All the reports and reviews I've read all say the same thing, with the OS61 and a Pitts muffler no tail weight was needed. I installed that same combo and had to add about 4 oz of weight to the tail. I added a Sullivan fueler but it doesn't weigh that much. I don't even have the wheel paints installed. I installed a Bisson Pitts muffler maybe it weighs more than others, Beat me. Oh well its still fun to fly.