Lawn Mower Maintenance: Oil, Blades, Spark Plugs, and Seasonal Storage

Oil Changes

Change the oil at the start of each mowing season and again mid-season if you mow frequently (weekly mowing on a half-acre or more). Most walk-behind mowers hold about 20 ounces of oil. Use the weight specified in your owner's manual — SAE 30 for warm climates, 10W-30 for variable temperatures. Synthetic oil works but is not necessary for a mower engine.

To change oil: run the engine for 5 minutes to warm the oil (warm oil drains faster and carries more contaminants out), stop the engine, disconnect the spark plug wire, and tip the mower on its side (carburetor side up to prevent oil from flooding the air filter) over a drain pan. Or use the drain plug if your mower has one. Let it drain completely, then refill to the full mark on the dipstick.

Check the oil level every time you mow. Pull the dipstick, wipe it clean, reinsert it fully, and read the level. Add oil if it is low. Running a mower on low oil overheats the engine and causes premature wear. Running it with no oil destroys the engine in minutes.

Used motor oil is hazardous waste. Do not pour it on the ground or put it in the trash. Most auto parts stores and municipal recycling centers accept used oil for free. Drain it into a sealed container and drop it off. A gallon milk jug works for transport.

Blade Maintenance

A sharp blade cuts grass cleanly. A dull blade tears it, leaving ragged, brown-tipped edges that make the lawn look stressed. Sharpen the blade at least twice per season — once at the start of the season and once mid-season. If you hit rocks or stumps, sharpen afterward because even small impacts dull the cutting edge.

To remove the blade: disconnect the spark plug wire, tip the mower, and use a socket wrench on the blade bolt. The bolt may be very tight — a breaker bar or impact wrench helps. Hold the blade with a thick glove or a block of wood wedged against the deck to prevent it from spinning while you loosen the bolt. Mark the blade (a paint mark on the bottom) so you reinstall it right-side up.

Sharpen with a bench grinder, angle grinder with a flap disc, or a flat file. Match the existing bevel angle (typically about 30 degrees). Remove equal amounts of material from both ends to maintain balance. Check balance by hanging the blade on a nail through the center hole — if one end drops, sharpen that end more.

Replace the blade if it is bent, cracked, deeply nicked, or has worn thin from repeated sharpening. A bent blade vibrates and damages the engine bearings and crankshaft. A cracked blade can break and send a fragment at high velocity. Replacement blades cost $15 to $30 and are available at any hardware store — cheap insurance against expensive engine damage.

Spark Plug and Air Filter

Replace the spark plug annually — at the start of each mowing season. A new plug costs $3 to $5 and ensures reliable starting and efficient combustion. Remove the old plug with a spark plug socket, check the gap on the new plug (the owner's manual specifies the gap — typically 0.030 inches), and install finger-tight plus a quarter turn with the wrench.

Clean or replace the air filter every season, more often if you mow in dusty conditions. A dirty air filter restricts airflow to the engine, causing poor performance, hard starting, and increased fuel consumption. Paper filters are replaced. Foam pre-filters are washed with soapy water, dried, and lightly oiled before reinstalling.

Check the air filter before every mow in dusty conditions (dry soil, gravel driveways, construction areas). A clogged filter in dusty conditions can cause the engine to ingest unfiltered air around the filter seal, which lets grit into the engine. Five seconds of checking beats a ruined engine.

While the spark plug is out, pour a tablespoon of engine oil into the cylinder and pull the starter cord a few times to distribute it across the cylinder walls. This prevents rust during storage and provides initial lubrication on first start next season.

Seasonal Storage

End-of-season storage determines how easily the mower starts in spring. The biggest enemy is stale fuel. Gasoline with ethanol (E10) absorbs moisture and degrades within 30 days, gumming up the carburetor and fuel system. Two options: run the engine until the tank is completely empty, or add fuel stabilizer to a full tank and run the engine for 5 minutes to distribute the stabilizer through the fuel system.

Clean the mower deck after the last mow of the season. Scrape off accumulated grass clippings from the underside — dried grass retains moisture and promotes rust. A putty knife or paint scraper removes dried clippings effectively. Hose off the deck and let it dry before storage.

Store the mower in a dry location — a garage, shed, or covered area. A mower stored uncovered outdoors rusts, and water in the engine and fuel system causes problems. If indoor storage is not available, cover the mower with a fitted cover or a tarp secured against wind.

For battery-powered mowers, store the battery according to the manufacturer's instructions — typically at about 50% charge in a room-temperature location. Do not store lithium batteries in freezing temperatures or fully depleted. Most battery mower manufacturers recommend removing the battery from the mower during storage.

Frequently Asked Questions