how much force is needed to hold down a waterblock?

[Biggerhammer]

I'm contemplating building a waterblock out of silver. I need to determine how thick the base needs to be in order to not crush under the force of the hold-downs.

So, how much force, in inch-pounds or whatever measurement is convenient, is applied by a 'standard' four-spring hold-down for an Athlon board?

Thank you.