Friction loss and heat transfer enhancement measurements were obtained for double-sided, partial height, strip fin arrays in a high aspect ratio (AR = 8), rectangular channel. Fins were arranged in a staggered array configuration with channel height to fin thickness ratio H/W = 9.6, spanwise spacing distance to fin thickness ratio S/W = 8.0, and streamwise spacing distance to fin length ratio X/L = 1.0. Within the channel, shortened strip fins of equal length are positioned directly opposite to each other on two heat transfer surfaces with three gap size to channel height ratios considered G/H = 0.2, 0.3, and 0.4, respectively. The thermal performance of each fin configuration is determined from the measured pressure drop across the array and regionally averaged heat transfer coefficients at flow Reynolds numbers ranging Re = 20,000–80,000. Partial height strip fin results are compared to the baseline cases of full height strip fins and the smooth channel. Correlations of friction loss, heat transfer enhancement, and thermal performance are provided as functions of flow Reynolds numbers for all cases. Full height strip fins provide the greatest heat transfer enhancement of all cases but introducing a gap size can significantly reduce friction losses. The thermal performance is greatest when using full height strip fins for Reynolds numbers ranging Re = 20,000–30,000 and using partial height strip fins with the gap size of G/H = 0.3 for Re = 40,000–80,000.