I am going to buy a ryzen 1600 and was curios if I could overclock it well with the stock cooler it comes with or if I will need to buy an aftermarket cooler. I want to try and hit 3.9ghz on it.
You're most likely not gonna hit the temperature limit, because Ryzen CPU's are not good overclockers and 3.9Ghz is probably the most you're gonna get.
If you wanna buy a CPU Cooler and feel safer, buy the one Dan suggested.