I purchased this Outerwears - Pre-Filter that is designed for the UNI - Compact version 2-Stage Foam Air-Filter (part# PT-3429) that I also purchased. Without the use of this Outerwear - Pre-Filter your leg will get foam filter oil all over it while riding & ruin your pants, so I had to also purchase this product too. It works well to keep your air-filter cleaner & keeps larger debris from ever getting to the foam section of the UNI - Air-Filter. It also worked well to keep the foam air-filter oil from ruining my pants, which was such a pain & the entire reason why I had to purchase one. This is an excellent must have product for the UNI- Compact version 2-Stage Foam Air-Filter (part# PT3429) & I highly recommend it to other customers that have the exact same problem as me with ruining your pants from the foam air-filter oil getting all over them.