Chances are it's a 25 cc. Although there are many variations of the Homelite string trimmer, 999 out of a thousand were 25's. On some of them, on the exhaust side of the cylinder on the next to the top fin, there is a number. On my 30 it say's 4730A. My 25 is not convienent but it has a number in the same place such as 3625B.

Jim