My home inspection of Speedmaster heads

12many said
The SM hole is drilled into the chamber and threaded. A straight threaded hole into chamber. The TF’s take the additional step of tapering or flaring it out to the chamber surface like I said to about a 20mm x 16mm oval, radiused.

have you done all the spark plug holes on you head?
IF not could you take a picture that shows more of what your trying to say.