An espresso puck screen brings optimum flavor in every shot by allowing even water flow thereby reducing water channeling.  The water pressure is spread across the screen to eliminate the hole drilling effect in the puck.  Plus a nice side benefit is the brew group remains free of coffee grounds and needs less cleaning.

Thickness
With a thickness of 1.7mm the water is allowed to spread evenly before hitting the coffee.
The initial water in the first pull will be cooled a few degrees by the screen so pre-warm the screen before use.

Diameter
If you don’t know the screen size for your portafilter a size table is provided below.  If your model isn’t shown then you can google a portafilter for your machine and the diameter in millimeters is usually given - then select a screen with a smaller diameter.