A nonstandard flanged and dished head is frequently used in the horizontal storage tank for quick and full access to the internal in oil and gas industry. The head is forged into an elliptical shape with a flat edge at its peripheral. The flat edge serves as a flange while the other mating half coming from the tank shell. The flange pair is installed within a C-shaped clamp and secured by compression bolts at its head side. A gasket is sandwiched between the mating surfaces of the flange pair to provide proper seal. While the head can be removed easily by unscrewing the compression bolts, this disintegrated structure does increase the complexity in component design and unique requirement for installation. The bolt compression load not only affects the pressure capacity of the storage tank, but also governs the stresses in flange pair and C-clamp. In this study, the flanged and dished head assembly has been modeled and analyzed by finite element method for stresses and gasket seal performance subject to installation and operation loads. Both elastic and elastic-plastic analysis has been performed. The tightening force of the bolts is examined against component stresses and gasket seal performance. Optimized bolt load is recommended based on acceptance criteria for stresses and leakage prevention.