The exhaust manifold is one of the engine parts in internal combustion engines that operates under the influence of high temperatures. To prevent issues arising from the expansion movement due to high temperature effects, the exhaust manifold of a 6-cylinder commercial diesel engine has been designed in three parts. Sealing rings that can withstand high temperatures are used in the three-part exhaust manifold to prevent gas leaks. For the exhaust manifold, 1-dimensional, thermal, flow, and structural analyses have been performed. Initially, based on the structural analysis of the exhaust manifold conducted without sealing rings, sealing ring selection was made according to the deformation values of the exhaust manifold. In the structural analysis, the PEEQ (equivalent plastic strain) method, which is utilized to determine the thermomechanical fatigue crack initiation area, is employed. In this study, an approach using this method was undertaken, and the crack initiation area was examined.