Stock coolers are cooler, having the hardware components like CPU, GPU, etc. These hardware components came along with it as manufacturers added them with it. They are known as small heatsinks with fans attached to them. They will be enough for i7 8700k if you live in an area with moderate climates; however, overclocking would be complicated with such cooling devices. We would recommend Cooler Master Hyper 212 EVO, a cooler which will provide adequate cooling to your system. It comes with the SickleFlow PWM fan and 4-pin PWM fan header that provides excellent air cooling.