Can You Run Mower Without Blade? - don't do it!

By: Author John Cunningham. Published: 2020/12/18 at 12:40 pm

Many people ask me about starting their mower engine without the blade, so much so I wrote a post explaining all.

Many small engines are designed specifically for mowers and as such require the extra mass of a moving blade to help rotate the crankshaft on the compression stroke past top dead center (TDC). Starting such an engine without a blade may cause personal injury and damage to the engine.

In this post, you’ll learn why most mower engines require a blade to run correctly. If you need video help, check out “Replacing mower blade video.”

Most mower engines won’t start without a blade attached.

Small Engine Types

There are two common engine configurations: the Vertical crankshaft and the Horizontal crankshaft layout. Vertical and Horizontal crankshaft is self-explanatory. The orientation of the crank will naturally lend itself more suitable to one task over another.

Horizontal Engine

It’s not possible to turn a Horizontal shaft engine vertically and simply call it a Vertical engine. These engines have been designed to operate in their particular configuration.

For example, most small engine oil systems are splash lube, which relies on splash paddles hitting a puddle of oil at the base of the engine. You can see the problem here. But the differences go deeper than that, as we’ll see shortly.

Horizontal small engines are designed to suit many applications, unlike vertical engines. The Horizontal engine is more common generally. They are best suited to driving power washers, water pumps, cement mixers, tillers, snowblowers, etc.

They are not designed specifically for any single purpose and, as such, will run just as well with or without an attachment on the crankshaft.

Vertical Engine

Mower engines, on the other hand, are a little different. They are known as vertical shaft engines. Vertical shaft engines, by their layout, are perfectly adapted to sitting on top of a mower deck with a blade fastened to the crankshaft underneath.

Most of these types of engines are designed to have a blade fitted, meaning the mass of the rotating blade is factored into the overall design of the motor. I say most mowers because some manufacturers employ a feature like Toro’s Spin Stop.

They use a separate control lever to engage the blade. This feature allows the engine to run without the blade spinning, useful for propelling the mower across graveled areas.

Higher-end Honda mowers also sport this feature. If your mower has this feature, then your engine will comfortably run without a blade attached. Likewise, tractor-mower engines will run happily without blades attached since they aren’t designed to have a blade fitted directly to the crankshaft.

Why Does Mower Need Blade To Start

Before explaining why a mower requires a blade, it’s helpful to know a little about the four strokes of an engine cycle. Once understood, the purpose of sufficient mass to rotate the engine becomes clear.

Stroke 1 Induction – As the piston travels down the cylinder the intake valve opens. Gas and air mixture enter the cylinder.

As the piston nears the bottom of the cylinder, the intake valve closes.

Stroke 2 Compression – The piston meets resistance as it travels back up the cylinder, and all valves are closed. Gas and air mixture is forced towards the combustion chamber at the top of the cylinder by the piston.

(It’s momentum, helped by the mass of the rotating crankshaft, flywheel, and blade that powers this stroke).

Stroke 3 Power – As the piston passes the very top of the cylinder travel (TDC), the plug fires, and the explosion drives the piston downwards towards the bottom of the cylinder.

This, of course, also drives the blade, which cuts the grass.

Stroke 4 Exhaust – This is the last of the four strokes of a complete cycle. The piston begins to travel back up the cylinder, and the exhaust valve opens to allow spent gases to exit through the muffler.

As the piston reaches the top of the cylinder, the exhaust valve closes, and the whole cycle begins again.

Mower Engine Needs A Rotating Mass

Flywheel Adds Mass

All small engines (vertical and horizontal) require a rotating mass to drive the piston when it’s not under power. This is especially true on the compression stroke, where the piston meets lots of resistance as it squashes the fuel mix into the combustion chamber.

To help the engine achieve sufficient rotational momentum, it employs weight in the form of a crankshaft and flywheel, and of course, the blade is factored in when the engine is designed for a mower.

If a moving object has sufficient mass (weight), it will carry momentum. Without the required mass rotating at a sufficient speed (the mower blade spins at over 200 mph at the tip), the resistance the piston meets on the compression stroke would simply overcome the rotational force on the crankshaft, and the engine loses its momentum.

Firing the plug now easily causes the engine to reverse track, aka kickback.

Spinning Blade Adds Mass

Running Mower Without Blade Is Dangerous

Attempting to start a mower without a blade is dangerous; the lack of crankshaft momentum causes the engine to kick back. Kickback occurs when you pull the starter cord to start the engine, and the cord is snapped from your hand sharply.

The cord often whips you as it snaps back and sometimes breaks the pull handle. This can be a painful experience and is more commonly associated with a broken shear key.

You can read more about a shear-key here.

Broken Shear-key

Running Mower Without Blade Risks Engine Damage

Running the mower engine without the blade risks damaging the crankshaft or shear-key. Blades aren’t universal; a mower engine is designed to run with a specific blade; by specific, I mean not only correct length and width but also mass and lift.

As an engine approaches top dead center, and if it isn’t carrying sufficient momentum. As the plug fires, it causes the crankshaft to rotate in the wrong direction, snapping the cord from your hand as it does so.

If you need video help, check out “Fitting mower shear-key video” which covers the complete process from diagnosis to replacing, or check out a post I wrote here, “Replacing a shear-key.”