Many American V-twin motorcycles on the road utilize a wet primary drive system. The oil within a wet primary case is tasked with cooling and lubricating the critical components housed inside. In Sportster/Buell applications the primary oil is also shared with the transmission. Primary oil tends to become quite dirty since several of the internal components are wearable. Once this contamination becomes excessive, the oil’s ability to do its job is compromised. This is why primary oil should be changed at the recommended intervals (typically 5,000 or 10,000 miles) at the very least. A bike that is ridden hard should have this oil changed a bit more frequently.
Changing primary oil is a rather uniform procedure and will not vary much between generations or models. Torque specifications, tools utilized, gaskets, specified oil and capacity will be what varies. The specific service information for a given primary can be found rather easily. There are two phases to a primary oil change which can be universally applied to the majority of wet primary configurations.
The first phase of the process is to drain the old oil. (The primary oil should be drained when warm whenever possible. A good warm up period followed by a one mile test ride should be sufficient to warm the oil. Upon returning, allow the bike to cool for a few minutes before you start the process.) Then, begin by getting the bike level. Remove both the clutch (aka derby) cover and the primary drain plug. The presence of fine, metallic particles on the magnetic tip of the plug is completely normal. Should you notice any abnormal particles or shavings on the plug, it would be wise to remove the primary cover for further inspection. If the particle accumulation is not concerning, simply clean the plug and prep it for installation.
You will want to install a new o-ring or apply fresh sealant onto the plug depending upon which style plug you have. Allow the oil to drain to a slow drip before moving on any further. If the bike you are working on is equipped with a cable actuated clutch this would be an ideal time to check its adjustment. Also consider checking the primary chain tension at this point if the primary housing is equipped with a chain inspection cover.
The second phase of the process is to refill the primary with fresh oil. Install the drain plug and torque it to the appropriate specification. Fill the primary with the correct amount of the specified primary oil. If you are unsure of how much oil to add don’t sweat it. There is a little hack you can use to ensure you add a sufficient amount. Simply fill the primary until the oil touches the bottom of the clutch diaphragm spring. Clean any oil or residue off of the gasket mating surfaces. Install the clutch cover with a new gasket. Torque the clutch cover bolts to specifications using the proper sequence. The torque sequence will vary based upon the number of bolts used to secure the cover.
Minor variables aside, that is all there is to changing primary oil. Certain things are best kept dirty. Primary oil, or any oil for that matter, is not one of them. Keep your oil clean and your throttle twisted.
Tinker, Shred, Destroy, Repeat