The Mopar steering boxes have one place on the splines where the spline was not cut. The coupler also has to that a matching position on the splines to fit. Then its a matter of getting the two spline places to line up with each other. They have to be exact, one spline off won't work. I'm betting your splines are not matching up correctly. Once the splines line up if everything is clean, most of the ones I've dealt with slid together pretty well with a jiggle or very light tap with a hammer. Just make sure the locking set screw is backed out far enough its not dragging on the splines.
I understand what you are saying, but there are no cuts to line up on either of the splines of the steering box nor the steering coupler. I even tried putting the coupler on the box by going around 1 spline at a time and pushing it on with no success. These are aftermarket parts from sellers on E-bag.
Did you try the old coupler on the new box and vice versa?
Did you compare the two?
Should be splined as mentioned By Poorboy, If I remember correctly the box has the Female slot and the coupler is male. ( KEEP IT CLEAN LOL)
The same arrangement was done on the steering wheel to shaft, It was done to keep the steering wheel, shaft and box aligned during assembly.
