Oil leak from engine to transmission

I have a wonderful motorcycle, 1971 750 Roadster. One problem which is starting to get irritating though is that I have to drain oil from the transmission every 50 km. If I fail to do this, it drools out from behind the clutch.
When I put the transmission together two yers ago, I was very carful putting in the seal nice and square, using some silicone sealant between the seal and the crancase. (the problem is not new). The sealing surface on the axle is not particularly worn.
I guess my problem is a combination of a flexing crancshaft and not so good engine ventilation. (both are standard from the factory)Both issues would need an engine rebuild, which I have some loose plans for. How much will new high strenght crancshaft bolts help? What is the best ventilation system?

Is there anyone out there who had the same problem and found some effective measures to fix it?

Welcome! Get a super slick and effective comnoz breather and solve leaking with an easy bolt on mod. new-product-t10721.html You may need to re-do that crank seal, just requires primary chaincase dismantling. When you mention "leaking into transmission", it's assumed you meant into the primary chaincase. Certainly while chaincase is apart, measuring (DON'T GUESS!) crankshaft axial and radial play would be prudent.

Thanks for the welcome!
Yes transmission=chaincase. That device looks like a real nice solution!
My possible plans for an engine rebuild is partly based on this oil leak, so your advice to measure the crankshaft play is welcome. It might help the decision. But if I make the rebuild, I assume I can put a reed on the blind plate on the back side of the timing chest and open up between the timing chest and the crankcase as I think is described elswhere on this forum?
There were some questions abot the price of this breather on the thread you linked to but no straight answer. Is the price fixed now?

Oil can also escape via the 3 bolt holes that hold 1' case on up there.

PM comnoz. He was selling the sump breathers himself a while ago and I don't see them on the CNW site yet. I think they were less than $200USD and it's just a bolt on item, no machining. Solves a lot of leak problems and gets rid of your other breather system. The object is negative crankcase pressure.

It may not fit on the early bikes (68-70) with the frame brace right below the sump bolt, like mine. Believe me, I tried.

Dave
69S

You state that you have to drain the gearbox oil every 50km?

because tranny fluid is coming out from behind the clutch or the I assume you mean between the gearbox and the inner primary?

well, this has nothing to do with ventilating the engine breathing as the gearbox is separate.

maybe you are overfilling the gearbox and the excess fluid is leaking into the primary, but you did not say your primary is overfilled

when you put gearbox fluid in do you have the bike on its side stand? Is so then then by the time you have filled it to the top of the spring or level plug then it is way way overfilled, and hence the leaking

You should check the level only when the bike is on the center stand or its two wheels and not the side stand

the tranny would have to be pretty darn overfilled for the fluid to rise to the level of the mainshaft seal to leak from there, I vote you are simply overfilling

We are not talking about an overfilled primary so can't see why a new crank seal is needed

I think he means engine to primary, Sounds like a blown crank seal to me. That much oil would be a more than just the bolts holding on the inner primary cover. Was probably caused by a bad wet sump event. Check your crank seal, not that hard to do.

it drools out from behind the clutch.

Click to expand...

maybe he is confused between what is the primary and what is the tranny

he says he has to keep filling the gearbox, said nothing about the primary overfilling

Who ever heard of over filling Norton volumes just going by the manual or the level plugs, or gasing up close to cap sealing, give me a break and stop mis leading an innocent into destructive practices and loss of ego bonding by intimate daily attendance. They only thing that might be damaging to run dry as a bone for a short while is the engine itself and then only the crankshaft stuff. So 1st test is rn dry primary, if stays dry thin external tranny leak, Then run the AMC dry, if primary stays dry think engine leaks getting blown back on oustside. If primary oils up with just engine oil its getting in via crank &/or small bolts or more rarely the middle top case seam wicking along charger cord into grommet or more so on 1' case outside. If runing with engine dry and tranny dry and primary dry and still drips off primary consider the fork seals, unless they are already to dry to leak.

Thanks for a lot of input!
I don't have a side stand, and I seldom have to top up the gearbox oil. It is way below the amount of oil I have to drain from the chaincase/primary transmission. (and I would not check any fluid level if the vehicle is not horizontal) So yes, I am 100 % certain that it comes from the engine. The three bolts is a good advice, I will make sure to seal these, but as someone said the leak is too large for these being the only problem.

I am aware of the wetsump problem. I allways leave the engine with the pistons near top dead position, if it is standing still for some days. The oil pump is fairly new AND carefully machined to have as little side play as possible, to minize the problem. (I think my timing side cover is too early to have the anti wetsump valve fitted)
I also take care to let the engine idle for a minute or two if it has been standing for some days. Nevertheless, I assume it could have blown out the seal at such an occasion.

I guess the conclusion is to dismantle the primary chain/chaincase and check the seal and axle play. Depending on the result I will consider the nice breather device.

In taking the primary off one first removes the cover, so why not start it uncovered and see what shows up, once nice and warmed after a ride to keep engine oiling rev's up prior. In actual down and dirty practice likely better for the bike to just let it weep engine oil into the primary as keeps the engine oil fresher by top offs and hardly ever have to change it the harder normal way.