I wanted to change the O-ring in the oil level finder (viewing glass) during the next oil change, but since I was doing other maintenance under the bike (gear shift linkage, side and center stand cleaning and greasing) I replaced the O-ring as well.
I emptied and re-used the oil; it was changed only about 1200 km ago. The two flange bolts cracked open with relative ease, I kept track of which one goes where, just in case, but they are both the same size. The oil level finder cover only fits one way and the O-ring sits in its groove behind the cover. After removing the old O-ring, I cleaned the cover thoroughly, mainly the groove the O-ring sits. I suspect the O-ring I took out was the original factory one, since it was very rigid (hard plastic texture and very little flexible rubbery feel), thinner than the new one, had three dimples embedded in it (see photo, inside of the O-ring), and left lots of dirt and grim in the groove. The window glass was clean, may be because I change the oil every 5 - 6 K kilometers.
Now, the groove that O-ring sits in has three dimples, 120 degrees apart. These dimple marks can be seen in the interior of the old O-ring in the photo. A new O-ring sits in its groove tightly, but make sure the new O-ring sits flush. I forgot to take a photo of the new O-ring for comparison. Assembly was the reverse, with correct torque settings above.
This is one of those leaks that is so slow and gradual that took about 4 months for me to notice! The oil oozes out ever so slowly on the side of the crankcase and to the bottom of oil pan and drips, one drop every few days! But it takes a long time to get to that point. I noticed it because the bike was winterized for a few months and I always put a clean pan under the bike, just in case, and after 3 months I noticed a couple of drops!
