Photo shows three types of spinner nut. The one on the engine has the shape for most electric starters. The other two are Harry Higgly nuts, the brass one is used for planes that are a bit tail heavy. Cheap and easy to use, they just replace your stock engine nut. A trainer really doesn't need a real spinner, just something for the starter to grab onto. A good spinner cost between $20.00 and $30.00, a nut is just a couple of bucks. These are much better then any of the plastic spinners.