20 Inch Lawn Mower Blade Replacement Guide | Do It Safely In 6 Steps

This 20 inch lawn mower blade replacement guide walks you through the job safely, whether your mower is gas, cordless, or corded. The process is straightforward, but skipping the safety steps or buying the wrong blade can turn a simple swap into a frustrating afternoon. Here’s the exact order that works, plus the mistakes to avoid.

What You’ll Need Before You Start

Gather these items first so you don’t have to stop mid-job:

• Heavy work gloves to protect your hands from the blade’s sharp edges.
• A socket wrench with a 15 mm socket — the most common size for blade nuts, though some mowers differ.
• A block of wood, like a 2×4, to wedge between the blade and the deck.
• The correct replacement blade for your specific mower model.

Consumer Reports stresses removing all power sources before touching the blade: disconnect the spark plug wire on gas mowers and remove the battery on battery-powered models. Skip this step and the blade could spin when you least expect it.

How To Remove The Old Blade Safely

With the power source disconnected and the blade stopped completely, tilt the mower on its side so you can access the blade underside. The fuel cap and air filter should face up on gas mowers to avoid leaks.

Wedge the wood block between the blade and the mower deck — this stops the blade from turning while you loosen the nut. Then, using the 15 mm socket, turn the blade nut counterclockwise. Most mowers use standard threading, but some retaining bolts are reverse-threaded and loosen clockwise, so confirm the direction before applying force.

Once the nut is loose, carefully remove it, then the spacer and the blade insulator if your mower has one. Note the exact order of these parts as you take them off — reassembly is easier when you know which piece goes where. Many guides recommend snapping a quick photo before disassembly to use as a reference.

Slide the old blade off the mounting hub and set it aside. Before installing the new blade, inspect the old one: if it has cracks, dents, or visible damage, it was due for replacement, and Husqvarna’s guidance agrees that damaged blades should never be reused.

Installing The New Blade Correctly

Place the new blade on the hub, aligning it with any alignment features or posts built into the mounting area. The blade must sit flat and match the mounting pattern — many 20-inch mowers use a single blade, but the hole shape and size vary by model.

Blade orientation matters. The cutting edge must face the direction of rotation, and any markings on the blade, like “grass side,” should face away from the deck when installed. When in doubt, compare the new blade’s angle to the old one before you remove it.

Reinstall the parts in reverse order: the insulator, the spacer, then the nut. Wedge the wood block back in place to hold the blade steady, and tighten the nut with your socket wrench.

Torque matters more than most people think. If your mower’s manual gives a specification, follow it rather than guessing at “tighten firmly.” If no spec exists, tighten evenly and firmly — under-tightening lets the blade wobble, and overtightening can strip the threads.

If you’re still deciding whether to replace the mower entirely instead, our tested roundup of the best 20-inch models covers the top options worth considering.

Buying The Right Replacement Blade

Don’t buy a blade by length alone. Measure the old blade from one corner diagonally across the center hole to the opposite corner, then match the hole shape and dimensions to your mower model. A blade that fits loosely or doesn’t align with the mounting hub is a safety hazard, not a bargain.

Look up the exact OEM part number for your 20-inch mower and check the replacement listing against it. Prices vary by retailer and manufacturer, so compare a few listings before ordering. Popular Mechanics notes that the right blade for a corded 20-inch mower will differ from a gas model’s even when the deck width matches.