My question for the guru's is why do we need to set the valve lash anyway?
For traditional valve trains that utilize camshaft followers to open the valves and springs to close the valves, it is important that there is a gap between the valve tip and the follower/adjuster to ensure that the spring can fully close the valve, otherwise compression would be lost and the valve seats would become damaged. It is also important to avoid excessive camshaft lobe wear, which would occur if they were always under tension.
Over time, that gap changes with wear as the valve seats slightly deeper in the head (as the valve/seat contact surfaces wear), the valve stem stretches (due to fatigue - not common), the valve tip compresses (due to repeated strikes by the follower/adjuster - more common), the camshaft lobe wears, the follower wears, etc. So, there needs to be a means to adjust that gap to keep the valve train in spec.
The actual ideal clearance gap is determined by many factors, in particular the acceleration forces imparted upon the valve tip by the follower/adjuster (which affects the amount of 'bounce' that the valves experience against the springs and seats), the precise timing upon which the valves open and close (seat-to-seat valve timing), and how much the gap changes due to temperature rise when the engine is at normal operating temperaure, or beyond if running too hot.
It is fairly common to set a slightly larger gap for the exhaust valves compared to the intake valves, the reason being that the exhaust valves run hotter than the intake valves, and so this allows the gaps to achieve their ideal setting while at normal operating temperature.
