The final drive translates the rotational motion of the driveshaft, which rotates along the longitudinal axis of the motorcycle, to the rotational motion of the rear wheel - essentially, it turns the rotation 90 degrees. It does this using a pinion gear, much like the pinion gear used in the differential of a car:
The gear oil in your rear drive is essential to the smooth operation and long life of your rear drive unit. Without it, the pinion gear would wear out quickly. Because the oil is subjected to extreme shearing forces, special hypoid gear oil must be used:
75W-90 hypoid gear oil is the correct gear oil to use in the final drive. The shearing forces of the pinion gears break down the gear oil, so it should be changed regularly, and at least once a year.
1. Ride the motorcycle several miles to warm the rear drive, which thins the gear oil and makes it easier to drain. Too cold out to ride? In my case, when I did this (during winter maintenance), there was snow on the ground, so I instead positioned a hair dryer on the final drive, and let it run for an hour, occasionally rotating the wheel to distribute the heat evenly.
2. The final drive has two plugs - a fill plug (upper in the picture) and a drain plug (lower in the picture).
3. You will need a suitable drain pain in order to catch the old, draining gear oil.
4. You will also need a 17mm wrench in order to remove and replace the fill and drain plugs.
5. The fill plug should be removed FIRST, before the drain plug! Why? Because if you remove the drain plug first, draining all the gear oil, and then discover that for one reason or another you are unable to remove the fill plug, your motorcycle is now out of commission!
6. It may be easier to access the drain plug with the right muffler out of the way - to do this, remove the bolt holding the muffler to the bracket next to the final drive, and gently pull the muffler out of the way to gain access to the drain plug.
7. Once the fill plug is out, remove the drain plug. Make sure your drain pan is underneath, as the oil will drain out quickly.
8. Allow the gear oil to completely drain from the final drive.
9. While waiting for the gear oil to drain, clean any oil and dirt from the fill plug, including around the rubber seal.
10. I like to use brake cleaner as a solvent to ensure all of the old gear oil has been removed. This is an optional step.
11. Spray the brake cleaner into the fill port while rotating the rear wheel. It will quickly clean the remaining gear oil from the final drive. When the gear oil draining from the drain port becomes extremely thin, you know you have got most of it out. Once this occurs, I let the final drive sit for 15-20 minutes to ensure the volatile brake cleaner has completely evaporated.
12. My preference for gear oil is full-synthetic Valvoline as shown. 75W-90 is correct for use in the GL1500 final drive.
13. Clip off the end of the gear oil bottle and attach a small hose.
14. Insert the hose into the final drive fill port and gently squeeze some gear oil over the gears while rotating the final drive. This fresh gear oil will drain from the drain port, which should still be open. This flushes any remnants of dirt, brake fluid or old gear oil from the final drive.
15. Install the drain plug by hand (to avoid cross-threading) and tighten with a wrench. It does need to be quite snug, but not extremely tight - remember, you are threading into the relatively soft aluminum final drive housing.
16. Squeeze gear oil into the fill port until it starts to spill out the fill port as shown. Rotate the wheel slowly, then squeeze more gear oil into the fill port until it again starts to overflow out the fill port.
17. Replace the fill plug by hand, and tighten with a wrench, as was done with the drain port.
18. Position the muffler into its bracket and replace the mounting bolt. Clean any spilled oil from the exterior surface of the final drive (brake cleaner works great for this!). Ensure no gear oil has gotten on the tire - if it has, clean it off completely.