How do I make this cut?

[Arich0908]

Im trying to recreate a part I have ive figured most of it out but this.
The two parts you see fit together in a head kit to a 2 stroke motor . The cylinder part I think I can recreate using a boring bar and rotary table (thats the compression Dome) but Im unsure how to make the other part that is the complete opposite.

[packrat]

To me there both lathe jobs, except for the slot. But in the mill the part with the calipers could be done with rotary table or dividing head and a ball end mill. If the concave radius is what your talking about.

What engine are these for?

[Arich0908]

The one with the calipers is done with a boring bar and rotary table.
The part with the slot in the first picture goes to a head kit that is roughly 4.5 x 4.5 in .
But maybe I can cut a lathe bit to match the profile of that angle .
The second and third pic are the same.

[Fastest1]

Is this engine a secret?