Adjusting the valves in an engine seems to be a simple chore. And it can be simple with plenty of practice, but adjusting valve lash (or clearance) without experience and patience can be frustrating. It can also be less than accurate.
Types Of Adjustment
Let's look at the different types of adjusting mechanisms that you might encounter. First are the valves that cannot be adjusted, including those on some Fords with rockers that look like those on a Chevy but are bolted solid. The intent is for the hydraulic lifters to take up any clearance. When that's the case, valve lash can only be adjusted by using aftermarket parts that allow adjustment.
The common Chevrolet adjustable rocker uses a pivot ball. This rocker can pivot any direction. Its side angle or tilt is located by the pushrod and the valve. Adjustment is performed by tightening or loosening a pinch-type locknut-one that has been pinched together slightly on one end. This makes it tight enough that it will stay where it's put.
The trouble with this arrangement is that after being removed and replaced a few times, the pinch is stretched and the nut becomes loose. This can allow the valve lash adjustment to change. Be aware, however, that plastic insert locknuts will fail from heat.
There are several solutions. One is to throw away the nuts after three or four adjustments and/or each removal from the stud. I have a large collection. Many years ago, I would re-pinch the locknuts with a hammer and a punch. I won't bore you with stories about what I messed up by doing this.
Another solution-and a better one, I might add-is to use what are commonly called poly locks. These are hardened nuts that are several times longer than standard nuts. One end is screwed onto the rocker stud and adjusts the rocker's lash setting. A set screw is then threaded into the other end and tightened down against the stud. This locks the nut to the stud by wedging the threads. It takes some patience to do this and get the lash setting you want. When the set screw is tightened, it often moves the nut and/or pulls against the threads, so you have to do this several times to get it right.
Another thing you might encounter is the adjustable pushrod, which is made mostly in the aftermarket for applications in which the engine did not originally have any adjustment. They adjust with a screw on end and a nut for locking.
There are also some rocker arms that have an adjusting screw on one end. Here the adjusting screw usually has a screwdriver slot so it can be turned. The nut then locks the screw in place.
How To Adjust
It would be simple to insert the appropriate feeler gauge between the valve and rocker, then tighten the adjuster to it. But that's easier said than done. Feeler gauges are thin strips of metal, usually in a set, of an exact thickness. They get their name from being used as a feeler between two parts. If the gauge feels loose between the valve and rocker, then clearance is looser than its thickness. If the gauge is tight and can't be easily moved between the valve and rocker, then clearance will be too tight. With a little experience, you will develop a feel for where the gauge can be moved without having slack in its movement.
I have used a technique with two feeler gauge strips. If the desired adjustment is 0.020 inch, select one gauge of 0.019 inch and another of 0.021 inch. When the larger drags hard going in and the thinner one slides easily, you have it right. This method is less dependent on feel and might be easier for someone learning the skills. However, with experience, using the single gauge of the correct thickness will be faster.