A sheared flywheel key is a fairly frequent problem for small engines, particularly - but not limited to - those used on lawnmowers. Symptoms range widely from a little mis-fire to a no-start condition.
The flywheel key is a small metal part (see Photo 1) that locates the flywheel in the correct position relative to the crankshaft, and thereby to the piston. A magnet mounted on the rotating flywheel moves past the ignition module (see Photo 7), induces a magnetic field in the module, which in turn delivers a spark to the spark plug. The spark must be correctly timed to occur when the piston is near top dead center. The flywheel key determines the timing between the spark and the piston.
The end of the crankshaft to which the flywheel is mounted is a precision machined taper. In the taper there is a groove, called a keyseat, cut lengthwise into the shaft (see Photo 2).
The flywheel has a matching tapered hole at its center, in which is cut a matching groove, called a keyway (see Photo 3). During assembly, the flywheel key fits into the keyway and keyseat, thereby locating the flywheel in the correct rotational position relative to the crankshaft.
The flywheel is then tightened down onto the crankshaft with a nut or bolt (see Photo 4). The tapered fit between the flywheel and crankshaft wedges the two parts together, providing the mechanical hold. The key has little to do with keeping the parts in position after assembly. That is the job of the tapered fit.
The amount of torque to which the flywheel bolt or nut is tightened determines how tightly the flywheel and crankshaft are held together. The torque must be sufficient to hold the two parts in the correct position relative to each other during normal operation of the engine. However, it is desirable that the hold should slip in the event that the crankshaft is forced to suddenly accelerate or decelerate relative to the flywheel. In this situation the flywheel may slip on the crankshaft, shearing the flywheel key in the process. The ignition timing will now be out of synch with the piston and the engine will misfire or not run at all. This slippage may, hopefully, prevent damage to the crankshaft and other internal engine components. The flywheel nut or bolt tightening torque is chosen to give sufficient holding power, yet enable the flywheel to slip in extreme loads.
A sheared flywheel key is a frequent cause of a lawn mower engine no-start condition or poor running, often the result of hitting a rock, stump or other non-mowable ( non-movable? ) object. Flywheel keys are more likely to shear on lawnmowers because the crankshaft is solidly connected to the cutting blade which may strike an obstacle such as a rock or a stump, compared to other situations where the engine crankshaft is connected to the load ( e.g. pump, mower deck, tiller tines, drive wheels, etc. ) via a v-belt or coupling. The blade, crankshaft, and flywheel are spinning at about 3000 rpm, so when the blade hits an immovable object, the crankshaft decelerates suddenly but the flywheel doesn't want to stop so fast. Thus it shears the flywheel key. Since the flywheel continued to rotate relative to the crankshaft, the ignition timing is advanced. When pulling the starter rope the spark will fire during the compression stroke causing the piston and crankshaft to spin backwards. When this happens the rope handle may yank violently out of your hand and the recoil starter may possibly be damaged. This is a sure sign of a sheared flywheel key.
If you suspect that an obstacle was hit, check to see if the lower end of the crankshaft, where the blade mounts, has been bent. If so, the mower might not be worth fixing. Also, check if the blade has been bent or the blade holder is damaged.
Another cause of a sheared key on a lawnmower is a loose blade. The blade may hammer back and forth and eventually cause the key to shear. The same effect can occur on any engine when the load, or the drive train between the load and the crankshaft, bucks, skips, or otherwise varies violently. Similarly, starting or stopping an engine while under load may shear a flywheel key, especially if done repeatedly. An intermittent spark due to a faulty ignition module, or an electrical problem in the safety interlock system might cause such an effect.
A flywheel that was not tightened sufficiently, or one that has become loose over time can shear the key. A flywheel that is not properly seated on the crankshaft taper will not hold tight. The crankshaft and the flywheel must be free of dirt and grease. Before reinstalling a flywheel, use a file or emery cloth to remove any burr that was raised by the shearing of the key. Lightly clean any rust or corrosion. Check to see that the flywheel does not rock or wobble on the crankshaft, and that it rotates true, without runout.
Occasionally, flywheel keys shear on engines used on lawn tractors or an application other than a lawnmower. Typically this is caused by an intermittent spark, such as flipping the ignition keyswitch on and off, or bouncing on the seat (think seat interlock switch). The ignition timing will probably be retarded as the crankshaft is propelled forward when the spark resumes and the flywheel fails to keep up with the crankshaft.
The symptoms of a sheared flywheel key range widely from a barely noticeable misfire to a no-start condition. In between these extremes the engine may misfire noticeably, run rough, backfire, be difficult to restart hot, or lack power. Advanced timing can cause engine overheating. A muffler glowing cherry red probably indicates retarded timing. These variations depend on just how far the flywheel has spun out of time and in which direction. The key may have only partially sheared, or the flywheel may have advanced a half a revolution. If the sparks occurs during the compression stroke, the engine will backfire, and the crankshaft will reverse direction violently. If the operator was pulling the recoil starter, the starter rope would be pulled back sharply, along with the operator's arm. This is a common and painful occurrence, and a sure sign of a sheared flywheel key.
On a lawnmower, if you suspect a sheared flywheel key, check to see if the blade or cranksahft is bent. Either condition would indicate an increased probability of a sheared key. To see if the blade is bent, remove the spark plug and ground the spark plug wire. Observe the position of the tip of the blade relative to the mower housing (see Photo 5). Rotate the blade a half turn and compare the position of the other blade tip (see Photo 6). Both ends should line up the same. To check the crankshaft, again remove the spark plug and ground the wire. Tip the mower on its side, carburetor side up. Close the engine control handle and pull the starter rope (you might need a helper to pull the rope) while focusing your eye on the end of the crankshaft. Any wobble means the shaft is bent. If the shaft is bent the mower might not be worth fixing.
The only way to verify whether or not the flywheel is at fault is by inspection. To do so, the blower housing, recoil starter, and any decorative covers and trim pieces must be removed. Commonly, the fuel tank and oil fill tube must also be removed or moved aside. On many engines the keyways can be seen after removing the flywheel nut or bolt. If the keyway groove on both the flywheel and the crankshaft are visible and aligned exactly, then the key is good.
#7 shows the position of the flywheel magnet relative to the ignition module on a Briggs vertical crankshaft engine, while the piston is at top dead center. Note that the magnet is just about to exit the area of the ignition module.
In #8 the key is partly sheared and the timing is slightly retarded. The effect on the running of the engine was barely noticeable.
#9 shows the position of the magnet and #10 shows the sheared key in the flywheel with the timing retarded by about forty-five degrees. The keyseat can be seen near the twelve o'clock position and the keyway near the ten-thirty position . The engine still started right up, but ran rough and the muffler got cherry red in about two minutes.
Often it is not possible to inspect the flywheel key without removing the flywheel from the crankshaft and sometimes it is very difficult to remove the flywheel. To get a good indication of the condition of the flywheel key, position the piston at top dead center and note the position of the flywheel magnet in relation to the ignition module. Top dead center can be found by removing the spark plug, touching a long screwdriver to the face of the piston and rotating the flywheel slowly by hand to the position where the screwdriver is extended out the spark plug hole by the maximum amount. A page showing the position of a few various flywheels at top dead center is included as a reference.