there are three different common issues that plauge the 6. 0. turbo failure, egr failure , and head gasket issues. egr failure is fords baby for the most part, turbo and gasket failure is navistars baby. the head has a 10 bolt design, and is a weak point. major egr failure can lead to a head gasket failure, partly cause of the weak head bolt design. i have seen gasket failures in both international trucks, and even more fords, usually they have programmes added, or have had egr issues. the 6. 0 engine itself is a very stout design, but as mentioned had gasket issues. latter 6. 0 engines were much improved. as for the 6. 4, i have seen very few with issues, in fact i have seen more 6. 7 cummins issues then, 6. 4's .