This is a cap that bolts over the intake manifold on a Harley Panhead that uses a Linkert Carbeurator and a 4-bolt intake. The cap has a hole in the middle for a schrader valve. This fits snugly and if a gasket is used and the surfaces are smooth, it should be airtight.

Applying 12-15 PSI of air to the valve after this is installed in place of the carbeurator will allow to test the intake manifold for leaks. Just spray some soapy water around the intake boots and look for bubbles.