I've put the stock heat sink on my E6400, but I honestly have no idea if its properly fastened. When pushing the tabs in, i hear light clicking noises, but nothing to make me thing it was fastened in nice and tight. How can I know its fastened in properly? Its running 35C idle right now, but easly jumps over 50C when i do a stress test.
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35C at idle and 50C at 100% load is not something to worry about. It's normal.
When it goes over ~40C at Idle, and ~55C at full load, you have trouble.