Under a tree, in a remote corner of the earth, there is a guy adjusting his hydraulic valve lash with the valve cover off and the engine running. Yes, it's true. We've seen it. Proof is that you can buy valve covers with the tops cut off so that very same guy won't get oil sprayed all over his headers and T-shirt. At the same time, dyno shops and engine builders are sending engines out into the world with the valve lash pre-adjusted. So what gives? How is it done? We will show you.
Cold Engine BasicsValve lash is simply the clearance between the tip of the valve and the tip of the rocker arm. Too much lash causes noise and potential valvetrain damage and too little can potentially cause the valve to stay open when it should be closed or to bottom out on the lifter.
To adjust valve lash, be it with a hydraulic or mechanical cam, you need to have the lifter on the heel or base circle of the camshaft lobe, and there are a couple of ways to get there. The first is the exhaust opening and intake closing method (EO/IC). Starting with cylinder No. 1 (usually the head that is farthest forward on the block is No. 1, or see our handy charts) with the valve cover off, rotate the engine until the exhaust pushrod begins to move upward. This is the point at which the exhaust valve is opening (EO), meaning the piston is through with the power stroke and nearing BDC, and it's about to begin the exhaust stroke. At this point, the intake valve is closed and the lifter is on the base circle of the camshaft and can be adjusted. To adjust lash with hydraulic lifters, simply grab the pushrod and back off the rocker-arm adjusting nut until you feel lash between the pushrod and the rocker arm. Turn the rocker arm adjusting nut while rotating the pushrod until there is no more play between the pushrod and the rocker arm, then tighten the adjusting nut one half turn. That's it, you're done. To adjust the exhaust lash, turn the engine over until the exhaust valve has completed its lift cycle and the intake valve is closed about three-quarters of the way (IC). Now, the exhaust lifter is on the base circle because the piston is finishing the intake stroke and about to begin compression. Set the lash on the exhaust valve and you're done. This needs to be done for each cylinder. But there is another way.
The Companion Cylinder (or 90-degree method)With the EO/IC method, you must move the crank 16 times to get all the valves adjusted. This is fine if you haven't a clue what the firing order is or how the cylinders are numbered, but if you do, you can use the companion cylinder method (also called the 90-degree method), which allows you to adjust all the valves and only turn the engine over once. This method is actually preferred by OEs, and you can usually find it as the recommended method in most repair manuals. The downside is that we've heard that with an aftermarket cam, this method can be slightly off, and you have to know both the firing order and the cylinder numbering of your engine. You do, don't you? You also need to understand companion cylinders.
In a four-stroke engine, companion cylinders are those that reach TDC and BDC at the same time, though they are in different phases of the four-stroke cycle. Let's say you have the No. 1 cylinder at TDC on the compression stroke, which means the No. 6 cylinder is also at TDC but is in overlap. That is true on nearly every V-8 you are going to see, regardless of the firing order or the cylinder numbering (the exception is the GM Gen III and Ford 5.0 HO, 351W, 351M, 351C, and 400). It also means there are other cylinders at various points in the cycle (some waiting at 90 degrees) just waiting to be adjusted. Going back to No. 1 on TDC on a GM, you know that both the intake and exhaust valves can be adjusted because both lifters are on the cam base circle. Nos. 4 and 8 are both at different points on the compression stroke, and No. 3 is on the intake stroke. Each of these events indicates that their exhaust lifters are on the base circle and can also be adjusted. Also, No. 2 is on the power stroke, and Nos. 5 and 7 are both at different points of the exhaust stroke. This allows you to adjust their intake valves.
Rotate the engine to TDC on the compression stroke for No. 6, and you can adjust the intake valve on Nos. 3, 4, 6, and 8 and the exhaust valve on Nos. 2, 5, 6, and 7. Since Ford just switched the firing order and the way it numbered the cylinders compared with GM, you can switch the exhaust and intake numbers and adjust an old Ford the same way.
Hot Lash Gone ColdSo that covers hydraulic lifters, but what about the mechanical lifter? The big difference is that mechanical lifters call for a hot lash adjustment between the valve tip and the rocker arm. The overall method is the same (90-degree or EO/IC) in terms of cam timing, but the adjustment at the rocker arm nut is not. The ideal is to warm the engine and set lash using a feeler gauge at the rocker tip, but what if you have a new engine? Speaking with a couple of dyno shops, we found that with a small-block with aluminum heads and a mechanical camshaft, the lash can be set 0.003-0.004 tighter than the cam card calls for. Big-blocks with aluminum heads can be 0.005, and all-aluminum blocks and heads as much as 0.010-0.012. The reason is that when the heads get hot, they expand and outgrow the pushrod and increase the lash. Once the valves are set, you can safely run the engine to operating temperature and readjust the clearances.
Quick SpecsChevrolet (except Gen III), Oldsmobile, Buick, Pontiac, Mopar, and AMCNo. 1 cylinder at TDC compressionAdjust exhaust valve: 1, 3, 4, 8Adjust intake valve: 1, 2, 5, 7No. 6 cylinder at TDC compressionAdjust exhaust valve: 2, 5, 6, 7Adjust intake valve: 3, 4, 6, 8
Ford (except 5.0 HO, 351W, 351M, 351C, 400)No. 1 cylinder at TDC compressionAdjust exhaust valve: 1, 2, 5, 7Adjust intake valve: 1, 3, 4, 8No. 6 cylinder at TDC compressionAdjust exhaust valve: 3, 4, 6, 8Adjust intake valve: 2, 5, 6, 7
Ford (5.0 HO, 351W, 351M, 351C, 300)No. 1 cylinder at TDC compressionAdjust exhaust valve: 1, 2, 3, 4Adjust intake valve: 1, 5, 7, 8No. 6 cylinder at TDC compressionAdjust exhaust valve: 5, 6, 7, 8Adjust intake valve: 2, 3, 4, 6
4770 Lamar Ave.
3402 Democrat Rd.