The model can be downloaded here: 7191S Cylinder Head - Ricardo Version Rev (D).SLDPRT - Google Drive 

I used a surface loft to model a flathead engine combustion chamber.  If you look at sketch 1, you'll see a representation of the cylinder block top surface.

A surface extrude, a couple of trims and fillets are used to provide clearance for the valves when they open.  All of that is good.

You'll see that I used the knit feature but it didn't seem to result in a single surface.  I say that based on what happens when editing the appearance (color).

I used the edit appearance feature to change the color from grey to red.  To do that I had to select the loft and the faces that make up the fillets.  I was thinking that if the combustion chamber was represented as a single surface, I would only need to make a single selection.

Questions:

Does the model consist of a single surface?

If not, is it possible to make it a single surface?