Lawn Mower: Leaks Oil

There are several gaskets that form seals in your lawnmower’s engine. If a gasket fails it can create an oil leak. Depending on the design of the engine there will potentially be gaskets between: the crankcase and sump; the cylinder and crankcase; the cylinder and valve head; and the valve head and valve cover. Any one of these can potentially leak. The gaskets can wear out of time due to constant exposure to oil and fuel. A gasket can fail much sooner than normal if the engine is overheated or used beyond its designed capacity. A gasket should never be reused. If part of the engine is disassembled for repair and then reassembled using the same gaskets, they may develop leaks. Repairing a leaking engine gasket can be a very simple repair or an extremely complicated one depending on which gasket it is. Replacing a valve cover gasket is a simple repair, only requiring you to remove the bolts that secure the cover in order to access the gasket. The sump gasket or cylinder gasket requires much of the engine to be disassembled in order to replace them. Once the gasket is accessed it is important to remove all traces of the old gasket. The new gasket will leak if you don’t. Use a scraper or razor blade to remove the old gasket. Extreme care should be taken to not nick or score the engine castings when removing the old gasket. Once the surfaces have been cleaned the new gasket can be installed. Refer to the manufacturer’s service manual for the correct torque specs and torqueing sequence when reassembling the engine. Over- or under-tightening the fasteners or unevenly tightening them can lead to oil leaks.

If your lawnmower is leaking oil it could have a bad crankshaft oil seal. There is an oil seal on both ends of the crankshaft where it passes through the crankcase. The seal is pressed into the engine castings and forms a seal against the crankshaft. As the crankshaft rotates it will wear the seal. Under normal conditions, the seal will be trouble-free for years. If the engine is operated in very dusty conditions or if dirt gets into the engine due to filter damage or improper filter maintenance, the seal can wear much faster. Occasionally, a seal will be installed improperly, which will cause it to leak. Replacing the seal is a somewhat advanced repair. The top seal is easier to replace than the lower. To replace the top seal, the engine shroud and starter will need to be removed. Next the flywheel will be removed. Once the flywheel is out of the way the seal should be visible. The seal can be stubborn to remove. It is best done with a seal removal tool. It can be removed by driving a small screwdriver into it and prying it out, but care must be taken to not damage the seat or the shaft with this method. Once the seal is removed, the new one can be pressing in place and the mower reassembled. The lower seal requires the engine to be removed from the mower to access it. Drain the engine oil and fuel from the engine. Then remove any cables or linkages attached to the engine. Remove the mower blade and any covers on the underside of the deck that cover the engine mount bolts. Unbolt the engine and remove it from the mower. The seal can then be pulled from the bottom of the engine. Once the seal is installed the engine can be remounted and the mower reassembled.