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How to Change Oil in a Lawn Mower

Updated on March 7, 2017
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Eugene has a keen interest in DIY and gardening. Over a 30 year period he has also become self taught in garden power tool maintenance.

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Maintaining Your Lawn Mower Engine

Motor oil is an essential requirement of all engines. It has several functions including lubrication of metal parts in contact (reducing friction), cooling, keeping surfaces clean, sealing and corrosion inhibition.
Your 4-stroke walk behind lawn mower engine is no different. The oil inside it gets dirty over time and without an oil filter to remove metal particles and soot, these contaminants can build up over time and cause engine wear. So it is a good policy to give it an oil change after every 20 to 50 hours of use (consult the manual of check the engine block for exact details).


Safety First

  • Don't run a lawn mower in an enclosed space. Carbon monoxide gas which is present in exhaust fumes is odourless, colourless and tasteless, but poisonous and can kill. Ideally run a mower outdoors when testing. If the mower is run in a garage/workshop, it should only be operated for a short period and you must ensure all doors and windows are open in order to provide adequate ventilation.
  • Set controls to off and remove the spark plug lead before working on any moving parts on an engine e.g. the blade. Ideally you should also remove the spark plug to be totally sure that the engine won't fire if you turn the blade.
  • Keep gas away from any sources of ignition.

What Tools are Required to Change Oil?

  • Socket wrench set
  • Spark plug wrench
  • Shallow container for collecting drained oil. A cookie tin is ideal
  • Some cloths for mopping spillage and wiping hands
  • Some newspapers/plastic sheeting for covering the floor or ground surface

A shallow tin like this one is ideal for draining oil into. Metal tins aren't totally leak proof and oil will eventually ooze out, so dispose of it safely
A shallow tin like this one is ideal for draining oil into. Metal tins aren't totally leak proof and oil will eventually ooze out, so dispose of it safely | Source

How to Change Oil in a Walk Behind Lawn Mower

  1. Pick a suitable location for working on, ideally the concrete floor of your garage, or you can work outdoors on a driveway.
  2. To avoid mess, cover the working surface with a few newspapers or a plastic sheet to collect any drips/spills.
  3. Run the engine for about 5 minutes to warm up the oil. This makes it less viscous and more fluid and it will drain easier. Don't run it any longer than this or the oil will scald you if it gets on your hands while draining!
  4. Drain the gas from the tank. You might get away without doing this if there is only a small amount in the tank.
  5. Detach the spark plug lead and ideally remove the spark plug from the engine.
  6. Place the engine on its side. Usually you need to keep the carburetor side of the engine uppermost to avoid oil leaking out through the breather. However check this out first.
  7. The oil in a 4-stroke engine is held in a reservoir at the lowest point called the sump. Usually an oil drain plug is screwed into the underside of the sump and when this is removed, oil can be easily drained. Any sludge which has settled should drain also. Not all engines have a drain plug and oil must be drained from the oil fill plug by tipping the mower on its side.
    Locate the drain plug. This may look like the hex head of a bolt in which case you can loosen it with a socket wrench. Alternatively it may have a square head and you will need to use an open ended wrench. Some plugs are recessed and flush with the sump with a square or hex socket head requiring an Allen key or the 1/2 inch drive of a socket wrench to be loosened. Turn the blade of the mower to move it out of the way if it is obstructing access. Alternatively remove it. In general the bolt which fastens a lawn mower blade in place is loosened by turning it counter-clockwise.
    Loosen the plug, but don't remove it fully. Turn the mower back onto its wheels and position it over the cookie tin. If you raise the cutting height to the highest setting, it should be easier to accommodate the tin under the deck of the mower. You can also raise the front and back wheels onto short slabs of lumber to increase clearance so that you can get your hands under the mower. Just be careful that it doesn't roll off these and drop down onto your hands while removing the drain plug.
  8. Now this is the difficult bit. You need to remove the plug and allow the oil to run down into the cookie tin. Disposable gloves will keep your hands clean but sometimes it is difficult to work with these. If you work bare handed, ensure you wash your hands well with a cleanser to remove oil and use moisturizer. Tip the mower if necessary so that you can reach the plug. Fully unscrew it and be prepared to get your hands out of the way. Allow the mower to drain for several minutes and then tip it slightly from side to side to remove any remaining oil. Turn the mower back on its side and replace the drain plug. Don't over-tighten it, especially if it is made from plastic. Wipe away any oil from the underside of the deck.

Usually you need to keep the carburetor side of the engine upright to avoid an oil leak when you turn the blade
Usually you need to keep the carburetor side of the engine upright to avoid an oil leak when you turn the blade | Source
The breather, a vent to the oil containing sump on the underside of the engine, is usually on the side of the engine adjacent to the air filter and carburetor. Keep it upright to avoid an oil leak
The breather, a vent to the oil containing sump on the underside of the engine, is usually on the side of the engine adjacent to the air filter and carburetor. Keep it upright to avoid an oil leak
Source
Oil Drain Plug
Oil Drain Plug | Source

What Oil Do I Need for a Lawnmower?

Oil is replaced in a mower through the oil filler plug. If the oil bottle has a narrow spout, you can pour it in directly, alternatively use a funnel. For small to medium sized mowers, you need from 17 to 20 fluid ounces (U.S.) or 500 to 600 ml of oil. Usually there are graduations on the side of small bottles of oil so that you can ascertain how much you have poured out. However I find it more convenient to use a plastic milk container which I have placed a mark on the side of, indicating the required amount of oil to fill my mower. I keep this container from year to year and I fill up to the mark from the oil can, and then just empty this amount of oil into the mower.
Generally an SAE 30 or 10W30 multigrade oil is recommended. Sometimes 15W40, which is more viscous when hot is recommended for larger engines. Check your manual for the required quantity and type of oil.

Oil level should be between the high and low mark on the dipstick
Oil level should be between the high and low mark on the dipstick | Source

How Often Does Oil Need to be Changed?

Generally every 50 hours or every season, whichever comes first

Questions & Answers

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      • eugbug profile image
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        Eugene Brennan 4 years ago from Ireland

        Thanks for dropping by and the comments!

      • wilderness profile image

        Dan Harmon 4 years ago from Boise, Idaho

        This is something that is all to easy to forget, and more than a few people have ruined a mower by never changing the oil at all.

        Good instructions and photos - you've made it easy to follow the instructions.