HELP! The screw that came with my Magpul AK grip doesn't fit.

[BPTactical]

If the threads didn't match it most likely would not go in 4 turns before stopping. There is probably something obstructing the threads. Maybe a bur or a bad spot left from machining. If you can shine a light in there and take a look, that may reveal the culprit.

This is false.
Metric vs SAE threads: as long as diameter and pitch are similar between the two frequently you can get a "false start". An example is 1/2 x 28 and 12 x 1.5. They look very similar from outward appearances.
But when you try to mate them the threads are "out of time" with each other. Get 3-4 turns and the timing difference stacks up and the thread stalls out.

Rays method of checking threads is the way to go without a pitch gage as long as you know what your "gage" thread is.



Silly cstone, don't you know those are both male threads?
You've been in Colorado too long