Spur gears are the stereotypical type of gears that are round with teeth running around the outside. These gears were used in some older cars with manual transmissions because, although they are louder, they are very easy to slip in and out of mesh with other gears. Generally this type of gear would be used while the car was idling or running in reverse. More often these days, spur gears are used in the engines of small vehicles such as motorcycles and ATV’s. Replacing a spur gear is an easy process but it does require a few unusual tools such as a clutch-hub holder and a flywheel puller that can be found at any automotive store. Since they are rarely used in automobiles, this article will explain how to replace an idler gear (spur gear) in a motorcycle.
Step 1 – Remove the Flywheel
Using a socket wrench remove the engine cover. Disconnect the clutch cable by hand and then fit the prongs on the end of a clutch hub holder into the open holes on the flywheel. This will keep the wheel from spinning while the socket wrench is used again to remove the bolt in the flywheel’s center. Pull the flywheel out of the machine with a flywheel puller.
Step 2 – Remove the Engine from the Frame
With the flywheel disconnected, a screwdriver will be needed to remove the screws holding the motor to the frame. Also, an Allen wrench will be needed to remove the clutch actuator. Once the engine is loose, carefully lift it out of the frame and move it to a flat area so that it is easier to work on.
Step 3 – Remove the Clutch
With a socket wrench, remove the clutch cover on the opposite side of the motor that was originally being worked on and also remove the bolts on the pressure plate found below. When removing the pressure plate bolts do so in a star shaped pattern. This means if the bottom right bolt is removed first, then the upper left side bolt should be removed next, followed by the lower left hand bolt and the upper right hand bolt and so on. The bolts must be removed in this manner because the springs that surround them could bend or warp the pressure plate, if all the springs on one side were removed while the other side remained secure. Once the pressure plate has been removed, the clutch plates and lifter will come out by hand, and a screwdriver will be needed to bend the lock washer off the center of the of the clutch hub. Finally use the clutch hub holder again in conjunction with a socket wrench to remove the clutch altogether.
Step 4 – Remove the Idler Gear (Spur Gear)
Once the clutch is out, a socket wrench can be used to remove the inner engine cover. This will reveal the idler gear on the upper left side of the engine. A snap ring pliers can be used to gently remove the snap ring in front of the gear. The gear and its washer will now simply slide off.
Step 5 – Replace the Gear
Check to make sure the pin that the gear slides into is straight and has not been bent during removal. Then, slide the new gear into place meshing it with the starter gear next to it. Confirm that the gears can rotate together and that the alignment is correct. Put the washer back on top of the gear and use the snap ring pliers again to replace the snap ring that was removed.
Step 6 – Reassemble the Motor
Reassemble the rest of the motor by simply reversing the above steps. Loctite may be used on the bolts to guarantee the tightest possible bond.