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The effect of fin attachment on the thermal stress reduction of exhaust manifold of an off road diesel engine (Komatsu HD325-6) was investigated. For doing this, coupled thermo-fluid-solid analysis of exhaust manifold of the off road diesel engine was carried out. The thermal analysis, including thermal flow, thermal stress, and the thermal deformation of the manifold was investigated. The flow inside the manifold was simulated and then its properties including velocity, pressure, and temperature were obtained. The flow properties were transferred to the solid model and then the thermal stresses and the thermal deformations of the manifold under different operating conditions were calculated. Finally, based on the predicted thermal stresses and thermal deformations of the manifold body shell, two fin types as well as body shell thickness increase were applied in the critical induced thermal stress area of the manifold to reduce the thermal stress and thermal deformation. The results of the above modifications show that the combined modifications, i.e. the thickness increase and the fin attachment, decrease the thermal stresses by up to 28% and the contribution of the fin attachment in this reduction is much higher compared to the shell thickness increase.