How do I replace the blade on my Petrol Lawnmower

Replacing the blade on your petrol lawnmower helps maintain a clean cut and ensures the mower operates safely and efficiently. A worn, bent or damaged blade can reduce cutting performance, place additional strain on the engine and increase vibration.

This guide explains how to safely remove and replace the cutting blade on your petrol lawnmower.

Quick Answer

To replace the blade on a petrol lawnmower:

• Switch off the engine and remove the spark plug cap.
• Safely access the underside of the mower.
• Hold the blade securely and remove the blade bolt.
• Inspect the blade adaptor for damage.
• Fit the new blade, a new washer and tighten the blade bolt securely.

Always ensure the blade is installed in the correct orientation before operating the mower.

Before You Begin

Before replacing the blade:

• Switch off the engine and allow it to cool completely.
• Remove the spark plug cap to prevent accidental starting.
• Place the mower on a stable surface.
• Wear thick protective gloves when handling the blade.
• Use the correct replacement blade for your lawnmower model.

Tools and parts required:

• Thick protective gloves
• Socket wrench or socket set
• Replacement blade
• New blade washer (recommended)

To access the blade:

• Tilt the mower backwards by lowering the handles with assistance, or
• Drain the fuel and engine oil before carefully turning the mower upside down.

Only invert the mower if the fuel and oil have been drained or if this is recommended by the manufacturer.

Steps

Step 1 – Access the Blade

Position the mower so you can safely access the underside.

Ensure the mower is stable before beginning work.

Step 2 – Secure the Blade

Put on protective gloves and hold the blade firmly to prevent it from rotating.

If necessary, place a block of wood between the blade and the mower deck to stop the blade from turning while loosening the bolt.

Step 3 – Remove the Blade

Using a suitable socket wrench:

1. Turn the blade bolt anti-clockwise to loosen it.
2. Remove the blade bolt.
3. Remove the blade washer.
4. Lift the blade from the blade adaptor.

Keep the blade adaptor in position while removing the blade.

Step 4 – Inspect the Blade Adaptor

Before fitting the replacement blade, inspect the blade adaptor for:

• Cracks
• Excessive wear
• Damage
• Distortion

Replace the blade adaptor if any damage is found.

Step 5 – Fit the New Blade

Position the replacement blade onto the blade adaptor.

Ensure:

• The locating pins align correctly.
• The blade sits flat against the adaptor.
• The blade is installed in the correct orientation.

Incorrect blade installation can affect cutting performance and safety.

Step 6 – Install the Washer and Blade Bolt

Fit a new blade washer onto the blade bolt.

Insert the blade bolt and tighten it clockwise using the socket wrench.

Tighten the bolt securely to the manufacturer's recommended torque where specified.

Step 7 – Final Checks

Before using the mower:

• Check that the blade is securely fitted.
• Confirm the blade rotates freely without contacting the mower deck.
• Reconnect the spark plug cap.
• Start the engine and check for abnormal vibration or noise.

If excessive vibration occurs, stop the engine immediately and inspect the blade installation.

Frequently Asked Questions

How often should I replace my lawnmower blade?

Inspect the blade before each mowing season and replace it if it is bent, cracked, excessively worn or cannot be sharpened safely.

Can I sharpen my lawnmower blade instead of replacing it?

Yes. A blade that is only dull can often be sharpened. However, if the blade is bent, cracked or heavily worn, it should be replaced.

Should I replace the blade washer?

Yes. Fitting a new blade washer is recommended whenever the blade is replaced to help ensure the blade is secured correctly.

What happens if I fit the blade the wrong way round?

A blade fitted incorrectly may cut poorly, fail to lift the grass properly and could become unsafe during operation. Always check the blade orientation before tightening the blade bolt.

What should I do if the blade bolt will not loosen?

Secure the blade using a block of wood and use the correct size socket wrench to apply controlled force. Avoid striking the bolt or using excessive force that could damage the blade adaptor or crankshaft.

Why is my lawnmower vibrating after replacing the blade?

Excessive vibration may indicate that the blade has been installed incorrectly, the blade adaptor is damaged, the blade is unbalanced or the crankshaft has been bent following a previous impact.