For manual transmission vehicles with turbochargers, the axial force of the bolts securing the covers for the oil control valves, which are to control opening/closing the engine intake valves by hydraulic power, is low. If the vehicle is driven continuously with high load and high revs, negative pressure inside the engine may increase and cause the valve gaskets to deform, in which case the axial force of the bolts could decrease further. If this occurs, the bolts could become loose allowing engine oil to leak, which could in the worst case, lead to a fire.