616 KS stems varied in shank diameter over the years.
If yours is one of the uber-fatties
, know that difficulty increases almost exponentially as the diameter increases and length decreases.
In short, there's a reason that chubby Authors (and related shapes) are not made very often, and their stems usually look like shit when they are.
This pipe, for example, is one of the most pretigious and expensive brands in the world:
Here are some uber-fatties done right, but they are all quite old (virtually no one can do this today):
A possible solution is to get a blade-style stem made for your pipe. More comfy, and isn't nearly as difficult to make correctly:
Like this: