Peabs the diameter is 1 5/8 inches. They come in different lengths. My brother uses a 4 inch long filter. I prefer the 6 inch. I think the more element the more free flow. I bought my arrow and revival from Myron's Mopeds in Fullerton. We have a Myron's Moped Ride every month. The next one is Oct 14 starting in Costa Mesa by PCH and Superior going through Newport, going across to Balboa Island via ferry and down PCH to Dana Point. You are welcome to join us. More info on WWW.myronsmoped.com.
By the way you will have to jet up with the uni pod air filter.