Estes B6-6 Model Rocket Engine Pack - Includes 3 Engines with Starters & Plugs (1607)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 B6 6 engine pack includes 3 engines, 4 starters, and 4 starter plugs The B6 6 is an upper stage engine for model rocket flight it can also be used as a single stage engine in lightweight rockets Recommended for use with the Alpha, Alpha III, Viking, Xtreme, or other