I have just ordered and recieved a new cylinder cover. (The old one had 2 stripped spark plug tappings. The new cover came with tapered openings at the spark plug holes that are larger than the plugs. Is their a part or some other piece that needs to be ordered or did I recieve the wrong piece.

PS The top plug hole is actually two holes that intersect to incorporate the temperature sensor and the spark plug so I would need something for the top opening that would allow me to install both the Spark plug and the temperature sensor.

Should have recieved part number 8368A 1 as in this diagram Mercury Mariner Outboard Parts 115 (6 CYL.) 7208848 AND UP END CAP, MANIFOLD AND EXHAUST COVERS Diagram

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Sorry bro, the plugs screw into the cylinder itself not the cover. That motor is a blind hole bore and the tops of the cylinder are not accessible from the outside.
Put Heli-Coils in it, screw on an external mount temp sender and go boating!