Estes 1/2A6-2 Model Rocket Engine Pack - Includes 3 Engines with Starters & Plugs (1593)PLEASE NOTE: These are 18mm diameter engines. Please refer to the instructions that came with your rocket (or the rocket's product description) to determine engine compatibility. Estes 1 2A6 2 engine pack includes 3 engines, 4 starters, and 4 starter plugs The 1 2A6 2 is a single stage engine designed for flights in rockets weighing less than 2 ounces, including the engine Recommended for use with the Wizard, Alpha III, or other compatible rocket kits