Persistent Leak in CVT-Side Crankshaft Seal

[twitchytoes]

I made a thread before to get the sizes for the seals on the CVT side.  I have since replaced the oil seal on the crankshaft on the CVT side 3 times to no avail.  It may last a while before leaking but always makes its way back.  Is there anything else I need to look into?  I'm relatively forgetful and after checking my oil recently found out that I have not been keeping a close enough eye.  I was down 200mL of oil.

[gsx600racer]

If the crank bearings have excessive wear/play, or crankshaft seal surfaces are damage, or out of true crankshaft, that will decrease seals lifespan.

[twitchytoes]

Odometer is at around 1200 or 1300 KM if I'm remembering right. I think I've done 5 or 6 oil changes thus far. Last time I messed with the crank, I just kinda pulled and pushed on it from the CVT side to see if it had much play and it didn't seem like much. As far as damage on the surface, I know there might be a slight nick in the casing but I might be remembering wrong. I've not had the CVT cover off since I retapped the threads after 5 of them stripping out.

[GrumpyUnk]

Several things will cause seal leakage. 1)damaged shaft surface 2)damaged seal seating area 3)excessive runout of the shaft being sealed 4)excessive vibration 5)excessive pressure in the crankcase 6)excessive internal oil flow beyond the seals capacity, and 7) blocked or partially blocked drain passages behind the seal.
You can check for these things, some more readily than others.

One additional, no two: incorrect replacement seal that 'fits' but does not seal and cheap seals that are not constructed using the proper material, or have lax tolerances.

Get a good seal, of the proper size and design, fit it properly into a good seal recess, against a well finished shaft surface that does not have excessive runout nor is subject to excess vibration. Make sure the internal drains are clear and have been drilled to handle the flow, and make sure there is no crankcase pressure nor excessive oil being blasted against the inner of the seal. Got that? should work...
tom

[Moat]

I think Brent (90GTVert) had recurring crank seal leakage on his Taotao ATM-50, and (IIRC?) eventually cured it by fine-sanding/polishing the crankshaft's sealing surface (which can tend to be a bit rough/uneven from the factory).

Bob

[tocoo]

since the last service 500km ago, there is a drop of oil near the flywheel that comes back after each ride

When I check the level of oil, instead of being between A and B,


I get a level above the B mark


is it possible this drop comes from having too much oil?

[GrumpyUnk]

tocoo, does you oil smell of gasoline? If so, it may be that your carburetor is allowing fuel to seep into the crankcase. That dilutes the oil, making it less viscous, reducing its ability to lubricate.
Did you do the service yourself? Did you check the oil level after the service was completed, and, if so, was it above or below the "B" measure(immediately after service - could it have been overfilled then?)
When you check, it should be on the center stand. Question is, front wheel on the ground, and the rear wheel raised? If the rear wheel is on the ground, the lube will tend to move 'rearward' and indicate a bit higher than if the front wheel is on the ground.
Perhaps insure the front wheel is grounded, and check again? But, yes, a 'too high' level can cause foaming of the oil as the crankshaft whips the lube around as it turns. Foaming can raise the pressure in the crankcase and force oil out past seals and gaskets.
The drop could be from within the flywheel/blower housing, or could come from higher up on the engine. Check to see if there is evidence of seepage above the blower plastic, though I can't think of any place convenient to have leak...
tom

[twitchytoes]

I went back in my order history and see that I ordered a gy6 oil seal, not a 139qmb seal. I’m not sure if that would have anything to do with it as they have the same measurements. I may try fine sanding the crankshaft and ordering a 139qmb seal to see how it effects things.

The seal I ordered: www.partsforscooters.com/164-156_GY6_Crankcase_Seal

139qmb seal that was not in stock at the time: www.partsforscooters.com/151-2-Crankcase_oil_seal_qmb139

[edwardtennant]

the 139QMB is a GY6 engine. the GY6 Family of engines included the 139QMB (50cc,2.96BHP) , 152QMI (125cc, 6.8bhp), and the 157QMI (150cc, 8.6BHP)

[twitchytoes]

So there’s nothing different about those seals? I think I have one of the black gy6 ones left.

[tocoo]

Aug 7, 2018 12:31:58 GMT -5 GrumpyUnk said:

tocoo, does you oil smell of gasoline? If so, it may be that your carburetor is allowing fuel to seep into the crankcase. That dilutes the oil, making it less viscous, reducing its ability to lubricate.
Did you do the service yourself? Did you check the oil level after the service was completed, and, if so, was it above or below the "B" measure(immediately after service - could it have been overfilled then?)
 
tom

The oil does not seem to smell like gasoline. THe official dealer serviced the scooter like for all the previous services and the previous service the mark of oil was below B. AFter his latest service, I believed I checked and it was over B. I will try to remove a bit of oil with a syringe. The way I check is the way of the manual: scooter on central stand on a flat ground, and in my case the rear wheel is not in contact with the ground. Then I put the plastic cap  without screwing it.

I checked again and there is no wake of oil, everything is dry of oil, especially around the oil filter. The only thing a slightly wet of oil is the protuberance of the gasket (I do not know if this protuberance is standard on this engine) but I cleaned it now :


and the habitual mix of oil+ dirt near the rear wheel:


Is this oil stain actually normal ?


The other habitual oil leak since I purchased the scooter is at the valve cover (which is slightly on the left of the scooter): when you look above the cover in the seated position like when you drive, the left side of the frame of the scooter, below the valve cover, has the usual mixture of oil+dirt. 

[GrumpyUnk]

I doubt these machines can be sealed 100%, and will always have some bit of seepage.

I would suggest using some RTV, just a 'wipe', on the gasket surface of the valve cover. The O-ring seal should do the job by itself, but they get 'tired', and harden so they no longer are in compression between the cover and cylinder head. A swipe of RTV will act as a 'rubbery' seal hopefully taking up that smidge of clearance allowed by a seal that is not 100% any more.
To find leaks, it seems you have to inspect carefully, maybe even remove or loosen the ductwork that forms the engine shroud so you can see the crankcase and cylinder readily.
If you think about it a bit, there are a few sources of oil leakage possibility: crankshaft seals, crankcase half gasket(center gasket), drain plug, strainer/filter O-ring, cylinder-to-crankcase(base) gasket, chain tensioner gasket, cylinder head gasket and valve cover O-ring.
They can all be observed directly with a little effort and part movement. I think.
One last thing is the oil filler/dipstick O-ring. Left loose or with a hardened O-ring, seepage can occur.
A final last thing is the vent tube from the valve cover. Any blowby products will exit through that tube. If there is excessive blowby and the tube is connected to a location that does not allow pressure relief, crankcase pressure can cause leakage at the weakest gasket/seal/O-ring of the engine.
tom