Abstract: This paper presents a formula, which is derived from the theory of strain wave, to calculate the seating stress for the low-mass fuel injector without push-rod. It is illustrated that the calculating results by the formula well tally with those in practice. Therefore, the formula could be used as a basis to design a nozzle and to select the needle valve opening pressure for the fuel injector without push-rod. Meanwhile, the new formula presented in this paper also solves the contradiction that the needle va...