Oil Change in Final Drive

Even though very simple to do, here is a post on the proper procedure:

Draining and changing the final drive oil.

 by Blindstitch2002 » Thu Jun 10, 2010 6:38 am

Draining and changing the final drive oil. 1. Breather Hole 2. Fill hole and maximum fluid height 3. Drain hole

Pretty simple. Drain the fluid through bolt 3 and replace bolt when draining is complete. Fill the final drive through hole 2 up to the edge with hypoid gear oil. That's the maximum amount of oil it needs.

Regarding the breather's purpose: Without the breather, the final drive case would be a sealed enclosure. The gear oil and especially the air inside it expand as the drive heats during use and the internal pressure would rise significantly. Likewise it shrinks and the internal pressure drops when it cools. The vent allows the pressure to remain constant in either situation. Without it, seals designed to hold oil with no pressure differential would fail prematurely. I think synthetics have a lower Thermal Coefficient of Expansion, however the air in the case would still expand and pressurize when hot.

As for viscosity and synthetics: It's mostly about temperature also. The manual says to use a thicker oil for higher ambient temperatures. This is likely because oils based on natural petroleum change viscosity in inverse proportion with temperature. The gear lash is set for the thickness the oil will film at when at operating temperature. Synthetics are more stable across their temperature range - they don't get as thick when cold, but they also don't get as thin when hot. Correct me if I'm wrong, but I think the manual says that straight 90 weight is called for in 70F & up temps. I can't even find a non-synth straight 90W, and I believe a 90W synth oil will be even harder to find. While synthetics outperform old-school oils, I'd be sure the oil I would use films at an appropriate thickness for my gear lash and riding conditions - whether synthetic or not.

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Here is another description of how to do this with some recommendations.

• • Shaft Drive Gear Maintenance

There are two types of maintenance you must do on a shaft drive bike. At your rear wheel there are pinion gears where the spinning drive shaft motion is turned 90 degrees into the rear wheel motion. These gears are bathed in oil which much be changed after break in, and about every 10,000 miles thereafter. Honda says this gets done the first time at 12,000 miles. Don't believe it. Do it at about 1,000, and when you see what comes out you'll decide to do it again in about 250 miles to flush the rest of the garbage out. Also, a shaft drive bike will have splines on each end of the drive shaft, and on a large ring which transmits the power to the rear wheel. These splines must be lubricated. This is a rather tricky subject, as the lubrication requirements of splines are very different from gears or bearings. Rear Wheel Spline Service

• • Gear Oil
  • The Recommended Synthetic Gear Oils||

AMSOil AGL 75w 90	Mobil 1 75w 90	Valvoline 75w 90

• • AMSOil AGL sae 75w-90 synthetic gear oil, about $8 / quart.
  • Mobil-1 75w-90 synthetic gear oil, about $8 / quart at Autozone.
  • Valvoline 75w-90 synthetic gear oil, about $8 / quart at Autozone.

How to Change Your Drive Gear Oil

Put the bike on the center stand. Place a large flat metal pan under the real wheel, like a cookie sheet. This process is a bit messy. Remove the 10mm bolt on the bottom of the rear wheel hub. Next, remove the inspection plug, 17mm large bolt at the rear of the hub. If your drain plug is magnetic, make sure to clean it completely. Replace the drain plug. To fill, get 70 or 80 weight gear oil, and put it into the inspection hole until it comes back out. About 160cc, about 1/6 quart. You may as well buy good synthetic oil, a quart is good for 5 changes.