Outdoor Power Equipment, Small Engine, Lawn Mower Repairs, Troubleshooting and Safety

Flywheel removal is necessary in order to replace a sheared flywheel key, to service ignition points and condenser on older engines, to replace a stator, or to remove the crankshaft. There are several techniques that may be employed to accomplish removal. The best technique requires a special tool - a flywheel puller; the least preferred method requires nothing more than a hammer and a screwdriver, but can easily result in serious damage to the flywheel and / or crankshaft.

Regardless of the method you are going to use, first access must be gained to the flywheel. This requires removing various shrouds or covers or trim, and sometimes the fuel tank, recoil starter, oil fill tube. If the flywheel fan is plastic it may or may not be easily separable from the flywheel. If possible, it should be removed. The fan's mounting screws may use the same holes as those intended for the flywheel puller. Remove the flywheel nut or bolt and washer, and possibly the starter cup, as applicable. If there is a flywheel brake, as usually found on homeowner's lawn mowers, it must be released from the braking position. This can be accomplished by tying off the engine operator presence bail. Be sure to remove the spark plug wire from the spark plug and secure it to an engine ground. On some models it may be necessary to remove or loosen ignition module in order to gain clearance to remove the flywheel.

The flywheel is mounted on one end of the crankshaft (see Photo 1). This end of the crankshaft is tapered (see Photo 2) and the flywheel has a matching taper in its center hole (see Photo 3). The tapered fit is held tight with a nut or bolt. The tapers of the flywheel and crankshaft wedge together and provide a strong hold which can be very difficult to separate.

Striking a Flywheel or Crankshaft Can Be Dangerous

Some of the methods described below mention striking the flywheel or crankshaft. Please, be advised that striking a flywheel or crankshaft can cause cracks or other damage which might not even be visible. A cracked flywheel can fly apart violently when running at operating speed. You or others may be hurt. Even a tiny crack can grow and lead to failure. Damaged threads might not hold tight. A warped or unbalanced flywheel can shake itself to destruction. The methods described below are what is often done. It doesn't mean they are good.

Wheel Puller

Most flywheels have holes intended for a puller. #4 shows a Tecumseh flywheel with three blind holes ( cored holes ) cast into the flywheel. These holes were not threaded at the factory, so they were tapped to 1 / 4 - 20.

#5 shows a Briggs & Stratton flywheel with two 5 / 16 - 18 tapped holes and a wheel puller in use. This type of puller is a 'Harmonic Balancer / Steering Wheel Puller' and will fit most flywheels used on small engines. It will accept either two or three pulling screws as needed. It costs about twenty dollars at any auto parts or hardware store. This one is a Craftsman from Sears (stock no. 47626). To use this type of puller insert the correct thread screws through the puller, screw them into the flywheel equally and tighten down the large center 'forcing' screw. In most cases the flywheel will pull free without a struggle. If it won't come free easily, a couple of sharp blows with a hammer to the end of the forcing screw will often do the trick. For engines with horizontal crankshafts it is good idea to leave the flywheel bolt / nut in place, but loosened a few turns, to prevent the flywheel from falling.

Special Puller

#6 shows a Briggs and Stratton flywheel puller. Other engine manufacturers also make available pullers made specially for their engines. Often these special pullers are preferable because they are small and can be used in confined areas.

Some Briggs engines are equipped with an extended crankshaft and a ratchet recoil starter clutch, which can make the use of any other puller difficult (see #7). To use this puller, thread the flywheel nut so it is flush to the top of the crankshaft threads, then place the body of the puller over the crankshaft. Thread the two puller screws into the holes on the crankshaft and lock them in place by tightening the two lower nuts down to the flywheel. Next, tighten the two upper nuts down to the puller body, alternately and equally. Continue tightening until the flywheel pulls free.

Gear Puller (Jaw-type puller)

A second type of puller is shown in #8 . This is a two- or three-jaw type puller. Engine manufacturers specifically recommend against using this type of puller. However, in many situations, it works nicely. It's your choice. Rather than pulling on screws threaded into the flywheel near its center, this puller catches the outside diameter of the flywheel. Use a jaw type puller only on flywheels that are constructed solidly enough to handle the stress at the circumference, and only with great care. Apply only a modest amount of force to the forcing screw. Usually this is enough. A few taps with a hammer on the end of the forcing screw often helps.

Flywheels with ring gears ( for electric start ), as in #9 , are not good candidates for a jaw type puller because of the risk of damaging the ring gear.

Strike Socket and pry up

For engines that use a flywheel bolt rather than a nut, you can place a socket, or other object, just smaller than the flywheel hole on top of the crankshaft end and strike it with a hammer while prying upward on the flywheel (see #9).

Tecumseh flywheels used on homeowners' lawn mowers, such as shown in #10 , are also not good candidates because of their light construction. The force applied to the outside of the flywheel will easily bend, crack or otherwise damage the flywheel.

Knock-off Tool

#11 shows a Tecumseh flywheel knock-off tool, available in four sizes: 7/16" x 20 (part # 670103), 1/2" x 20 (part # 670169 ), 5/8" x 20 (part # 670314), or part 3/4" x 16 (part # 670329). The tool is simply a piece of hex stock with a blind hole drilled and tapped to the correct thread. It protects the end of the crankshaft and distributes the striking force over the full length of the crankshaft threads. After removing the flywheel nut, washer, and starter cup, thread the appropriate flywheel knock-off tool on the crankshaft until it bottoms out, then back-off one complete turn. Using a large screwdriver, lift upward under the flywheel with sufficient force to take up the crankshaft endplay. Using a 1-1/2 lb. to 3 lb. ball peen hammer strike the knock-off sharply and squarely to break the flywheel loose. If necessary, rotate the flywheel a half turn and repeat until it loosens.

This method of flywheel removal has some drawbacks. It is quite easy to damage the crankshaft with an errant blow, bending the shaft or crushing the threads. The flywheel may be damaged by prying on it too hard. Be certain that your prying tool is clear of any charging stator under the flywheel. Avoid the flywheel ring gear.

Some Tecumseh engines (notably some two-cycle engines) have a ball bearing at the flywheel end of the crankshaft which may be driven out of position when striking the knock-off tool. To reposition the bearing, strike the opposite end of the crankshaft with a rawhide mallet or with a hammer and a block of wood. You should be able to feel the crankshaft endplay.

Strike Flywheel Nut

Similar to using a knock-off tool, you can thread the flywheel nut to be flush with the top of the crankshaft and while prying upward on the flywheel, strike the top of the crankshaft to pop the flywheel free. One might even omit the nut. This is method very prone to damaging the crankshaft or nut and should be avoided.

Strike From Rear

On horizontal shaft engines, especially snowthrower engines, it is often possible to strike the flywheel from the rear. Choose a solid portion of the flywheel, away from the magnet. Avoid the ring gear, if there is one. Keep flywheel nut in place but loosened, to prevent the flywheel from falling. Use a block of wood or other object against the flywheel to cushion the hammer blows.

Helpful Tips

Sometimes the flywheel just doesn't want to come loose. If you are using a puller, you might try putting tension on the puller and leaving it for several hours or overnight. With time, the tapered fit can creep. When you come back, you just might find that the flywheel has crept free. Applying heat to the flywheel can cause it to expand enough to loosen it. A hand-held hair drier can do the job, with a little patience. A butane or propane torch will heat the flywheel faster and hotter, but getting it too hot may distort it or loosen the charging magnets, if so equipped. Also, be careful not to cook the charging stator, ignition coil or any nearby wires. Penetrating oil like Rust Buster, Blaster, WD-40 and the like may help if there is any rust or corrosion.