They should be nice and put on a Cusil spreader (Copper/Silver alloy with higher thermal conductivity than either of them), seeing as how much we are paying for dirty sand (bad joke)........
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I have no idea why this works that way it does, but my water cooler works better with the shim on. I have a spring loaded lexan hold down so I can adjust the pressure and the distribution. I tried with and without several times and came to the conclusion that either the block won't set correctly on the core without the shim or the shim is providing cooling because it is 10C cooler with it on. I refuse to say that the shim is actually aiding in cooling but it is providing some form of assistance towards that goal, either in allowing me to be more confident with tightening the nuts on the holddown or in straightening the block on the core or the unmentionable. So if it is simply because of keeping it straight then I am all for a nice big heat spreader right on top of the core.