If your lawn mower engine seized up, it's a good bet that you forgot to add oil. Without the lubrication the oil provides, the piston that drives the shaft creates enough heat as it pumps back and forth to fuse it to the housing, although it's unlikely that would actually happen. What's more likely is that friction between the piston and the housing gets so extreme that the force created in the combustion chamber isn't strong enough to move it.
The result is that you can't pull the starting cord, and when you turn over the machine and try to turn the blade by hand, you can't do it. At this point, you may think it's time for a new lawn mower — or at the very least, an expensive trip to a service pro — but in fact, you can probably fix this yourself. It's not a sure thing because depending on how long the machine ran without oil, the piston or the housing may be partially fused, or it may be gouged, and there may not be enough compression for operation. It's worth a try, though.
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How to Free Up a Seized Lawn Mower Engine
Online demonstrations that this procedure works often feature a product called PB Blaster. This is a spray lubricant, but unlike other types, such as WD-40, it's also a penetrating oil, so it's the stuff you need for this job.
Step 1: Tip the Lawn Mower
Set the lawn mower on a level surface and tip it so the spark plug is facing up.
Step 2: Remove the Spark Plug
Pull off the boot from the lawn mower spark plug and unscrew the plug from the motor housing using a socket wrench. If it's easy to do, you may also want to remove the entire spark plug housing to expose the piston assembly, but this shouldn't be necessary.
Step 3: Spray PB Blaster Into the Spark Plug Cavity
Spray a generous amount of PB Blaster into the spark plug cavity. In this case, a generous amount is as much as 1/4 of the contents of the can. Leave the lawn mower in this position overnight to allow the lubricant to penetrate.
Step 4: Unstick the Engine by Turning the Blade
Rotate the blade back and forth to free up the seized piston. If it's too hard to turn it at first, affix a torque wrench to the nut that holds the blade to the motor shaft and use that to get it started. Keep moving the blade back and forth until it starts to turn freely. You may have to spray a little more PB Blaster into the spark plug cavity to get this to happen.
Step 5: Add Engine Oil
Turn the lawn mower upright and fill the crankcase with oil as recommended by the manufacturer. For extra lubrication, combine the engine oil with 20 percent Marvel Mystery Oil. Mix the oils in a separate container before pouring it into the engine to make sure you get the right proportion. At this time, check the gas and add more if necessary.
Step 6: Replace the Spark Plug
Pull the starting cord to be sure the engine turns and then screw in the spark plug and replace the boot.
Step 7: Start the Engine
Pull the cord to start the engine. As it turns over, it will probably release a lot of smoke as the extra oil in the piston housing burns off, so it's best to do this outside. If the engine won't turn over or runs rough, it's probably because of compression loss due to gouging in the piston chamber. If so, it's probably time for a new mower.