Once you have the adjuster nut at the right height to give you the lash you want, hold it in place with an open-end wrench and tighten the lock nut with an Allen wrench. Just tighten it down good and snug, there’s no need to really lean on it.
9. Once you have the cold lash set, run the engine just long enough to get it up to temp and then carefully go through and set all the rockers again at the hot lash.
Photos: Jeff Huneycutt