Walmart offers the Smithsonian Motor-Works Transparent Engine Kit, model no. 49013, for $26.56. (Amazon charges the same.) Opt for in-store pickup to dodge the $5.97 shipping fee. That's tied with our mention from a week ago and the lowest price we could find by $9. It includes all necessary parts to build a working model 4-cylinder engine. It requires two AA batteries (not included).