It's a gas line filter. It fits just like an OEM ... 'cause it's an OEM part! Bwah ha ha. Anyhow, it's the sort of thing one should replace every few years (or more if you mow a lot - ie, pour a lot of gasoline through the tank). Don't forget the&nbsp;<a data-hook="product-link-linked" class="a-link-normal" href="/Briggs-Stratton-5062K-Spark-Plug-Platinum/dp/B00004RB22/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Briggs & Stratton 5062K Spark Plug Platinum</a>&nbsp;-- we have a twin -- and&nbsp;<a data-hook="product-link-linked" class="a-link-normal" href="/Oregon-30-104-Paper-Air-Filter-For-Briggs-Stratton-394019-394019S-398825-4136/dp/B00070QWXM/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Oregon 30-104 Paper Air Filter For Briggs & Stratton 394019, 394019S, 398825, 4136</a>&nbsp;every year while you're at it. Yeah, sure they can last longer with cleaning / re-gapping / etc. but if you've got the money and you're already in there for Spring cleaning and oil-changing ... why not. BTW, it's better to change the oil in the Fall just before stowing your rider IMHO so your rider spends the winter sitting in nice clean oil.