Originally Posted by
CyclingFool95
Does M4.5 have the same pitch as M5? If not, when you try to drill and tap M5 where the M4.5 hole is, you will probably end up with a mess - there's not enough difference that the drill will clean out the M4.5 threads completely. If they are both coarse thread they are both .5mm pitch.
Unless the mfr is really cruel and wants to screw with the customer by not using metric standards, you likely don't have to measure. M5 course is 0.8mm pitch and M4.5 course is 0.75. But I agree with CyclingFool that if you just stuck an M5 tap into the thing, you might end up with a mess.
To throw another idea into the mix, if you are drilling a through-hole anyway, you might use a form tap rather than a cutting tap. The cutting tap would cut new grooves that may interlap with the old threads, and the interaction might tend to cause the tap to go in at the wrong angle. If that happens, the tap will likely break. Also, a cutting tap will remove metal without regards to whether that material is needed. A form tap will move the metal around. I think you'd get a better thread with a form tap.
That said, I'm pretty sure that this fastener is not super high stress (may be wrong- has anyone ever pulled one out or broken this type of screw in use?)