M3 and 4-40 thread
 

M3 and 4-40 thread, I have a 4-40 screw and a M3 screw in my hand and they look exactly the same. Are they interchangable?

is the thread the same?

It looks like its the same, one of the screws is 4-40 Associated, and the other is an OFNA M3, I am asking if the thread is the same, it looks like it is, one is a "Standard" the other is "Metric" but can they be used interchangably?

No.they're about the same diameter but M3 is a finer thread(more threads per inch/different pitch).Are you sure you haven't got 2 the same there? If you have a plastic part with untapped holes then you could probalby use either since the diameter is so similar,but a 4-40 screw will definately not fit a M3 nut/thread.
I use both alot and can tell the difference by looking at a screw.

I've found that 4-40 screws are actually slightly smaller in diameter. For example, when threading into plastic, the 3mm screws will catch the plastic and make a thread, but the 4-40 won't hold a thread (not enough plastic being caught by the threads).

What diameter is the hole your threading then? I just have metric drill bits in .5mm incriments so if i'm drilling a hole to be tapped i use the same drill for M3 or 4-40 since there's less than .1mm diameter between M3 and 4-40,hence why i said they're about the same.

I'm not sure, it's for RPM rod ends (for turnbuckles).