You don't need a coal forge oxy-acetylene would work great and faster. I built my coal forges years ago as I am a blacksmith also. I prefer to use a forge when heating tool steel because better control the degree of heat and observe the color of the metal as it heats. The 1/2 inch round bar I used is an old car spring I had laying around the shop. I had straightened it years ago to by heating in my forge so I could use it to make chisels and punches. The average car spring has 60 points of carbon making it tough enough for simple tools. If you want to use this method make sure and anneal the steel after you bend it to your final shape. Anneal by heating to a light cherry red and placing the steel in ash or lime (I use lime) which will allow it to cool very slowly. This process will relieve any stress caused in the steel by bending it. After that you can re heat the wrench to an even light cherry red one more time and quench it in a can of oil or water. I use oil so it will cool a little more slowly and have less chance of being brittle. This should give you a very strong and reliable valve wrench that wont snap and bust a knuckle. Maybe overkill for a low torque tool of this nature but that is the method I use when making all my tools. Besides, you never know when you may need it to break free some other PITA stubborn bolt elsewhere on you Benz.