Really? You want to know how hot something should be under load, and NOT tell us what is it? The answer to everything is 42, so there you are. If you want a real answer, try telling us details. It really helps.
You are asking about "core". Which core? Your CPU or GPU? GPU we know is a 1070, "i5" could be anything from an old 750, to a new 8600K. Is it overclocked? What cooler do you have on it? The more you tell us, the more we can help.
65C on a GPU is nothing to worry about. 65C on a CPU is my personal limit as to how hot I'll allow my CPU to get. The Thermal limit for those CPUs is 100C or 105C, not sure which. So 65C or higher won't harm it.