I replaced all my spark plugs, wires and coils today. While I was replacing the plugs I noticed an open hole above the two front spark plug holes that is threaded so I assume something is supoosed to go there and has been missing? Can anyone confirm what is supposed to go there, if anything? Picture attached. Thanks.

 

beats me. im assuming that its maybe used in the assembly of the engine and then not important to the functionality afterwards. i mean the car runs still right?

 

Don't worry,..its where the Knock sensor goes. If you look @ the rear housing you will find the knock sensor fitted @ this same hole.
Its A black round plastic thing.
Mazda uses only one knock sensor per engine,.but the housing are made with this hole on both housings,..I think casting issue.
So dont be alarm,..just dont mixed-up your spark-plug cables and you'll be fine.

9MM.

 

That hole may be for the "Far Trailing" plug, good for 1 more Hp. However the electrode is made from Unobtanium and costs more than the car, which is why the hole is typically filled with that spacer.