I personally have a love-hate relationship with valve lash adjustment. I love adjusting anything mechanical, getting my hands on it and fine-tuning it to perfection.
The part that I hate is how awkward, burdensome, and complex it often is to get to the lash adjustment. It seems that half of the engine and its accessories need to be removed to perform a 10-minute valve lash.
For this reason, I like engines with hydraulic valve lifters that, for the most part, require no adjustment. If the valve cover never has to come off an engine, that is a good day for me.
There are times when the hydraulic valve lifter needs to be adjusted. But instead of setting lash (as you would with a solid or mechanical valve lifter), a hydraulic system needs to have the preload set since there is no lash. This is usually only required if the cylinder head has been removed and is now being reinstalled.
THE NEED FOR SETTING LASH
The camshaft in an engine is responsible for the timing, lift, and the duration the valves stay open and closed. To accomplish this, it works through the intermediate components of the valve lifter (or tappet), pushrod, and rocker arm (in a cam-in-block engine).
With an overhead cam design, the intermediate components are different using some style of follower in lieu of a pushrod and possibly a tappet. This guide will be focused on a hydraulic tappet employed in an engine that has the camshaft in the block.