Lozza,
2 pieces there, there is the combustion chamber (inside piece) that bolts to the top of the liner and then the outer cover bolts down over that to the outside of the barrel/water jacket. The spark plug hole is raised on the combustion chamber part and the outer cover has a hole that fits over/around it, and there is an 'O' ring between the two for the water seal, my TSS post has a few photos that might help.