Engine builder Keith Dorton...
Engine builder Keith Dorton of Automotive Specialists shares a few of his tricks for optimizing valve lash gained over years of building winning race engines.
You've spent more money on your engine than any other single component of your race car. And if you are like most racers, you're fine with that as long as your engine makes good power and doesn't grenade on you unexpectedly.
After all, spending your hard-earned cash on repairs you didn't plan on making is never fun. Thankfully, you can avoid many engine problems with a few regular checks, and, if you're running mechanical (not hydraulic) lifters, one of the most important is checking and setting your valve lash.
By sticking to a regular schedule for checking your valve lash, you can potentially save yourself a lot of money by catching small problems-with easy fixes-before they become big ones. After all, replacing a lifter or a rocker arm that is in the process of going bad is a lot cheaper than the results of a broken valve chewing up a piston and combustion chamber, or lots of metal shavings from the camshaft getting flushed into the oil.
Valve lash is the available 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. In other words, when the valve is intended to be fully closed. Sometimes, it can be difficult to tell by feeling the rocker arm when the lifter is on the cam lobe's base circle. The easiest way to make sure you are checking lash at the right time is to use the "EOIC" method.
You can set your lash with...
You can set your lash with an Allen wrench and a standard crescent wrench, but a dedicated torque wrench like this can help your precision.
EOIC stands for Exhaust Opening, Intake Closing when working with the intake and exhaust valves in a single combustion chamber. To use this method, pull the valve cover so that you can see the rockers and valve stems and turn the engine over by hand. (You can also use a bump starter if you choose.) Work with only one pair of valves from a single combustion chamber at a time. When you see the exhaust valve beginning to open-you will see the rocker begin to push the valve stem down-you will know that the corresponding intake valve is fully closed with the lifter on the base circle of the cam lobe. Likewise, when you see the intake valve fully open and then start to close (Intake Closing), you will know the corresponding exhaust valve is ready to be checked. Repeat the process with each pair of valves on the engine.
Remember, lash is only an issue with mechanical lifters, either flat-tappet or roller. Hydraulic lifters are designed to pump up the lifter cup and get rid of any lash in the system. Typically, in a race application the lash adjuster in the rocker arm is tightened until all the lash is taken out and then tightened down an additional quarter turn before locking everything down. After a hydraulic cam and lifter set is properly installed, no further maintenance-at least in terms of lash-is typically needed.
To set the lash on a shaft-mounted...
To set the lash on a shaft-mounted rocker system, choose the tab of correct thickness on your feeler gauge and insert it between the valve stem tip and the end of the rocker while using the adjuster on the pushrod side of the rocker to adjust the distance. When it's right, you will just be able to slide the tab into place without any room to spare.
Lash with a mechanical system is much more precise which can yield increased power, but it also requires more attention. Valve lash can change even if the rocker adjusters are locked down tight and must be checked regularly. If you're using a pedestal-mount rocker system, valve lash should typically be checked after every two or three weekends of racing. Shaft-mount rocker systems are more stable and you can reliably delay checking lash much longer. On a shaft-mount system, most manufacturers say lash should be checked every 500 to 700 laps.
Once you've determined that the valve you are checking is fully closed, actually checking the lash is the easy part. Simply use a feeler gauge and slide the tab of the correct thickness between the tip of the valve stem and the end of the rocker. The camshaft manufacturer or your engine builder will give you the preferred lash range-it's usually between 0.010- and 0.024-inch for a race engine. So let's assume a hot lash spec of 0.017, so you will use the 0.017 tab on the feeler gauge. If the gauge won't fit between the rocker and valve stem tip, loosen up the adjuster nut until it will, then tighten down until the 0.017 tab just fits while a 0.018 tab would be too thick, then tighten down the lock nut.