This study evaluated the thermal performance of a 100 cc internal combustion engine cylinder through the integration of geometry–material–FEM criteria aimed at a techno-economic co-optimization. Four fin geometries (rectangular, circular, angular, and trapezoidal prismatic) and five alloys preselected using CES EduPack (Al 5182-H19, Mg Z6, lamellar graphite cast iron EN-GJL-300, Zn–Cu–Ti Korloy 3130, and Zn–Al ZA-27) were analyzed. The developed method comprised: (i) CAD modeling in SolidWorks, (ii) material preselection based on thermal conductivity (k), density (ρ), and cost, and (iii) steady-state thermal simulations in ANSYS, where minimum and maximum temperatures (Tmin/Tmax), heat flux, and fin mass were quantified. The results showed that the angular geometry maximizes heat dissipation, reaching a maximum heat flux of approximately 0.069 W/mm² (Al 5182-H19: 0.069172; Mg Z6: 0.069177), outperforming rectangular (≈ 0.056–0.063 W/mm²), trapezoidal (≈ 0.044–0.050 W/mm²), and circular (≈ 0.038–0.042 W/mm²) geometries. With the Mg Z6 alloy, fin mass remained low (0.360 kg for angular), offering advantages over aluminum (≈ 0.522 kg) and particularly over cast iron (≈ 1.424 kg). In relative terms, the angular + Mg Z6 combination increased maximum heat flux by approximately 85% compared with circular + Mg Z6 (0.069177 vs. 0.037448 W/mm²) and by 24% compared with rectangular + Mg Z6 (0.055613 W/mm²), maintaining Tmax ≈ 585 K and reducing Tmin to ≈ 552 K, thereby enhancing the thermal dissipation gradient at the fin base. In summary, the main contribution of this study is a reproducible methodology for form–material co-optimization based on the finite element method and a multicriteria selection (k–ρ–cost), enabling quantitative decision-making for small-displacement cylinder redesign. Consequently, the combination of angular fins with the Mg Z6 alloy is identified as the most favorable option for lightweight engines, balancing thermal performance, mass, and cost.