Estes B4-4 Model Rocket Engine Pack - Includes 3 Engines with Starters & Plugs (1602)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 B4 4 engine pack includes 3 engines, 4 starters, and 4 starter plugs The B4 4 is a single stage engine designed for model rocket flights weighing less than 3. 5 ounces, including the engine Recommended for use with the Baby Bertha, AstroCam, Cadet, Riptide, or other