Commercial Mowers

The blade in a Rotary Push Mower doesn’t need to be sharp as a carving knife or scalpel but serious dents and nicks will result in a less than perfect lawn (to say the least!). In addition, a seriously unbalanced blade can result in excessive vibration and eventually, possible internal damage as well.
First, drain the gas or remove the gas tank. If you will be filing steel, you get sparks. Sparks are not the greatest thing to have around gasoline vapor. Enough said. Disconnect the spark plug wire and tie it safely away from the spark plug or remove the spark plug entirely. Turn the mower on its side, I like to raise the carburetor side to keep oil from filling it up.

CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere. If there are any, you will need to work on the mower just propped up by 45 degrees or so. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out but it will take a little longer). Just don’t forget to refill the crankcase with fresh oil once you have completed work on the blade!

Check that the blade isn’t bent. Locate a reference point on one side and note the height of the blade tip at that location. Rotate the blade 180 degrees and check the height of the opposite blade tip. There should be no significant difference - say no more than 1/8″ or so. If it is greater, the blade is bent or the crankshaft is bent. Either will require further investigation as running the mower under such conditions will probably result in excessive vibration and can be dangerous.

Assuming this is fine, remove the blade:

For just some minor touch up, there is no real need to remove the blade. For major grinding and balancing, removal will be needed. Removal will also be required to inspect for a damaged or sheared blade lock key and to replace the blade or blade related parts if necessary.
In either case: disconnect the spark plug wire and tie it safely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely to prevent accidental starting.

If the nut holding the blade on is very tight, use a block of wood to prevent the blade from turning. Use a good quality socket wrench or box-end wrench of the correct size. An adjustable or even open-end wrench may not be enough and will damage the nut and tend to slip off, possibly quite violently. The nut usually unscrews counter-clockwise. However, check this out first! A careful inspection of the threads on the end of the crankshaft will reveal the direction. Or, determine the direction of rotation which will be designed to tighten, not loosen the blade. Most, if not all, single blade mowers rotate the blades clockwise as viewed from above which will therefore use a normal right-hand thread nut.

CAUTION: Make sure that if the wrench slips, your flesh will not contact the blade or other sharp sheet metal - liberal use of rags or newspapers is a good idea. Arrange your position and the mower so you are *pulling* towards you - this is a more stable controllable arrangement.

Pay close attention to the arrangement of the blade and associated hardware so you can replace everything properly.

Use some penetrating oil (e.g., liquid wrench or WD40) on the nut and threads if there are signs of rust or corrosion. Allow it to soak in for a few minutes before attempting to remove the nut.

A hammer or other more violent approaches should not be needed.

Once the nut is loose, unscrew it the rest of the way by hand and remove any washers or mounting plate and note their exact position and orientation. The blade and adapter should come off easily. Some penetrating oil (e.g., WD40) may help if it is difficult to remove.

If your adapter/blade doesn’t pop off after removing the nut or bolt, it may be mounted using a taper like the flywheel. This is somewhat unusual on a walk-behind lawn mower but might be present on a larger machine like a lawn tractor. A wheel puller is best for dealing with this situation but first see if it isn’t just gummed up or rusted in position - try the WD40.

Inspect the key or locking tab for damage. You may have:

A rectangular blade adapter with a welded-on soft metal ring with a tab protruding into a slot in the crankshaft.

A blade adapter that locks to the crankshaft with a rectangular or D-shaped (Woodruff) soft metal key.
If the adapter’s tab is broken off or the key is sheared or damaged, then replacement of the entire blade adapter or just the key (depending on your mower’s design) will be needed upon reassembly. For now, if you will be sharpening the blade, replace all the hardware in the correct positions (except the blade) and finger tighten the nut so you won’t lose anything.

WARNING: Do not install a hard steel key in place of the recommended blade lock key as you will lose the protection that the soft metal provides and the next incident may be the last… See the section:Why soft metal keys must be used.

Another way to remove a stubborn blade is to take it to your friendly garage or tire changer and have them put an impact wrench on it. Saves knuckles, tempers, etc. Just don’t put the blade back on without a touch of oil or anti-seize compound and of course, don’t use an impact wrench!

Lets Sharpen the blade:

Slight nicks and dents can be cleaned up with a file while the blade is still installed on the mower. Unless you have run into a curb, this is probably all that is needed on an occasional basis. Removing this small amount of metal will also not unbalance the blade enough to worry about.

If the damage is severe, consider replacing the blade entirely - they are not that expensive (usually under $10). Otherwise, you can use a file, a bench grinding wheel, or a grinding wheel mounted in an electric drill (there are special attachments for this specific application).

Since the rotating blade also contributes to the proper air flow, you do not want to upset the shape. Grind in such a way that the original blade angle is preserved. It doesn’t need (or want) to be razor sharp. A 1/64″ edge is fine. Anything finer will quickly be dulled by little bits of stone and dirt in any case. Safety is not the main concern here - if any part of your anatomy contacts the whirling blade, you **will** be in trouble no matter how dull or sharp the blade might be!

Attempt to remove approximately equal amounts of metal from both ends and in roughly similar areas if possible. If there are a few large nicks, it isn’t necessary to remove them completely - your lawn (and neighbors) will never know the difference.

Check the balance by positioning the blade at the center hole location on a pencil or other rod - you don’t need a fancy blade balancer but can use one if you like. If it tips one way or the other, remove more material from the heavy side a little at a time.

Replace the blade along with all its mounting hardware. Make sure you get all parts in the same relationship as they had originally. The blade must have its sharpened edges pointing downward. Don’t forget to install the key if it is separate and DO NOT substitute a hard steel key for the soft metal one that should be used, tighten the nut to the proper torque.

(The blade replacement procedures for riding mowers and lawn tractors generally differs significantly from that of walk-behind mowers. At the very least, it’s probably not possible (or at least easy or desirable) to turn them on their side. Some may require removing the deck or it may be possible to work from underneath (with appropriate precautions and accident insurance). Refer to your owner’s manual.)

Clean-Burning Fuel Reduces Mower Emissions

Beatrice, Neb. (March 30, 2009) – New LPG (Liquified Petroleum Gas) conversion kits make it possible for Exmark® customers to operate three of the company’s lawn mower models with propane, a clean-burning fuel that produces lower emissions than gasoline.

Conversion kits are available for:

Turf Tracer® HP (THP19KAE483 or THP19KAE523)
Turf Tracer® (T23KAE with FMD 604)
Lazer Z® AS (LAS28KA604)

To ensure the safety and high performance of the mowers, Exmark will install all conversion kits at its headquarters in Beatrice, Neb. Customers interested in LPG conversion should visit their local Exmark dealer, who will coordinate shipping and logistics.

Development of the LPG conversion kits is the latest example of Exmark’s commitment to offering environmentally-friendly products. In October 2008, Exmark announced that all 2009 model year diesel-powered Lazer Z XS mid-mount zero-turn riding mowers (serial #790,000 or higher) are approved to use B20 biodiesel fuel.

“More and more, landscape professionals are taking actions to reduce their impact on the environment. At Exmark, we have a similar focus,” said Mark Stinson, president of Exmark. “We feel strongly that it is our responsibility to provide products that are environmentally responsible, yet still offer our customers maximum performance.”

Propane has been approved by the Environmental Protection Agency and the California Air Resources Board as a clean alternative engine fuel. According to the Propane Education and Research Council, propane’s octane rating is 104 (compared to premium gasoline at 91-92), allowing for a higher compression ratio in the engine and greater engine efficiency. This ultimately leads to reductions in certain exhaust emissions, such as carbon monoxide.

In addition, propane is a naturally stable fuel and can be stored for long periods without going stale or varnishing within the carburetor.

Using propane does not negatively impact mower performance. The clean-burning fuel results in fewer carbon deposits, leading to reduced engine maintenance costs and longer engine life. Run time is similar to gas-powered mowers.

Exmark’s LPG fuel system is designed to conform to National Fire Protection Association (NFPA) 58-2008 Liquified Petroleum Gas Code for safe operation.

The following description applies to most small rotary lawn mowers with direct driven blades. The vast majority of these use either Tecumseh (as found a variety of Sears/Craftsman equipment) or Briggs & Stratton engines. However, similar comments apply to others as well including Lawnboy two stroke engines and the more modern Honda and other overhead valve type of engines.
The assumption is that the engine started and ran normally prior to the incident. Now, no matter how many times you yank the starter rope or run the electric starter, it will not start at all, bucks, kicks back, backfires, or fails to develop enough power to keep going on its own.

If the blade struck a solid boulder while the engine was set on ‘high’, more severe damage is possible as even with soft metal keys locking the blade and flywheel to the crankshaft, the inertia of the rotating blade is acting sideways against the crankshaft in addition to suddenly stopping its rotation. This can result in a bent crankshaft. The end of the crankshaft with the blade adapter could be bent without affecting the bearings or internal parts. This would need to be tested for as well. Not that such an occurrence is that much better - the crankshaft would still have to be replaced but at least the bearings in the crankcase will not be damaged.

If the starter will not turn the crankshaft (assuming you remembered in your haste to engage the safety bar) - it is seized or will only rotate part of a revolution before hitting against something solid inside - then you probably have serious internal damage that will require a complete strip down and replacement of some (expensive) parts. If it turns but much more tightly than you recall (assuming you do have the safety bar engaged!) then the crankshaft may be bent - again very expensive. Repair may not be worth it.

However, in most cases, what has happened is that either or both of the blade lock key and/or flywheel key have sheared to protect the crankshaft from serious (and terminal) damage.

If the blade lock key broke, the blade will no longer turn rigidly with the crankshaft and provide the inertia required by many small engines with undersized flywheels. In this case, the engine may try to start but die out with a few “putt-putts” or even kick back on the starter cord. (As a side note, attempting to use a lawn mower engine as a replacement on a piece of equipment that doesn’t have something to substitute for the blade’s inertia may not work for this reason.)

If the flywheel key broke, the ignition timing will likely be totally wrong and the result may be no ignition, backfiring, kickback, or weak or total loss of power.

To diagnose, proceed as follows:

First, pull off the spark plug wire and tie it securely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely so that there is no chance of the engine accidentally starting. Even though it will not start now no matter what you do, the underlying problem could actually be a flooded carburetor or something else which may correct itself while you are working. Never take chances.

Drain the gas or remove the fuel tank. This will prevent gasoline from spilling out the gas cap vent hole or flooding the engine through the carburetor since you will need to tip the mower to get underneath.

Set the mower on its side (carburetor side up).

CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere.

The mower can usually be set on its side for a few minutes without harm but if these occur - you will have to work with it tipped less than 45 degrees or so - propped on wood blocks. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out - it will just take a little longer). Just don’t forget to refill the crankcase with fresh oil once you have completed your work!

Using an old rag and/or proper work gloves, grasp the blade and attempt to rotate the blade and crankshaft.

CAUTION: despite your lack of maintenance, the blade may be sharp!

The blade and crankshaft should rotate together. If there is slippage, the key has broken and will require replacement of just the key or the entire blade adapter plate depending on design. If it appears to be intact, then you can assume the flywheel key has broken. The blade key may be broken as well but it is not likely the reason for your failure to start. You should remove the blade to determine this for sure before restoring the mower to service in any case.

You can possibly avoid removing the flywheel for inspection of the key by unscrewing the sparkplug, rotating the crankshaft so the piston is at TDC, and noting the location of the magnet on the flywheel relative to the magneto coil pole pieces. The magnet should be pretty close to the magneto in that position. If this is not the case or just to be sure, the flywheel will have to come off to inspect and possibly replace the key.

To get at the flywheel key itself, some disassembly is required.

Fly Wheel Clutch Tool

Briggs Fly Wheel Puller

• Try to use a mower with a control that stops the mower from moving forward if the handle is let go.
• Children younger than 16 years should not be allowed to use ride-on mowers. Children younger than 12 years should not use walk-behind mowers.
• Make sure that sturdy shoes (not sandals or sneakers) are worn while mowing.
• Prevent injuries from flying objects, such as stones or toys, by picking up objects from the lawn before mowing begins. Use a collection bag for grass clippings or a plate that covers the opening where cut grass is released. Have anyone who uses a mower wear hearing and eye protection.
• Make sure that children are indoors or at a safe distance well away from the area that you plan to mow.
• <>Start and refuel mowers outdoors, not in a garage or shed. Mowers should be refueled with the motor turned off and cool.
• Make sure that blade settings (to set the wheel height or dislodge debris) are done by an adult, with the mower off and the spark plug removed or disconnected.
• Do not pull the mower backward or mow in reverse unless absolutely necessary, and carefully look for children behind you when you mow in reverse.
• Always turn off the mower and wait for the blades to stop completely before removing the grass catcher, unclogging the discharge chute, or crossing gravel paths, roads, or other areas.
• Do not allow children to ride as passengers on ride-on mowers.