Article: Grip the end of the new valve cartridge (which should be identical to the old cartridge) with pliers and slide it back into its slot in the wall. Wiggle the cartridge back and forth if at any point it gets stuck and won't slide any further.  If your valve cartridge will not slide into the wall or seems too small for the slot, you may have bought the wrong size. Double-check the valve size and model to make sure it matches your old cartridge. If you notice severe rust and corrosion while changing the shower valve, then you may need to change out the entire tub and shower valve assembly. Using your needlenose pliers, pick up the new valve's metal clip and insert it into place. It should fit in the same spot as the old valve's metal clip and be inserted from the top. If you tightened the water stops, loosen them by turning or screwing them to the right. Turn on the water supply to the entire house if you turned off the entire water supply because you couldn't locate the stops. Set the trim plate back over the access hole and screw it in place on both sides. If it had a layer of calk securing it around the sides, apply a new layer. Screw the shower handle back in place, turn it on to make sure it works, and remove the cloth from the drain. If you enlarged the wall opening earlier, you should be able to put everything back in place so long as the hole is not larger than the trim plate. Fill the hole back in if you accidentally made it too large.
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Insert your new valve. Slide the metal clip back into place. Turn on the water supply. Put the trim plate and handle back in place.