Since this Duramax was an LB7, that meant it had removable injector cups. The later LLY ('04 to '05) Duramax has a different design that eliminates the injector cups as a weak link. Dmitri covered the O-rings with assembly lube and then put Loctite on the tapered fitting on the opposite end. If the friction fit fails, coolant is allowed in and compression finds its way out, so a dab of Loctite is used to help the situation. Injector cup leaks are a common problem, especially when replacing injectors.