I would replace the stem.it will have new washer and stripped screw will no longer be an issue.look insude the hole for debree. The seat can be reolaced. Too i f it diesnt cleanup

This is not a cartriage.it is a tub shower stem.phister will have this.unscrew the outside casing and you will see the spot where you can put an adjustable wrench on it to remove.just make sure you turn the stem where the screw is stripped counter clockwise all the way out then back 1/4. Turn.

make sure there is no furniture in front of the unit, that will cause the unit to short cycle because the temp is satisfied to soon.