Lash on a stud-mounted rocker...
Lash on a stud-mounted rocker system is adjusted by turning the adjuster nut on the stud that controls the height of the rocker on the stud.
Be aware, however, that some adjusters will actually change the lash clearance when the lock nut is tightened. On some designs, tightening the nut seems to pull the adjuster up into the body of the rocker and creates a thousandth or so extra lash. So you may need to make some trial adjustments and recheck your lash after tightening the lock nut. If you consistently see more or less lash after locking everything down, you will need to account for this when making your lash adjustment.
Of course, there's one last curve that we have to throw you before we're done. The lash number that cam manufacturers supply will be the "hot lash," which is the ideal lash after the engine is at operating temperature. The lash can change several thousandths of an inch between when it is at room temperature and when everything is warmed up properly. But lashing valves on a hot engine comes with plenty of difficulties.
The first, and probably most obvious, problem is it's no fun to work on a hot engine. Second, the engine starts cooling off as soon as it is shut down, so it's impossible to lash all 16 valves at a consistent temperature. And third, lots of engine builders are now flooding the valve covers to help control valvespring harmonics, making it impossible to pull the valve covers so that the valves can be lashed for several minutes after the engine has been shut off.
Dorton says it can be difficult...
Dorton says it can be difficult to feel with your finger when you have a needle bearing going bad in the tip of a rocker arm. But if you pay attention when lashing your valves, it can be easier to feel a rough spot in the roller when inserting a feeler gauge between the roller and the valve stem tip.
The solution is to determine how much the lash on your particular engine changes as it goes from room temperature to operating temperature. The easiest way to do this is to warm the engine up (getting the oil temp up to around 200 degrees Fahrenheit should do it), lash one intake and exhaust valve properly, allow it to cool and check to see how much the lash has changed. But that's impossible to do if the engine is brand-new and you need to at least get the lash close before cranking it. Keith Dorton of Automotive Specialists has done extensive research in this area and provided us with some guidelines when it comes to lashing a cold engine.
Because different materials expand at different rates as they are heated, you have to take into account your engine's construction when estimating how the lash will change as the engine approaches operating temperature.
For an engine with cast-iron heads and steel valves, you can expect the lash to tighten up a couple of thousandths of an inch as it heats up-so for the cold lash you would set the valve lash a couple thousandths larger than the cam manufacturer's recommended hot lash setting.
Different rocker designs work...
Different rocker designs work in different ways. From left to right, the first two rockers are the standard stud-mounted rockers (in aluminum and steel). The center pair are two shaft-mounted rockers. The final pair of rockers is a new design that's hitting the market. They are shaft-mounted but feature an eccentric built into the rocker pivot for adjusting lash.
If the engine has aluminum heads with a cast-iron block and standard steel valves you can expect lash to loosen up approximately 0.006. If you are running aluminum heads with an iron block and valves with narrow 6mm stems you can expect the lash to loosen up 0.010 to 0.012. And finally, if you are running an all aluminum race engine, the lash can loosen up by as much as 0.015.
With normal use, you should notice your valve lash slowly decreasing from one check to the next. This is especially true with a new engine. This is because of wear between the valve and the valve seat, which allows the valve to sit slightly deeper in the combustion chamber. You should consider this normal wear and it isn't a reason for concern unless you have one valve move 0.002 or more after a single race, which could be a sign you have a damaged valve or seat.
What is a cause for concern is if you notice the valve lash opening up. The most common reason for this is a failure of the needle bearings in a roller lifter or a wiped cam lobe. "Wiping" a cam is when a lack of lubrication or some other problem causes the crown of the lobe of the cam to be ground away. When this happens the cam, and probably the lifters as well, must be replaced.