Changing out guitar tuners.?

A buddy of mine asked me to fix his guitar. He had mentioned that he wanted the tuning heads changed out (idk why) but told me to put Grover modern nickel keystones on it. I though that would be easy and simple, but i had heard that most of Grovers tuners required a 10mm peghead hole for each tuner. Do you know how you would measure the peghead hole, Or could most Grovers fit perfectly without it?