Estes 3 Packs of C6-5 Model Rocket Engines - Total of 9 Engines with Starters & Plugs (1614PK3)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. 3 Packs of Estes C6 5 engines total of 9 engines (each pack includes 3 engines, 4 starters, and 4 starter plugs) The C6 5 is a single stage engine designed for flights in rockets weighing less than 4 ounces, including the engine Recommended for use with the Antar, Big Bertha,