Abstract: Oil leakage occurred many times at the aeroengine elbow joints with stop pins during operation. To solve this problem, a three-dimensional finite element calculation method was used to simulate the tightness of the pipeline. The effects of the stop pinstructure, material stiffness, radial offset of the ring centerline, and deviation angle of the cone-ball coaxiality on the tightness of the pipeline were verified. The results show that the softening process of material has negligible effect on the tightness of the pipeline. In contrast, the circumferential coverage of stop pin, the radial offset of ring centerline of stop pin, and the deviation angle of cone-ball coaxiality affect the distribution uniformity of contact pressure at the fitting location, exhibiting clear regularity. With the increase of these influencing conditions, the difference in maximum contact pressure shows an increasing trend. Therefore, the above three factors play a critical role in determining the pipeline’s sealing performance. Through assembly tests, tightening tests, and sealing tests conducted under both gently-pulled and softened states of the stop pins, it was concluded that the assembly of soft stop pins can improve the processing quality of elbow joints and reduce the problem of oil leakage on the sealing surface.