Paul, here's a simple (not to scale) drawing that I've just done to show the basic Valve, Bucket, Shim, Cam set up.
The Drawing on the left shows the individual components.
The Blue being the Shim, the green being the bucket that sits over the valve and has the shim sitting in the top of it.
Then above that, the camshaft.
The R/H drawing shows all these piece's as they would sit in the Engine (again, not to scale)
The Camshaft and valve are in a fixed position (relatively)
The "valve clearance" is determined by the thickness of the Shim.
Too thick a shim, .... no clearance.
Too thin a shim, ......excess clearance.
This Dwg. shows a shim too thin (exaggerated) so therefore excess clearance.
Hope this helps.
