Hello,
mine were 3mm but I couldnt find 3mm studs. I got some 3mm bolts, screwed them in and then cut off the head with my grinder. If you attach a nut on the bolt BEFORE you cut off the head, you can use said nut as a thread chaser(or a die) to clean up the bolt.
If you dont have a 3mm die or use the nut as a chaser you will not be able to thread a nut on after you cut off the head.
If this is not making sense I can post a pic