Abstract: Nonroad diesel engines have been widely used in modern agriculture in recent years. This study aims to investigate the impact of the pre-tightening deformation of the cylinder bore on the engine oil consumption. The pre-tightening force was also exerted by the cylinder head bolts. A series of tests was carried out to determine the axial force of the cylinder head bolt. The accuracy of boundary conditions was improved in the finite element (FE) simulation. A numerical model was established on the cylinder head bolt-cylinder head-cylinder gasket-body. The pre-tightening deformation of the cylinder bore was obtained under different spacing of the cylinder head bolt in the diesel engine. Two shapes of the cylinder bore were selected for the oil consumption test of the nonroad diesel engine. Furthermore, the oil consumption rate and the engine fuel ratio were then calculated according to the oil consumption. An evaluation approach of cylinder bore deformation was proposed after simulation. The maximum internal tangent ellipse was also fitted for the cylinder bore pre-tightening deformation of a diesel engine under different cross-sections using Python software. The average ovality, average elliptical leakage rate, sectional area gradient, and maximum concentricity were taken as the evaluation indicators for the deformation of the cylinder bores. The results show that the necking behavior was observed in the pre-tightening deformation of the cylinder bore in Projects A and B. There was a relatively small difference among the three indicators of the average elliptical leakage rate, the maximum sectional area gradient, and the maximum concentricity. The average ovality in Project B was only 25% of Project A, indicating a large decrease. There was little influence on the shape of the cylinder hole pre-tightening deformation of the nonroad diesel engine; There was also a small difference in the maximum concentricity index between Project B and C. There was a large difference between the average elliptical light leakage rate and the maximum sectional area gradient. The pre-tightening deformation shape of the cylinder bore was shifted from the hourglass-like of Project B to the horn-like of Project C. Therefore, the average elliptical light leakage rate and the maximum sectional area gradient greatly contributed to the deformation shape of the cylinder hole. They greatly improved the necking deformation of the cylinder bore, only when the center distance of the cylinder head bolt bore increased. The reason was that the axial force of the cylinder head bolt was characterized by the tension. Average elliptical leakage rate and maximum sectional area gradient were beneficial to the evaluation of the cylinder bore necking. According to the national standard, the test of engine oil consumption was conducted 800 hours for the nonroad diesel engine with the hourglass and horn-like cylinder bore in Project A and C, respectively. The average rate of the engine oil consumption was calculated from 0.142 g/kWh in Project A to 0.062 g/kWh in Project C. The center distance of the cylinder head bolt hole was improved, the cylinder bore shape from hourglass to horn-like. The deformation of the horn cylinder bore significantly reduced the engine oil consumption than that of the hourglass ones.