I have a 86 Mariner ( I think) and I checked the pressure in my cylinders after I found a broken oil line. I have 120 psi in 5 of my 6 cylinders. Could this be a blown head gasket? Could this be a bad piston, cylinder wall, or what could it be? I'm pretty mechanically inclined but my other question is. Do I need to tear apart my lower unit before I start working on the head and all that? Any help would be greatly appreciated.