FunkyKit posted a review on the Overclocking the Core i5-7640X (Kaby Lake-X) to 5.1GHz in 3 Simple Steps A quote from the article:

In this article, we'll show you how to overclock the Intel Core i5-7640X (Kaby Lake-X) from the default speed of 4.0GHz to a whopping 5.1GHz, by simply changing 3 BIOS options on the ASRock X299 Taichi motherboard. You can check out the review of the ASRock X299 Taichi motherboard here.

 Overclocking the Core i5-7640X (Kaby Lake-X) to 5.1GHz in 3 Simple Steps @ FunkyKit