The venturi size of these carburetors is larger than Series 1 and Dual System Carburetors. For sale is a piston kit for a STHILL MS251C chainsaw bought new but couldn't use due to bore damage on saw it was supposed to go in. (CAUTION: Overtightening will damage the tapered portion of the needle.) When replacing the fuel pump element, install with the slot opening at a 45o angle as illustrated.