Air filter replaces Stihl OEM# 4237-120-1800, 42371201800, 4237 120 1800 on HS81, HS81R, HS81RC, HS81T, HS81TC, HS82, HS86, HS86R, HS86T and HS87 Hedge trimmers. Rotary# 12785, similar to Stens# 605-819, 605819. Specs: 7/16" x 2-3/8" x 2-13/16".
Refer to your owner's manual to match by OEM part #.