Simply, hex bolt 5/16" http://www.amazon.com/Monel-Hex-Bolt-Length-Pack/dp/B001377Z20 can be opened by wrench key #13, and hex bolt m8 can be opened by wrench key #12. Rough bolt mean the pitch is bigger. But you need to test all of them. Sometimes not all bolt fit. Different manufactures, different size. Even just 0.05 mm, you can feel it when try to fit with 608z. Choose the good brand, usually they have good size.