MTD 921-0381  Oil Seal, 4

The purpose of the Oil Seal is to protect shaft and bearings from intake of dirt and debris, and prevent oil or grease from leaking out. Replacing a damaged oil seal can prevent the more consequential damage to the crankshaft and other parts of the engine. Improper assembly, excessive wear, poor lubrication, and contamination from debris can all damage the oil seal and require replacement. Be sure to adjust the oil seal tightness when installing to prevent leaks of rotating shafts.