The bush of the head idler gear for the camshaft is not properly manufactured, and therefore, too much play may occur between the head idler gear shaft and the bush, which may cause excessive stress on the base of the brim of the shaft. If this problem is left unattended, the brim of the shaft may break, and consequently, the gear may interfere with the surrounding parts, making abnormal noise, or engine oil may leak, and at worst, interlocking between the gears could be lost, in which case the engine would stop and would not restart.