Britbike forum Forums British Motorcycle specific Brands Norton bulletin board Commando crankcase vent blows oil

1969 Commando crankcase vent blows oil out onto the rear fender and the license plate bracket. It does it even after running some miles, so I doubt it is caused by wet sumping. The engine has just been professionally rebuilt, and unfortunately the builder is out of state. Otherwise the rebuild runs clean and strong. It just blows oil out the vent. Haven't measured how much, but enough to make a small puddle when parked. Please give me some advice what to look for.
Thanks in advance,
Rick

Fastback or 'R' type (with the RH oil tank) or 'S' type with central oil tank?



The crankcase breather should connect to a spigot on the top of the oil tank so any oil blown up the breather returns to the oil tank.

The oil tank should have a separate breather to 'atmosphere' so more details are needed.

S type, dead stock.

The shade tree mechanics fix for this, until you get on top of it, is to blow the oil into a coke can 'catch tank'.
This has the immeasurable benefit that you can measure how much is coming out, AND it stops it from going everywhere.
Done with neat zip ties, it looks like it belongs there ... ?

Seriously, Nortons had about 10 goes at their versions of oil breather systems, across Dommies and Commandos, and none of them were entirely satisfactory - for high speed motoring ! They didn't have the catch tank as a factory-fitting-of-last-resort. Catch tanks on race bikes are generally a compulsory fitting, oiling your back tyre (or someone else's) is a no no.

In that case, the central oil tank vents into the airbox (as the front plate of the oil tank is the backplate of the airbox) so that's the point where the oil should be leaking from?

Assuming it has the normal central oil tank dipstick what level on the dipstick are you filling the oil tank to?

Is the oil pump is NEW? If not, you might suspect a bad pump on the scavenge side which can allow way to much oil in the sump! Then it gets slinged up onto the cams vent holes and out the hose, Drain the sump after a run to see if the oil level is too high?
I've seen similar before....It gets worse as the oil heats up.

The scavenge side of the oil pump has a much larger capacity than the delivery side,
it would have to be REALLY REALLY worn for this to be a problem ?
Or have eaten a lot of grit for breakfast ...

Do Commandos do big enough mileages that this can become a problem ??
The oil pump is swimming in oil ...

"The scavenge side of the oil pump has a much larger capacity than the delivery side,
it would have to be REALLY REALLY worn for this to be a problem ?
Or have eaten a lot of grit for breakfast ...

Do Commandos do big enough mileages that this can become a problem ??
The oil pump is swimming in oil ..."

yes but not often...

The scavenge is 1.666 times the feed. That ratio is the same whether the narrow gear pump or wide gear pump.
Having tested pumps that absolutely would NOT evacuate the sump and gets worse as the oil gets hot.

So where is the wear that causes this ?

The ratio might be 1.666:1, but under pressure/pumping freely the scavenge side can deliver (scavenge) way more.

I DO recall now that folks used to put catch tanks on their Commandos. I will study that one this weekend! And I will doublecheck the routing of the vent and the dipstick marks. Thanks Rohan and LAB! I will report back in a day or so!
Best regards, Rick