Stens Pre-Filters improve the efficiency of your equipment by capturing dust and large particles before they reach the main filter.. 40 Pack Foam Pre Air Filter For Briggs Stratton Lawn Mower Engine Stens pre-filters are made of high-quality materials that effectively capture dust and other contaminants. Extending the life of the main filters is helpful because they tend to be more expensive than pre-filters, and most pre-filters can be cleaned several times before they need replacement.