How to change your lawn mower blades

Dull mower blades rip the grass to shreds leaving the finish rough and the grass stressed out. Sharp mower blades encourage healthier lawns to grow because the grass can heal quickly from a clean slice. Here’s a quick rundown of when and how to go about changing the blades on your ride-on mower.
How often you need to sharpen or replace your mower blades will depend a bit on the terrain you are mowing. Rocks, sticks and rough ground will cause your mower blades to deterate faster. But as a general rule your mower blades might need sharpening twice a season .
Blades will need replacing when:

  • they’re bent or warped
  • there are chips or dents in the cutting edge
  • they’re corroded
  • the edge opposite the cutting edge is thin

Sharpening or replacing the blades on a rotary mower is easier because you can simply flip the mower over and remove the one or two blades it has. A ride on mower is a bit different as getting at the blades can be more complex and for this you’ll need jacks and steady supports.
Before you attempt to change the blades refer to your service manual to confirm what safety precautions you’ll need to take, and if you do not have the appropriate experience to carry out the job, it is always recommended to take your lawn mower to a qualified repairer.
Step 1 – Remove the spark plug
Begin by removing the spark plug cable from the spark plug to make sure that no power is delivered to the motor during the process. To prevent damaging the spark plug lead, be careful not to pull on the lead itself. Instead, firmly grip the lead plug where it joins the spark plug, twist it side to side to ensure it moves freely, then pull straight backwards.
Step 2 – Drain the fuel
Next, it’s recommended to drain any fuel that you have in the tank and close off the fuel supply tap to the engine. This helps to prevent any unwanted fuel spills and makes the entire task a quick and easy one. Store the fuel safely away from where you are working in a clearly marked, sealed container intended for storing fuel.
Step 3 – Loosen the blade bolt
Next, you want to secure the blade by placing a block of wood between the mower deck and the blade itself. Wearing safety gloves, begin to loosen the blade bolt by turning it counter-clockwise. Depending on the type of lawn mower that you have, there may be one or several heat treated blade bolts to undo, which may or may not have special washers attached to them. Refer to your service manual.
Step 4 – Removing & Replacing The Blade
Now you have removed your blade make sure you know which way up it needs to go when reinstalled most blades have “this way up” embossed on them but if you can not see this then mark the blade with spray paint.  Inspect the mower blade for signs of damage, cracks or dents on the blade, replace it.
If you decide to sharpen the blades, start at the heavier end and just do a small amount at a time while paying attention to sharpening evenly across the face, until the blade is balanced. Never sharpen your blades to a knifepoint and try to allow for a 1mm edge. Before reinstalling, check the blades are balanced so they doesn’t cause any vibrations or worse, ruin the blade shaft or bearings. To do this, hang it on a nail like a propeller. If one side falls, that means it’s heavier and needs more taken off.
If installing a new blade, you need to make sure that the cutting edge of the blade is facing the direction of rotation. Ensure that the writing “this way up” is facing toward the deck and that the star on the mandrel is perfectly aligned with the centre hole on the mower blade. Giving the underside of the deck a good clean at his stage is a good idea  to help prevent any debris or clippings from obstructing the new blades.
Step 5 – Tighten the Bolt
Place the blade bolt into the centre hole and then tighten it. It is important to check bolt specifications for your individual mower’s torque settings to ensure it is not over or under torqued. Re connect the spark plug and add fuel, then start your lawn mower and listen for any suspicious vibrations or sounds that occur with an unbalanced blade.