Replacing a Delta Stem Unit- RP1991

Old one been leaking for 3 or more years. Hopey this cut down on water leak and higher water bill. Stem has to be pulled out by hand or with a tool to grap it and then pull. New one, push it in. Make sure it in correctly. They a gap to the left side. Yup, my finger are little too big to push them smaller parts in the back of it.