This kit blocks off the openings in the block when the manifestly stupidly located stock under plenum heat exchanger or "hex" is removed.? The primary effect of under plenum hex is to heat up the stock aluminum plenum bottom and increase air intake temperatures and decrease engine power.? Block it off with our kit and stop the hurt!? This kit gives you the machined pieces and o-rings necessary to seal up the block when the under plenum hex is eliminated.? Includes a weldable plug for the middle opening of the thermostat housing or, for an additional charge, a refurbished thermostat housing (cleaned, ceramic bead blasted, and new thermostat gasket installed) with the middle opening already professionally welded.