In the end I unbolted the 3 bolts holding the steering gear to the frame. That let me pry it out another couple inches which was enough to take the cap out, reposition the spring that had turned sideways, and then get the snap ring in with proper pliers with nice meaty 1.7mm tips. I was able to make sure it pressed in all the way with a large screwdriver pushing against some angled pliers. I had a little inspection mirror to help with all of this. Seems to be good now. I was able to get it back in place with one hand easily enough. The bolts are 70 to 80 Nm torque. Used regular STP power steering fluid this time.

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1987 300E