If you've used radius tools, or other form tools, especially on a small machine, you know how difficult it can be. Unless you're running quite large machinery, form tools chatter due to the large area of material in contact with the cutting tool. Rigidity is critical to good results, as is good lubrication. Often the form tool will cut pretty well, although leaving a rough finish. As you go for a good finish, allowing the tool to rub a little, up comes the chatter. The success enjoyed by the fellow machining 12L14 would likely be hard to get with almost any other steel. Lead is likely the secret to his success in this instance. On the other hand, you didn't mention the size of the radius, either. When they're small enough, it's not much of a problem.
Did you manage to take a pic of the engine? Love to see it.
Harold