Hello all. Back again and digging deeper with the 68 DT1. Finally got around to building a pressure test setup from compression test hose, bike pump and various parts. Sealed exhaust with 44mm rubber expansion plug and intake with metal plate & carb o-ring. At under 10psi the front motor mount bolt on left side began to drip oil. Tightened it up a bit and seemed to stop. But also getting major flow from transmission breather tube. From my searches this is a failed right side crank seal? Any help is appreciated. I do have the part in attached photo in my trusty parts bin. Thanks much!

EDIT: I’ll admit when I bought this bike and got it running, I had so much fun with it so I got overly optimistic and concentrated on cosmetic aspects. But my ignorance is slowly fading and ready to get into the beating heart of it! Luckily I have the xt250 to get my thrills at the moment!

Leaking "oil" from that spot does not make a lot of sense. I suspect that it is more likely some moisture, possibly mixed with some combustion gas/oil mix. Take a peek at the pic of my left case half, and you can see there really is not much of a path from the transmission oil cavity to that cavity the top front motor mount bolt goes through. If your testing pressure is forcing something out from that cavity, it is probably/possibly caused by leakage past the crankcase mating surfaces (sealed by some kind of sealer, Yamabond at the factory surely- have the cases been split before, to your knowledge?). 

Do use care when you are pressuring things up, I am thinking 4-5, maybe 6 psi at the most. I don't think the seals and bond agents can take much more than that, but I not the expert on using one of those tools. (paging RickC, paging RickC  )



  

I've seen front mounts leak like a sieve... Oil does come from there.  Can be slowed or somewhat "repaired" by sealing mount bolt but only real fix is splitting cases and resealing halves.

Thanks guys. So it could be leaking on the interior mating surface and then forced out thru the mounting hole? No leaks on outside of case anywhere. It was dark black oil, started dripping at about 5psi then I stopped. I’m almost certain the cases were split at some point, as my left case isn’t stamped. Believe I was told on this forum that sometimes the chain would come loose and destroy the case, and that replacement cases weren’t stamped? Suppose I should plan on pulling the motor and getting into it. Will start reading thru manuals and go from there. Any specific resources I should seek out?

Per the notes on the attached parts guide, several updates were made to both the right and left side crankcase half’s over the ‘68 model year. While the noted changes were the change to ISO threads and the increase in the rear mount bolt diameter from 8mm to 10mm it does appear from your photos that the two case half’s do not align properly at the front mount bolt location which makes me think that the new left side might not be correct. As nhsteve indicated it would be a unlikely path for oil to get from the transmission to the front engine mount cavity. That all said it makes me think that another possible source of the black oil is seepage from exhaust flange over the years some of which accumulated around the bolt or seeped into the mismatching case half’s.  Either way if it is leaking from this location while under pressure a case split would be in your future.  

 

 

Thanks jgersh. Working my courage up to start further dismantling. Think it’s safe to say the crank seal is shot. Is that the seal or what’s left of it identified by red arrow? There was a buildup of white/yellow powder behind the gasket in the cavity above, aluminum oxidation? I’m definitely in over my head but confident in taking my time to learn along the way. Is a copper color on the connecting rod normal, or a bad sign? Also figured out why my tach never worked...Thanks again to all

Photos of mismatched cases. Left case is longer in both directions. Pretty flush on top and bottom. Hopefully this isn’t the reason the seal went bad to begin with. Or chalk it up to poor workmanship/wear and tear?  

Looks as though you have several issues going on:

1 - The Clutch Primary Drive Gear is hitting the crank seal.  This could be simply due missing thrust washers/bearing beneath the clutch basket or more probably an improper case half was used when previous owner reassembled the engine.  I have attached photos of my ‘68  which clearly show the crankcase having two raised sections around the crank seal which result in the seal being recessed below the clutch drive gear.  

 

 

2 - Missing Tachometer drive gear assembly (white nylon gear and shaft). 

 

These engines are very simple. I always take lots of photos during disassembly but in your case they might just show you what a previous owner did wrong.  That said I would download the parts manual from the Tech Library and use this as a guide for proper assembly.  


 

I see the differences there. Hmm. Was hoping at least half this motor was original haha. I assume this mangled spring is part of the seal? I actually pushed the seal back in with my thumb and it clears the clutch gear. Maybe the seal just backed out and that’s why it was hitting the gear? I have a tach drive gear from a 72, it’s not nylon though. Would just need to find the “axle”.  

You did mention at the beginning of this thread that you pressurized to 10 psi. As previously stated most recommend not exceeding 6 psi as you risk pushing a seal out... either way the seal should not be able to be pushed back in that easily. If it were mine I would split the cases, verify if the correct left side was used and reassemble making sure that all surfaces match up and are properly sealed...