HELP PLEASE FELLAS!  I am at an impasse.  I've posted several times over the past 6 months on this same topic and have received excellent guidance; but the problem persists.  I've done many things (everything I could think of and what has been advised) and have been working on my DT360a that I bought new in Japan in 1974.  I won't rehash all that but I did find that the right crank seal was popped out on the forward side with a bent retainer clip...AHA!  Bought a new seal and 2 clips, installed the new seal, and put it all back together.  Rode it and it still ran very poorly and smoked a lot.  Plus, when I pulled the cam plate tensioner plug about 6 ounces of tranny oil came out.  I thought the seal must have popped out again so this morning I took the clutch case off and the seal was still in position.  On very careful inspection I noticed two things:  the circumferential spring imbedded in the seal was slightly bent and in the inner case right underneath the clutch there's a small irregular hole.  I took photos of both of these "defects".  Other than a bad right seal, how can tranny oil get into the crankcase?  Any advice will be greatly appreciated.  Many thanks!  Pat 

how did this oring look?


here is a pic of opposite side. you can see how the crankcase splits off the trans/crank.

You lost me at "Plus, when I pulled the cam plate tensioner plug about 6 ounces of tranny oil came out."

Do you mean the transmission is low on oil?  Did you remove the actual transmission drain plug?  

Anyway, this kind of thing is the main reason I learned to pressure test crankcase with about 5psi and then vacuum test if it passes pressure test to find leaks and then again afterwards to make sure it's fixed.  

For one, the new seal could have been defective or even incorrect.  Or possibly damaged during install.  Did you use an original Yamaha-spec dual lip SW-type seal? Or one of the generic single lip aftermarket seals sold on ebay as the original seals for that model are getting harder to find and can be expensive?

I've only had one DT360 with an air leak.  Seals held fine, the leak was the seam between the case halves.  

It took a hell of a lot of pressure to force the seal out and bend the retaining clip, almost certainly had to be hydraulic force so it's possible the crankcase filled with oil from another path and the popped seal was a symptom and not the cause?
 

Wow Mark, you are the High Priest of Enduro-Land.  I'm a 74 yo ex-fighter jock and still think I can do everything (spoiler alert: I can't).  My son had the bike for a while and put it in the back of a U-haul for 2,000mi.  Bike got loose from the tie downs and bounced around for the last 700 miles...I think that's how the oil originally seeped into the crankcase.  The seal I bought from Ebay was listed as NOS stock that I assumed to be OEM.  And here's where my ignorance will truly shine -  I never saw an O-ring where you indicated on my first disassembly.  I just went down to look but the primary drive gear is still in place so I didn't see anything awry, but the collar attached to the primary drive pinion is scored.  I'm thinking that the O-ring you described is not there and I never saw it the first time I took it apart.  Might be sitting in the bottom of the crankcase in pieces.  The photo you sent from the other side view cleared that part up for me.  I'd like to buy a proper crank seal and O-ring and try all of this again.  Do you know how I can get these?  Many thanks Brother, Pat

Hi Pat. That o-ring hides near the bearing race or it may have been stuck in the primary gear when removed.  So it may actually be there.
posting up a gasket set (is not OEM though. your NOS could be fine) I found useful on ebay.  O-ring on partzilla. DT360 Seals only or   DT360 Gasket Set

*  verify left side case bolts
*  very motor mounts not loose

The seal in question from the kit:

Pictures were not mine, pabdt posted them.  He's far more of an expert on the DT360 than I'll ever be!

Great that you went for a NOS seal.  As shown in additional pics that pabdt just posted, the inner side should have had the "castle turret" bumps.  But that doesn't really solve anything as the aftermarket single lip seals should and do work okay.  They just aren't as good of a seal and will likely not last very long compared to OEM.  Either type could be defective or get damaged during installation.  

 

Thanks for that vote of confidence, but not sure I deserve it! Speaking of vacuum/pressure testing, I would be VERY interested to read about your methods of doing this and what you use to do it. I would rather build a good system the first time.