I have a shower drain that the top screws into the stopper and there is a washer that fits into a grove that holds the stopper up when you pull it up and turn it. The washer came off and won't go back on the stem. The stem has a flat side (half moon) type and the washer is half moon type. I don't wan to force the washer back onto the stem. How do I re-install the washer and get it back into the groove around the stem?

If there is nothing to unscrew to get the rubber on you will have to stretch the o-ring enough to slip over the metal and go into place.