Standard hardware store M5x12 with .8 thread? I picked up a couple and all they had were .8 thread but it sure looks like a fine thread.The little spike pictured above mounts perfectly. I have seen Stihl spikes mounted as well, which seems more aggressive. The little Echo spike is a bit short in thick bark or an angled cut.
Make sure you get the right coarse thread screws. If you force screws that are too wide or long you will ruin your fuel tank.