Abstract: For nozzle replacement, repair and initial installation in ASME pressure vessels, disclosed are nozzles, nozzle assemblies and nozzle repair assemblies, all of which are mechanically attached and mechanically sealed to the vessel, i.e., without any welding to the vessel, and in most embodiments without any welding at all. For nozzle replacement, a part or the entire existing nozzle is removed and a partial or full replacement nozzle or nozzle assembly is mechanically attached and mechanically sealed to the vessel. For nozzle repair, the existing nozzle within the bore is not removed, and a mechanical seal is provided for the existing nozzle which may also be welded or mechanically attached to the repair assembly.