As you may tell because of the “K” at the end of the CPU’s name, the Core i7-7700K has an unlocked multiplier, which means you can overclocking it just by modifying its clock multiplier.
We were able to configure the CPU to run stable at 4.9 GHz (100 MHz reference clock and x49 multiplier), with the original voltages. It may be possible to reach higher frequencies if you “play” with the available adjusts.
It is clear that the Core i7-7700K has a great overclocking potential. However, it is good to use a high-end cooler, because it is a CPU with relatively high dissipation and, on overclocking, it may dissipate even more heat. If the temperature reaches high values, it is possible that the CPU automatically reduces the clock, which spoils the point of overclocking it.
It is also good to keep in mind that the overclock capability depends on pure luck, since two CPUs of same model can reach different maximum clocks.