Hydraulics – a descriptive term for a system operated or moved by a fluid.
The internal work of the hydraulic valve lifters, commonly found on lots of cars and some motorcycles too. the idea behind this is that LIQUIDS CAN NOT BE COMPRESSED. While some valve systems use screw-and-lock-nut OR shims (metal plates in different sizes) to adjust their valve clearances, engines with hydraulic valves do not need periodic valve adjusting.
This is how it works: engine oil is pumped constantly by an oil pump, enters through the one-way entrance (where the green ball seats) into the chamber. The oil is then trapped in this chamber and acts as a solid (that can’t be compressed remember?)- and by that eliminates any clearances in a chain of moving parts (between the cam lobe and the VALVE head).
In case of wear, there is a chance that when the engine is stalled, the oil pressure in this chamber will drop drastically, and that is why when you start such an engine once again, you will notice a ticking noise for a short while, maybe couple of seconds. Noise is clearances, and the noise stops as soon as the oil pump will push enough oil to fill the tappet chamber again. In a good engine, pump keeps feeding the system oil, always equal the volume of oil leaking out of the system (of natural clearances between the parts, wear, etc).
The pros of this system is that it rarely needs maintenance and an engine that has such a system will need no periodic valve adjusting . In motorcycles most Harleys have hydraulic valves, and some KAWASAKI cruisers too.
The cons of the system is that a contamination (little metal particle, etc) can cause the hydraulic tappet to leak oil from the chamber , allowing it to be susceptible to squeezing = therefore not closing clearances = causing valve system to make rattle/ticking noises. Repair then, is sometimes relatively complicated. This can happen in old engines where the wear is too much and the parts shown here (tappet lifter, plunger, body) cant hold enough oil, that leaks from the sides (where ever it can).