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Before you pull it for the o-ring. May sure transmission oil level is correct with bike level. Make sure primary is correct level to lower outer clutch disk and not to top edge of derby cover. (primary vents thru transmission) Be sure transmission vent hose is open. (little puff of air thru vent tube with transmission dip stick out) And most of all (and is what causes this a lot) is if you have an outer loose protective cover over clutch outer sealed seath that runs all the way up front, make sure it did not get a dose of oil that ran in it from the last filter change. Then it runs back from the air stream and appears to make it look like it is leaking a transmission. If it is actually at bottom of side plate below cable, it could be the two bolts for the side cover that attach the exhaust mounting plate are loose if you have that attachment.