How to Repair a Lawn Mower Oil Leak
The Lower Bearing Ring Seal
The bottom bearing ring seal (also known as an oil seal) is placed on the engine assembly's underside. This ring seals the crankshaft bearing, which rotates the mower blade. OIL LEAKAGE UNDER THE ENGINE (when it is sitting flat on its wheels). Always tilt the carburetor up. Otherwise, it will leak gas. Tilting some mowers causes gas to leak from a vented gas cap. Gas can sneak through the carbon filter in modern mowers with carbon filtered gas caps, destroying it. Both vented and carbon filtered caps must be drained before tilting the mower. The seal's inner face usually wears out. To test if the bottom bearing ring seal is the source of the leak, wipe away any oil buildup and reposition the mower (flat on its wheels). The seals frequently leak only during and after use. Heat makes the oil less viscous. The mower will cool off the leak.
Upper Bearing Ring Seal
Similar to the lower bearing ring seal, but positioned on top of the engine. This ring seals the crankshaft's entry into the crankcase. Damaged upper bearing ring seals cause oil leakage from the crankcase top while running. The seal is covered by several components, including the flywheel, which should not be removed without the right tools. Removing the flywheel from the crankshaft will allow you to inspect the area just below it where the crankshaft joins the crankcase. The upper bearing ring seal is likely the source of the oil leak if oil accumulates around it or if it is visibly damaged. In most cases, upper oil seal leakage causes oil to pool on the engine deck.
The valve gasket seals the valve cover to the valve chamber. If oil accumulates around the valve chamber where the cover connects, the gasket may be worn and have to be replaced. To find the leak, remove the oil buildup and run the engine for a few seconds. If oil returns to the same spot, a worn valve gasket is the likely culprit.
Seal for Oil Fill
The oil fill seal is located at the fill tube's base. While oil can leak through the dipstick seal, it is unusual due to the distance from the crankcase oil.
Filling Tube for Oil
The oil fill tube delivers oil to the crankcase. This tube links the crankcase to the sealed screw-on cap on one end. An accumulation of oil on the mower's deck, around the tube's connection to the engine, or on the tube itself could indicate a cracked or worn tube. To find the cause of the leak, wipe away any extra oil and run the mower for a few minutes. Find out where the oil comes from. If it is coming from the tube, it is probably cracked or damaged. If oil is leaking from the tube to the crankcase, the seal may be worn or cracked and has to be replaced.
The gasket on most lawnmower engines seals two sections of the crankcase rather than a cover. Normally, the seam is in the bottom third of the crankcase, or diagonally across the crankcase. Some engines employ a liquid gasket, while others use a paper or rubber gasket. Trace the circumference of the crankcase cover where it attaches to the crankcase housing. If oil collects around this seal, the crankcase gasket may need to be replaced. To determine if the leak is coming from the crankcase gasket, remove any oil and buildup from the area and look for leaks while driving.