The OS AX engine line is great. They run great, and last a long time! I am still using a 46 AX I bought fifteen years ago. They stay in tune and very rarely need any low speed needle adjustment. I own seven OS 55 ax engines, they all run great. They weigh the same as a 46 ax so I use a 55 AX in all .40 size planes. Here is a tip to make them last. When you find the Ideal needle valve setting after starting the engine go back one or two clicks to slightly richen the engine. Many flyers say you do not need after run oil. These are the guys who's engines need new bearings.Remove the glow plug and put three or four drops of oil in the cylander, then do the same in the carburator and turn over the engine with a starter to distribute oil. One old timer who insisted you do not need to use after run oil now uses it because my engines never fail!