Are Lawnmower Blade Bolts Reverse Thread?

Are lawnmower blade bolts reverse thread? -Yes, on some models, they are!

These bolts hold the blades of mowers in place and they’re known as reverse threaded. When a bolt is reversely threaded, it’s turned to the left or counterclockwise when tightened and clockwise or on the right to be loosened.

Standard fashion bolts are turned to the right or clockwise to be tightened and on the left or counterclockwise to be loosened.

Reverse thread bolts are used in lawnmower blades to secure the blade on the shaft so that the blade’s rotation doesn’t loosen up the bolt.

Also known as left-hand bolds, these anticlockwise bolts are the same as regular bolts, except for the fact that the ridges wrap around the cylinder in an opposite direction.

These bolts are less common and used only in specific situations, such as in lawnmowers rotating on the right side.

A man repairs a lawn mower in the backyard of the house

Are Lawnmower Blade Bolts Reverse Thread?

Some bolts that hold lawnmower blades are reverse-threaded, but not all of them. This depends on the manufacturer, model, the direction of rotation of the blade, etc. A reversely threaded nut is used to keep the blade firm onto the engine’s drive shaft.

When the blade rotates in a certain direction, the bolt won’t be affected in any way or loosened because it’s tightened in the opposite direction.

Considering there are blades with a reverse thread bolt and blades with a regular bolt, you need to know the proper direction of the shaft prior to unscrewing any bolts and replacing blades.

Most blades of lawnmowers do rotate clockwise or counterclockwise when seen from below and their cutting edge is on the right side. When you’re not sure how the mower blade rotates, see the discharge chute position-if it’s on the right side and angled back, it’s a clockwise blade.

Prior to the unscrewing, tip the mower on the side with the engine’s air cleaner upward and keep the blade blocked with a wood piece to prevent it from turning while working. Don’t forget to keep the bolt in a safe place so that it’s there when you need to set it back!

While undoing the bolt, pay close attention to how the blade fits into the adapter on the shaft, which side of the blade is toward the mower deck, and the order in which the blade washers are installed on the retaining bolt.

Ensure you’re fitting the new blade into the adapter with the proper side upwards and that the washers are replaced in the right way.

To screw it on, rotate it on the opposite side to which you’ve unscrewed it. If the blade rotates on the right, screw in the bolt counterclockwise. If it rotates on the left, screw in the bolt clockwise.

Rotate it to a torque value that is specified in the manual of the machine.

Why Extra Cautiousness Is Needed When Removing Blades on Lawnmowers?

The blade which cuts the grass on many types of lawnmowers is secured by a threaded, by a hex head bolt that’s screwed into an adapter that’s on the drive shaft of the engine. If a blade needs to be removed or sharpened, the bolt which retains the blade needs to be unscrewed.

Before this is done or any other type of maintenance on the lawnmower, the spark plug wire must be removed from the plug for safety!

In addition to knowing the direction of the bolt so as to prevent any injuries if you try the wrong way, you may also need an additional tool for the removal if it’s been tightened too hard.

Moreover, it’s also pivotal to wear work gloves while unscrewing bolts; hand slips if you’re tackling a stubborn bolt may lead to a serious cut by leading to contact with the blade!

Left- vs. Right-Handed Bolts

Bolts are left- or right-handed and the handedness is defined by the direction in which the helical thread is wrapped around the shaft of the screw. 

The right-handed ones have a thread that runs on the right or in a clockwise direction while the left-handed ones run in the opposite, anti-clockwise direction.

Screw-handedness origins are in human physiology; most humans are right-handed and the right hand rotates in a clockwise direction which makes the installation of screws ergonomically suited for right-hand usage.

The left-handed ones are rare, with a lower prevalence, although once they’re used, it’s for a specific reason, like in the case with lawnmower blades.

There are cases when reverse-threaded screws are used to achieve the mechanical benefit of reverse threading.

When to Replace Old Lawnmower Blades with New Ones?

A standard blade of push lawnmowers, if it’s been cleaned properly between mowing sessions and sharpened every 20 to 25 hours, usually has a work span of between 100 and 200 hours.

Some blades, made of stronger metals or alloys, may actually have an up to 400-hour working lifespan.

Final Thoughts

Although most bolts are right-handed and they’re rotated in a right, clockwise direction, there are also left-handed ones or bolts that are rotated in a left, anti-clockwise direction when tightened.

The former is commonly used, although there are cases when the left-handed ones are implemented to enable the benefits of reverse threading, like in the case with some lawnmower blades.

If these blades rotate in a clockwise direction, the bolt has to be fastened anti-clockwise to prevent loosening of the blade caused by the torque of the blade and vice versa.

Considering this, when replacing lawnmower blades with new ones or when you have to unscrew them for some other reason, it’s pivotal to check the shaft direction first to know if the bolt is reverse threaded or not and unscrew it properly, preventing any damage and injuries!

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