I have a 2004 Ford F150 with a 5.4 3valve engine.I am trying to do A compression check but none of my adapters will work in the spark plug holes I have an M12,M14,M16 & M18 And none of them will work.Is there anyone that can help me with the adapter that I might need to screw into my spark plug hole so that I can do a proper compression test.Thanks

 

The size should be 16mm.
Are your adapters possibly too short?