Help! Need some dimensions!

The neck screws on most Fenders are #8 x 1 3/4" long flat head wood screws. Sometimes people swap in larger #10 x 1 3/4" screws if the holes are worn out.

But if you're swapping necks around, you need to measure the actual combination to be safe. First, check that the head of a #8 screw fits correctly in the countersink in the neck plate. Then, try threading a #8 screw partway into the holes in the neck. It should thread in fairly easily. That will verify that the holes are sized for a #8 screw.

Finally, check the length needed by holding the neck, body, and neck plate together, and stick a piece of stiff wire down through one of the holes until you can feel it bottom out inside the neck. Put your thumbnail against the wire, even with the surface of the neck plate and pull the wire out. Compare that against the length of the screw. The screw should be slightly shorter than that length. The tip of the screw should not be able to contact the bottom of the hole when it's snugged up. If it looks like it might, use shorter screws, or grind the tips of the screws back a bit.

Play it safe and check them. Screws that are too long can do serious damage to the neck by pushing up the fingerboard. Screws that are too tight in the holes can split the neck (see other thread nearby).

 

It's better to be slightly too short than slightly too long. Measure twice, screw once.
Rocky