Add Center Axis to both of your parts and use it for concentric constrain then you can use your top /front / right planes to fully constrain it.
When working with round parts it is always good idea to add center axis and place it as high on you feature tree as far you can drag it, and then use it for constrains in assembly, instead of simply selecting round faces as the SolidWorks demo / tutorial would like you to think.
Using reference geometries like planes and axis will make your assemblies more stable ( hence less chance for "Trees of Blood" ;) )