Abstract: A compression tester adapter assembly is provided for use with both 12 millimeter and 16 millimeter spark plug sockets in an internal combustion engine cylinder head. An adapter has a leading cylindrical portion including one set of external threads configured with a nominal diameter of 12 millimeters, and a trailing cylindrical portion spaced from the leading cylindrical portion and including another set of external threads configured with a nominal diameter of 16 millimeters. The axial spacing between a first leading full thread on the 12 millimeter threads and a first leading full thread on the 16 millimeter threads lies in the critical range of 0.636?-0.737?. A hose has opposite ends attached to the adapter and a connector.