The assembly of the pelvis part is a bit non-intuitive. Indeed to assemble the abs_y motor and the pelvis part, you have to spread it a bit (about 5 mm):
Also, to do it easily, you should also remove the screws holding the MX28 horn.
If you begin by fixing the l_hip_x and l_hip_y motor, it will increas the rigidity of the pelvis part. You will be unable to bend it enough for passing the abs_y motor. If you insist you will probably break the part !
So first you add the double rotation for the abs and then you fix the hip motors !
Note:
The pelvis part will be completely changed in the futur release of Poppy.