This is the next installment in a series of How-Tos covering some basic routine service procedures on an old motorcycle. As before, the guinea pig will be an ’83 Honda GL650.
The shaft drive system used on this bike is similar to what you’ll find on other shaft-drive bikes. Like a car, there’s a differential filled with oil that needs to be periodically drained and replaced. This bike also has a grease fitting on the shaft housing, again very much like what you’d find underneath a car.
The procedure is simple and you should be able to complete the whole job in about 10 minutes or even less.
Tools needed: Basic metric socket set and ratchet driver; catch pan; rags/paper towels.
Step 1:
Get enough fresh gear lube to refill the differential. A quart is typically more than enough to do the job. I like to use Silkolene (80w-90) 
because it’s a high-quality lube and also because it comes in a handy container with its own built-in fill-tube. One of the few sometimes challenging aspects of changing differential lube – in a bike or a car – is getting the fresh lube into the fill hole, which is typically hard or awkward to access. In the past, I’ve jerry-rigged my own solution using a length of fuel hose that I press on on to the nib of the lube container’s fill top. This let me squeeze the fresh lube into the differential. The Silkolene people provide a ready-to-go solution that works the same way. There’s a fill tube built into the cap; just pull it out to extend, and you’re ready to go.
Footnote: While you can use car-type differential lube in a shaft-drive motorcycle differential, it’s best to use a motorcycle-specific product or at least one that meets the recommended specifications you’ll find in the bike’s owner’s manual or stamped into the bike’s differential housing, near the fill hole.
Step 2:
Ride the bike for about 15 minutes to warm everything up; this will make it easier/faster to drain the old lube and should help get more gunk out of there.
Step 3:
Park the bike and (if it has one) raise it on its center stand. Locate the drain and fill holes/bolts on the rear differential housing. The fill hole/bolt 
will typically be larger and mounted toward the top of the differential while the drain hole/bolt 
will usually be found near the bottom of the housing.
Step 4:
Place a catch pan underneath the drain plug. Then loosen and remove the larger fill plug first. You do this for the same reason you remove the oil fill cap on the top of a car engine before you drain the oil. It will make the draining process go faster. Now loosen and remove the smaller drain plug and allow the old lube to drain out. 
Step 5:
When the old lube has fully drained, reinstall the drain bolt; be very careful not to overtighten. Like many parts on a motorcycle, the differential housing and bolt threads are made of aluminum. Don’t go crazy. Snug – just a bit more than hand-tight – is plenty.
Step 6:
Fill the differential with fresh lube. Usually, “full” means lube is just barely trickling out of the fill hole, or about even with the fill hole. Be careful not to over (or under) fill. If you can’t see inside the housing, use your finger to check the level. Reinstall the fill plug, again being very careful not to overtighten it.
Step 7:
If the bike has a grease fitting on the shaft housing, use a grease gun to pump some grease into there. Don’t go crazy; two or three full pumps is plenty.
That’s it, you’re done!
Total cost: About $10.
