So I am in the middle of performing a valve cover replacement job, and just finished pulling and cleaning the half moons, along with the mating surfaces, and I have to ask... Why do those things exist?

I know there a lot of other Toyota engines that have these as well, but why is there a part of the cylinder head that necessitates having these to plug them? Is they for some other option the engine had in Japan or elsewhere?

 

I’ve always been under the impression the void is a byproduct of the machining process.
The tooling cuts into the back edge of the head as the bearing journal supports are cut.

Pic is a random example not sure what manufacturer.

 

All cam journals are bored with a single tool process is known as line boring, cam caps installed. This means also machining holes in outer casing.

 

Ah! That makes sense. Seeing that picture with the cams removed shows how it lines up.