Get .50 size power into your .40 size aircraft The 55AX is better than ever, with an improved carburetor and muffler. You get the same phenomenal power, with the convenience and safety of a rear angled needle valve on the carb and a squared off muffler for easier cowling installation. If you want .50 scale power that fits conveniently in .40 size planes, get the 55AX and get flying! Comes with the E3071 muffler for quiet efficiency. 40K carburetor has a diagonally placed needle valve so you don't need a remote needle valve. Mounts directly into the bolt pattern for the 46AX. Advanced Bimetallic Liner (ABL) uses two alloys to provide a durable layer between the brass liner and piston. Recommended prop sizes: 12x7, 12x8, 13x6, 13x7. Thumbnail label:--> The large-chambered muffler reduces sound without reducing power. Thumbnail label:--> The 40K carburetor includes a diagonally placed needle valve so there's no need for a remote needle valve.