yup high end .21 race engines are so much fun to break in. loosen the glow plug and heat up the head before you go to start it and make sure when you start it the piston is at bottom dead center that way the engine will get a full stroke when you go to start it. and when you break it in get the engine up to around 200 run it for 3-5 min. let it cool and do it again.(heat cycle) that is the only way to run in a nitro mill. give it time!!