Estes 3 Packs of B4-4 Model Rocket Engines - Total of 9 Engines with Starters & Plugs (1602PK3)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 B4 4 engines total of 9 engines (each 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,