I know the feeling, FRUSTRATION!!! but we’ll get it figured out. The blade bolt can be stuck for a few different reasons. Usually, it’s a combination of rust and over-tightening.
The easiest way to remove a stuck blade bolt is with an impact tool; they make the whole job look easy. Other options include:
- Use Tool Leverage
- Hammer & Chisel
You may not have an impact, so I’ll show you a few different options. Some of these options may not suit you; it’ll depend on what tools you have available. Best to don a pair of work gloves. Stuck bolts usually mean slipping tools.
Removing A Rounded Bolt
Over-tightening is common. Mower blades are designed to be tightened to a specific torque, which isn’t as tight as you might expect. That’s because they’re designed to slip if they hit a solid object. The slipping protects the engine from serious damage associated with a curbstone strike.
Also common is turning the bolt the wrong way; hey, it could happen to a bishop. All single-blade walk-behind mowers will have what’s known as a right-hand thread. That means, to loosen the bolt, you turn it to the left. (counter-clockwise)
I cover all you need to know pretty well in this post, but if you need more help, check out the following videos:
“How to use a torque wrench video”
“Sharpening a mower blade video”
Blade Bolt Torque
Mower blade bolts should be torqued to spec. These bolts are usually over-tightened, and when you add corrosion, removing them can be a headache.
Only some large twin blades walk-behind mowers and some lawn tractor mowers are likely to have one only left-hand threaded blade bolt; the other bolt will be a regular right-hand thread.
How do you know which is which?
Simple, if the blade is designed to cut turning right (viewed from above), then it will be a right-hand thread; this is the most common type. To loosen a right-hand thread, you turn it to the left.
The same idea applies to twin-blade tractor mowers. However, a left-hand thread is common on some lawn tractor mowers.
So, if the blade cuts grass turning to the right, as before, it’s likely a regular right-hand thread (left to loosen). But it’s not uncommon for a tractor mower to have one of the blades turn to the left when cutting, and that usually means it’s a left-hand thread (check your owner manual) to loosen a left-hand thread, turn it to the right.
L/H – R/H Thread
A r/h thread loosens to the left. This is the most common type of thread. (counterclockwise)
A l/h threaded bolt loosens to the right. (clockwise)
Typical torque specs for blade bolts are anywhere from 35 ft. lbs. to 90 ft. lbs., you’ll need to check the spec of your mower, it’s important to get it right.
Most of the time blade bolts just get buttoned uptight and aren’t torqued to spec, and that’s OK, but you run the risk of bending the crankshaft if you hit a solid object. I advise using a torque wrench, it’s a lot cheaper than a new mower engine.
Torque wrenches are easy to use, they come in inch-pounds for smaller torque specs, but for mowers, you’ll need foot-pounds. A torque wrench from 30 to 100 foot-pounds is about right.
If you don’t have or can’t borrow one, check out this post on my 1/2 drive Teng Torque, it won’t break the bank, it covers 30 to 150 ft. lbs., it’s simple to use, calibrated from the factory, and has a flexible working range.
I get my torque wrenches calibrated every year but it gets a lot of use. If you set your torque wrench to zero after you use it and don’t throw it around, it should stay calibrated for years.
Damage – The bolt on the right has a rounded head, this kind of damage happens when a tool slips on a bolt head, or corrosion deforms it. Getting the bolt out presents a challenge.
A rounded bolt head is a real pain in jacksie. It usually happens when the bolt is old and corrosion has deformed it. Worn or damaged tools will give you the same result.
It can also happen if the wrong size tool is used. An American mower may use imperial size nuts and bolts, I know the more modern kit is metric and some mowers are a mix of both. If your mower is European or Asian it will be metric sizes.
The trouble is you can get an imperial wrench to almost fit a metric bolt, but it’s loose and will slip, which rounds the bolt head. Typical bolt sizes for mower blade bolts are Imperial 1/2″, 5/8″, 3/4″ and Metric sizes 13mm, 14mm, 15mm, 16mm, and 17mm.
Imperial or Metric, be sure your tools are a good fit.
Tools You’ll Need
Impact power tools are designed for this exact job. They cause a hammering action which helps reduce the bolt thread friction and breaks any corrosion loose. So if you have an air or battery impact tool, you going to feel like a superhero when that bolt just walks out.
Basic tools needed assuming you don’t have an impact tool: wire brush, wd40, ratchet & sockets, selection of wrenches.
Other tools you’ll need if things don’t go exactly to plan: breaker bar, hammer & chisel, butane torch, and if everything goes to crap, a Mig welder. In my workshop, I use an air impact tool, if you haven’t got one or can’t borrow I have other solutions for you.
But the tool I am least likely to be without is an impact tool, it just makes life really easy and saves so much time. The coolest thing about the latest generation impact tools is their mobility, cordless now packs the power of an air tool. Great for around the home and for flat wheel emergency, use it to run the jack-up and take the nuts off.
Although I still use air in the workshop, I bought a 20v Ingersoll Rand cordless for mobile repairs, I know they ain’t cheap but you won’t ever need to buy another.
If you do buy an impact tool, you’ll need to buy impact sockets too. Sure you can use regular sockets, but you run the risk of them shattering. Anyway, you’ll find all these tools on the “Small engine repair tools page”.
Tool Up – Most stuck bolts won’t need all these tools, but some do.
Removing The Bolt
Removing a stuck bolt involves trying different solutions until you ring the bell. In the first attempts, we’ll try the simple stuff and if that doesn’t move it, I have lots more ideas.
Before we start any work on our mower we need to make it safe. Pull the plug wire off and set it away from the plug. Turn your gas off if you have a gas tap, if you don’t know where your gas tap is check out “Gas tap location”.
WD40 is my favorite tool, it solves lots of problems, I also like a product called nut buster, it’s formulated for dissolving rust. Try spraying the bolt liberally above and below the blade, and allow it time to work into the threads.
Disable Mower – For safety, let’s remove the plug wire and turn off the gas.
Turn the mower over with the carburetor side facing up, stops gas leaking on the floor. (see tilting mower over)
Wire Brush to remove any rust. Wd40 Spray front and rear of the bolt and give it some time to soak in.
Impact Tool – By far the preferred way to remove a bolt. An Impact gun hammers the bolt as well as twists it, this loosens the corrosion between the threads.
An impact tool will remove the bolt in seconds and you won’t need to lock the blade. But if the bolt head is rounded, the impact tool is of no use. You’ll need a different solution.
Check out the Amazon link, some of these impact wrench prices surprised me.Amazon Impact Wrench
Lock Blade – If you are not using an impact tool we’ll need to use a piece of timber to lock the blade against the body. Longer timber is better than shorter. Cut a length to suit.
Good Fit – Select a socket (6 points preferably) and check the fit. Turn the ratchet left to loosen. Using a breaker bar, or if you don’t have to improvise with your ratchet and some pipe.
Pushing down on the pipe will give you the extra power you need to break it loose. Just be sure the socket is a good fit, and it stays on the bolt head when you’re applying force.
Wrench Leverage – Turn the Wrench left to loosen. If you don’t have a ratchet and breaker bar, try 2 interlocked wrench’s for extra leverage, or use a hammer to shock the bolt.
If it still won’t budge, try tightening it slightly, this often helps, odd I know!
Striking – Try striking two hammers sharply (wear eye protection) while one is placed against the bolt head, this can help break loose any corrosion on the threads. If the head of the bolt is rounded, move on to the next solution.
Rounded Bolt – If your bolt head is rounded, try a vice grip. Get it as tight as you can, and try hitting it to the left sharply with a hammer.
Not all vice grips are the same, for this application you’ll need a flat jawed set. Check out this post on Vice-grips tools.
Chisel – This method is pretty effective, but you’ll need a new bolt, sharp metal working chisel, and a heavy hammer. With the chisel and hammer, take a sideways and downward aim at the bolt, we’re attempting to loosen it by turning it left. This will require good aim, so now’s a good time for those gloves.
Heat – Ordinarily I’ll tell you to get some heat on the bolt, the reason I haven’t introduced it earlier is that it comes with the risk of damaging the crankshaft nylon seal, which would cause the engine to leak oil.
The risk of this is fairly small, once you direct the flame and only use a small amount. We’re not going to redden the bolt, just going to heat it up.
Maybe 2 minutes with a butane torch directed at the bolt. You can now try heat with any combination of the above methods. Heat is very successful at helping move stuck bolts.
Welding – This method will obviously require a welder, when I get a really stubborn bolt with a rounded head, I take a new bolt and weld it to it. This gives me a not-so-pretty but clean bolt head to work with.
This solution has never failed me yet. You’ll need to replace the bolt. Blade bolts have a fine thread, they are a specialized bolt, getting one in the hardware store isn’t advisable.
Torque – Finally, you’ll have to move your timber to lock the blade in the other direction and torque your new bolt to spec.
Check out this post to see why it’s important to torque your blade bolt.
Check out “How to use torque wrench video”.
The spindle turns when removing the blades? The easiest way to prevent the blade from turning while loosening the blade bolt is to use a large block of wood to lock the blade against the mowing deck.
Lawnmower blade bolt direction? Turn the mower on its side, carburetor side up, turn the blade bolt to the left (anticlockwise) to loosen.
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.