I have followed the debate in this forum and others about the utility of shims with much interest. I would like to offer my opinion.
I was almost seduced into getting a Tweakmonster Athlon CPU spacer, which are often cited for their quality and precise tolerences, but had a technical question: The Athlon XP Data Sheet on AMD's website gives the distance between the top of the CPU core and the upper surface of the ceramic substrate as between 800 and 880 microns, in other words up to an 80 micron (0.08 mm) variability in thickness. Presumably Tweakmonster chose the thickness of their spacer based on these published dimensions or determined them from direct measurement. Add to this the thickness of the thermal paste layer between CPU and heatsink, perhaps the thickness of a sheet of paper, or about 130 microns. The shim thickness quoted on the Tweakmonster website is 0.027 inches, which is aproximately 680 microns (several other manufacturers quote this thickness). Based on these verifiable measurements we can confidently predict that a gap of between 250 and 330 microns (0.25 to 0.33mm) will exist between the upper surface of the spacer and the bottom surface of the heat sink. At this scale one quarter to one third of a millimeter is a lot.
Clearly the heatsink will not be supported by the shim, only by the CPU.
How can these anomalies be explained, given that "CPU spacers prevent cracking of CPU cores while installing the heatsink, and help distribute weight evenly over the core surface", to quote Tweakmonster? In addition, the large and variable gap between shim and heatsink would seem to make the concept of having a precise shim tolerance somewhat redundant.
Needless to say I remain unconvinced. I suspect that this manufacturer of CPU spacers and others, purposely undersize shim thickness to ensure they do not "fry" their customers CPU, as would be the case if the CPU sank below the top level of the shim. However, in terms of function, these items appear worthless, except perhaps as New Age earings.
Any thoughts?

Acorn