I have 335 (purchased new) and suddenly started leaking in location described above. I removed the 4 bolt cover to inspect the oil seal and found the seal laying in the bottom of the water pump cavern. (No water pump exists on 335 as is oil cooled.) I bought an old 335 engine which wasn't running for parts/ cheap . Removed same cover and also found the oil seal laying inside. Yikes!Air cooled with oil cooler. The four bolt plate on right side of engine (straight in from foot brake) is where I have a leek. It seems that the 500 and 335 has the same design, except the 335 does not have a water pump. So there is a oil seal behind the plate that is leaking. Seems like it is pretty easy to replace. I ordered one last night for 20.00 w/shipping. If you search the internet for a 500 water pump leak, you can find more info on the oil seal. Seems it is highly recommended to replace it if you have to replace the water pump. Not much info on it for the 335. Both are the same though. Hope this helps.