Stress tests made on the i5-8400 with an AIO (Corsair H110) appear to reach just 45C (http://www.legitreviews.com/intel-core-i7-8700k-core-i5-8400-processor-review-coffee-lake_198473/10). There was also one who tested the i5-8400 with the stock cooler at full load and reached 72C (https://pcpartpicker.com/forums/topic/248001-temp-for-coffee-lake-i5-8400).
It's perfectly safe, though, some prefer a rather much lower temp during gaming by just getting a ~$35 aftermarket CPU cooler (which will also produce lesser noise and looks more aesthetically pleasing). For example:
CPU Cooler:CRYORIG - H7 49.0 CFM CPU Cooler ($34.89 @ OutletPC) Total: $34.89 Prices include shipping, taxes, and discounts when available Generated by PCPartPicker 2017-10-20 04:26 EDT-0400