Replacing the primary and transmission fluid should be done with every motor oil change.
While the other members of the Harley-Davidson motorcycle line utilize separate fluids, the Sportster’s transmission and primary drive systems share a common lubricating fluid. The primary, or gear, oil used by a 1992 Sportster reduces harmful friction and heat within the primary and transmission unit, preventing premature wear as it coats the unit’s internal moving parts. Replacing the fluid regularly is a simple task that should be done whenever you change the motor oil, extending the job by only a few minutes.
1. Start the motorcycle and let it idle for five minutes, allowing the transmission fluid to warm up and expand. Stop the motor after the primary fluid has warmed and place the motorcycle on a stand to support it in an upright position.
2. Locate the primary fluid drain plug on the bottom rear corner of the primary housing. Place an oil pan below the primary housing and remove the drain plug with a socket wrench. Allow the primary fluid to drain completely. Remove any debris present of the tip of the drain plug with a shop towel and replace its O-ring before reinserting it into the primary housing.
3. Remove the clutch adjustment plug from the center of the primary housing, using a Phillips screwdriver to unscrew the pair of bolts that secure the plug. Using a funnel, pour up to 1 qt. of fresh primary fluid into the primary housing. Ideally, the fluid level should be just below the clutch diaphragm spring within the primary housing. Remove the funnel and wipe away any spilled primary fluid with a shop towel. Reinsert the clutch adjustment plug into the primary housing and tighten its bolts with a Phillips screwdriver.