How To Fix An Oil Leak – Lawn Mower Repair
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How To Repair Lawn Mower Oil Leak

About this repair:

  • Rated as REALLY EASY
  • 6 step by step videos

Lower Bearing Ring Seal

The lower bearing ring seal (sometimes referred to as an oil seal) is located on the underside of the engine assembly. This ring seals the bearing that the crankshaft is fitted through, which then spins the blade of the mower. A damaged lower bearing ring seal will cause oil to leak directly under the centre of the engine (when it is sitting flat on its wheels). Make sure that the carburetor is always tilted up. Otherwise gas will leak out of it. On some mower models tilting the mower will cause gas to leak from a vented gas cap. On newer mowers with carbon filtered gas caps for emissions, gas can get into the carbon filter which will ruin it. With both vented caps and carbon filtered caps it is important to drain the gas before tilting the mower. Wear usually occurs on the inner face of the seal. To confirm if the lower bearing ring seal is the source of the leak, wipe away any oil buildup around the suspected area and return the mower to its normal position (flat on its wheels). Often the seals will only leak during and immediately after use. The oil becomes much less viscous when it has been heated. The leak will slow as the mower cools.

Upper Bearing Ring Seal

The upper bearing ring seal has the same function as the lower bearing ring seal, but is located on the top of the engine. This ring seals the point at which the crankshaft enters the crankcase. A damaged upper bearing ring seal will cause oil to leak from the top of the crankcase while the engine is running. It may be hard to verify that this is the source of the oil leak, as the seal is covered by a number of components including the flywheel, which should not be removed without the proper tools. If you are able to remove the flywheel from the crankshaft, inspect the area directly under it where the crankshaft enters the crankcase. If there is a buildup of oil around the seal, or if the seal itself is noticeably worn, the upper bearing ring seal is likely the cause of the oil leak. Oil leaking from the upper oil seal will typically run down the sides of the engine and pool on the mowers deck.

Valve Gasket

The valve gasket seals the connection between the valve cover and valve chamber. If there is a buildup of oil around the outside of the valve chamber where the cover attaches, the gasket may be worn and require replacing. To confirm the source of the leak, wipe away the oil buildup and run the engine for a short time. If oil returns to the same area, it is likely that a worn valve gasket is the cause of the oil leak.

Oil Fill Seal

The oil fill seal is at the base of the fill tube where the tube meets the crankcase. While oil can leak past the dipstick seal it is unlikely because of how far the opening is from the oil in the crankcase.

Oil Fill Tube

The oil fill tube allows oil to be delivered to the crankcase. It has a sealed screw-on cap at one end and connects to the crankcase at the other. If there is a buildup of oil on the deck of the mower, around the tube’s connection to the engine or on the tube itself, the tube may be cracked or its seal with the engine may be worn. To confirm the exact source of the leak, wipe away all excess oil from the affected area and run the mower for a few minutes. Check to see exactly where oil is originating. If it is coming out of the tube itself, the tube may be cracked or damaged. If oil is leaking from the bottom of the tube where it connects to the crankcase, the seal may be worn or broken and in need of replacing.

Crankcase Gasket

With most engines used on lawnmowers the gasket seals two halves of the crankcase rather than sealing a cover. The seam will usually be within the bottom third of the crankcase or diagonally through the crankcase from back to front. Many engines will use a liquid gasket while other manufactures use a traditional paper or rubber gasket. Once the crankcase cover has been located, trace the perimeter of where it attaches to the crankcase housing. If there is any oil buildup around this seal, the crankcase gasket may be worn and need replacing. To confirm if the crankcase gasket is the source of the leak, wipe away all oil and buildup from the area and check for leaks during usage, as that is when they are likely to occur.

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