Mechanical reasoning is crucial for many engineering fields, yet undergraduate engineering students struggle to understand discipline-specific formalisms from their courses that model mechanical concepts. The current investigation observed undergraduate engineering students' speech during mechanical reasoning and the benefits of attending to students' speech for assessing their mechanical reasoning and learning. Results from discourse analysis of two undergraduate engineering student dyads enrolled in a Mechanics of Materials course revealed students' mechanical reasoning through both course-specific and non-course-specific speech. Students generated speech that described functional or dynamic behaviors, as well as structure that described the static features. Students with highly developed formal knowledge of mathematics and physics often reason about mechanical systems in ways not captured traditional formalisms has implications for instruction and assessment.