10-32 thread x 3/16″ long. Standard thread size for most set screw type low profile gas blocks. Uses 3/32 hex or allen wrench drive. Sold in multples of 2. 

These AR-15 gas block set screws are knurled on the tip for a secure grip. You could upgrade your standard screws to these low profile gas block set screws.

How often have you lost one of these when building and crawled around on the floor looking for it? it’s bound to happen. Grab a couple gas block replacement screws just in case.