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Hydraulic tappets
A persistent question is ‘how do hydraulic tappets work?’. I thought I knew but I have found it impossible to explain in a way that others understand. So here is an explanation with pictures and video.
They are not new although they were unusual when the RV8 was first fitted to the Rover P5B.
Their main benefit over solid tappets is that they are maintenance free. In the 50s adjusting tappets was a 3-monthly - weekly if you had a BMW twin motorcycle - task. If you ignored it you would find the performance dropping away steadily. Most Rover V8s will go through their life without any fiddling being required on the tappet side.
The Rover V8 has a camshaft in the block of the engine, or in other words it does not have the virtual universal overhead cam of more modern engines. The rotational force of the camshaft lobes is transmitted to the rockers in the head via pushrods. So far this is identical to the tried and tested method of engines such as the BMC A-series and that fitted to the MGB. However, instead of the adjustment being taken up by a screw and locking nut, hydraulics are used.
The pushrods qre not directly acted upon by the cam lobes. There is a device that goes between the base of the pushrod and the cam lobe. It is called by various names. The one I’ve been used to is cam follower but will now use the common term of hydraulic tappet.  
Right is a picture of an hydraulic tappet or cam follower. It is inverted. The bit at the top of the tappet is what the cam lobe acts upon. This particular tappet is well worn. If it was new it would be nice a shiny.
As can be appreciated, even with modern oils where metal acts upon metal wear will take place. The amount of wear is remarkably low when you consider that if your engine is running at 3000 revs per minute, the cam raises the value and hence actuates the hydraulic tappet 25 times every second.
Right: Top view of an hydraulic tappet with all the normal internal gubbins removed. As can be seen it is little but a metal tube with one end sealed and a little hole drilled in the side. See picture below.

Relative size of an hydraulic tappet is shown on the right. It is being held by a young woman who just happened to be passing. The vent hole can be seen level with the top of her thumb.

The internals. Don’t worry too much about what bit does which. This is all explained in the video below.

The clever bit and one that confuses many people, is how the hydraulic tappet works. In other words, how does it take up the wear that., quite naturally, takes place from the first time the engine is started.

This video explains how. It is a little patronising but run with it. It will all become clear.