Abstract: An internal combustion engine including a coolant jacketed body surrounding a combustion cylinder with a tubular wall extending through the coolant jacket and surrounding an ignition device exposed through a firing deck area to said cylinder, has a circular recess axially of said ignition device and encompassing said firing deck area with an axially disposed, internally threaded bore extending from said recess into said tubular wall, and a circular plate insert having an axially disposed, externally threaded boss threadable into said bore to axially secure said insert within said recess, the diameter of said insert being less than the diameter of said recess to enable expansion by heat absorption of said insert independently of and without transmitting stress and strain to said engine body.