For just under $300 and offering decent performance at it's stock clocks of 3.7GHz and 4.6GHz Turbo, the i5-9600K is an attractive chip for many looking to build a new system.  However, by overclocking it you can get even more bang for your buck, which is exactly what [H]ard|OCP has been looking into.  They attached a RX480 V3 Radiator, and D5 Photon Reservoir/Pump Combo V2 to cool the chip which let them hit 5.25GHz perfectly stable with some noticeable results.  See the settings they used as well as some tips in their full review.

"The Intel Core i5-9600K Processor will likely hit the the sweet spot for a lot of desktop PC enthusiasts and gamers. We have a solid 6-Core count with a Turbo Boost clock of 4.6GHz coming in for right around $270. What kind of overclock will the new 9600K CPU support and remain 100% stable?"

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Processors