M3 x .5.
I bought 8mm long. They stick thru about 1mm.
Flange head, socket head. Couldn't find a Phillips head large enough in dia to use without a washer. The Allen socket seems small but I got them tight enough so that when removing them that they made a snapping sound. Final install was with red loctite.
From mcmaster carr.
This is for the secondary butterflies. I assume the primaries would be the same.