Today. I change the engine oil and transmission oil together .
I notice there is very little amount of transmission oil came out . may be 200 cc.

But the engine oil came out  800cc. 

So i fill new oil in and ride for 1 hour . i found engine oil level increase about 100cc per 1 hour of ride.


Transmission side breather hose looks ok ,No oil came out the hose looks clean.

Is this the Crank seal problem?

 

Hi -I'm sure this has been raised before. If you search the forums I think the answer is on there somewhere.

Good luck  

Here it is: 

 

http://www.trialscentral.com/forums/topic/60890-transmission-oil-seeping-into-engine-side-on-my-2012-montesa/

Indeed it would have been useful if 12Mont had come back to let us know if he had any success. Is he out there somewhere?

Thank you for your comment.

Update.  
Oil seal 's rubber deteriorate. No groove on the metal sleeve.

I don't know how this happen but this bike parked for 2 years before i bought it. only 20 hours of use.

after change ,I let the engine run at idle for 10 minute. engine oil level still the same  





 

Edited December 16, 20169 yr by menglovekai

It would be interesting to know what kind of oil was used that caused such swellings.

Or if the oil seal was made out of the wrong kind of rubber.

I would suspect it was using the wrong kind of oil based on only 20 hours of use. 

 

And yes if you know what type of oil was in the bike please let us all know so we can avoid it.

Hi,

I swapped out the crank seal and it rectified my problem of transmission oil leaking into engine compartment.

I have two friends with earlier 4RTs and they have not had any oil migration issues.

The CRF versions of these engines are known for shaft seal failures that allow oil to migrate between engine/tranny. 

3 months  after crank seal changed.
I notice the engine oil level is rising again.  and the transmission oil is decrease. 

the engine looks clean ,no oil leak

Is it possible that the engine oil pump's oil seal is defect ?   

 

On 4/6/2017 at 7:42 AM, menglovekai said:

... Is it possible that the engine oil pump's oil seal is defect?   

 

May be possible, but I think it's unlikely because:

1) Pressure should be higher in the oil pump, which would push engine oil into the clutch / primary drive case rather than the opposite, and,

2) The oil pump seal is fairly high, above the transmission oil level (unless transmission is overfilled). There could be splash that gets to that seal, but it's also recessed and away from oil slung by the primary gears.

Back to the first point, the pressure should be higher in the oil pump than the clutch / primary drive case. But if the transmission breather hose is blocked or pinched, pressure could buildup in the transmission / primary drive. That would increase potential for transmission oil migration to engine oil at the various seals or at the center-case gasket.

Sorry... probably not much help.

I would agree that a pressure issue caused by some kind of blockage sounds more likely the cause than the oil pump.  The other option could be that you got a defective seal.  Not common but can happen...