Topic: CB750 K6 USA oil tank leak

[Aurelia B20]

I worked on a K6 imported from USA in 2014, it had a oil leak problem very hard to identify !

Finally I understood that the leak came from the oil tank, the tube of the breather which passes in the tank was probably cracked or a weld was failed, the oil of the tank ran in the tube then in the pipe of the breather, the leak came from the connection of the pipe on the engine, not envisaged for such a flow... some pics so that it is clearer !

The engine crankcase breather is connected at the bottom of the oil tank to a steel tube, next to the oil discharge connection:




The other side of the hose normally connects to the engine crankcase at the rear :




The tube on which the breather is connected goes up inside the tank to the top with a vent system: normal, so that the oil does not go down in the tube! A small drawing to make it clear :




I tested in the following way to check the leak : plug the 2 oil outlets in the bottom of the tank, suspend it high enough to slide a container under the breather tube, fill with oil and wait :




30 minutes later, already a few centilitres of oil have passed :




I repared with a second hand oil tank found on ebay USA, and no more leak problem until today, 7 years later : same problem on the same bike ! 
Question :
- is this problem well known in USA, in France very few K6 were sold and I am not sure that the oil tank was the same with this breather tube ?
- if yes, have you solved the problem by sealing the tank with a resin like for gasoline tanks ? What do you think about this idea ?

Thanks ! 

[newday777]

"The other side of the hose normally connects to the engine crankcase at the rear "
Was the hose not connected to the rear hose nipple?
Did you plug that hose outlet on the tank or leave it open to drip when you did the hanging test?
I haven't heard of a leak there in the tank.
I'll add the notification to this as I have a K6 waiting to rebuild as its sat for 32 years.

[Aurelia B20]

Was the hose not connected to the rear hose nipple?

It was connected, but not sure that the hose is original part, maybe a little larger or maybe too old ?? I'll buy a new original one

Did you plug that hose outlet on the tank or leave it open to drip when you did the hanging test?

I of course left it open to let the oil flow out, showing that the tube inside the tank has a leak!

Thanks for your answer, so this problem seems to not be well known on K6 ?
Other advices ? 

[PeWe]

Oil tank overfilled?
Oil level in tank over the max level on dipstick?
Eventual oil drpping out from this vent will flow back to engine.

[Aurelia B20]

Thanks PeWe, oil level was correct, and below the correct level when tested ...

[BenelliSEI]

I had a similar problem, years ago...... From memory, I cut a length of 1/4” OD steel tubing to the same length as the original. Drove it up inside the existing tube, and brazed it shut at the bottom. I left it sticking out about 1/2” longer and welded the existing nipple to the OD of the “new” pipe. Sold the bike years ago, so no idea how long it lasted!

[PeWe]

Interesting problem.
This will make oil to flow to engine when parked, "leaking oil pump" will get the blame when oil disappear from tank.

Fixing the pump with its valve and rubber to work fine will not help.

Strange that the vent pipe can crack inside. Abuse by owner when cleaning the tank? Or bad quality from the very beginning?

I can imagine that the pipe connection to tank can crack if abused outside so oil will drip on the floor or leave oil spray to the rear while driving.

[Aurelia B20]

you are right Pewe, the problem with this kind of leak is that you can't see it if the pipe between the oil tank and the engine is perfectly tight ...
So you have to test the oil tank like I did to check it

About the origin of this leak, I think it's more a bad quality of the tube or welding, I had twice the same leak on two different tanks, that's why I asked you if this failure is known and recurrent on the K6 USA ?

[bryanj]

It would suggest to me short infrequent journeys  causing acid in the oil and this eating away at the pipe, especially iftank does drain to engine via oil pump

[newday777]

It would suggest to me short infrequent journeys  causing acid in the oil and this eating away at the pipe, especially iftank does drain to engine via oil pump

Or acids from old used oil left in the tank/bike for many years.

[PeWe]

Aha, corrosion caused it together with old oil?

This kind of oil leak from tank is good to know.

[Aurelia B20]

Thanks to all for you comments, I will apply a resin treatment as for a fuel tank and buy a new hose with the right size