This Model Rocket Accessory Package is An Engine Mount Kit for "D" and "E" Engines by Estes. With the parts in this accessory kit, you will be able to construct two D and/or E engine mounts. An adapter is included to allow you to fly D engines in the E engine mount. The spacer ring provided allows you to "field" convert an E engine mount to use D engines. You can build these mounts to fit BT-55, BT-60, or BT-80 body tubes. There are extra parts, so save them for future use. Two different size engine hooks (one fits D engines and the other fits E engines). The engine mount tubes provided with this kit are heavy duty and thus thicker walled. Fully detailed assembly instructions with illustrations.