Excess revs equals oil leaks?

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May 22, 2004
I hear and read alot of stuff.. or guff.. about 5,000 plus rpm on the Commandos.

My 73 850 is mechanically just fine and additionally I enjoy quite a bit of 'Mountain highway climbing" , but I find that if I reach up in to the 4500- 5000 rpm PLUS range.. the breather from the oil tank to the (standard) air filter sucks up oil turbulence , and this excess leaks out the bottom of the filter!!!

My bike does not use oil at any other point.. plugs are exemplary colour.. and the oil tank is NOT over filled..

So either I have a problem or others are just ignoring this as they hit the high rev ranges.. ????
1973 Crankcase breather

The 1972, and some 1973, breathers are an issue because of their location. High RPM's are where the problem is most noticable.

On engines with this type of breather, the crank winds up throwing oil against the back of the case, where it pools behind the cast in baffle. Positive crankcase pressure forces this oil back to the oil tank - via the breather hose.

Read Fred's tech article: www.oldbritts.com/n_c_case.html to learn more about it.

However, even if you have the less-than-perfect breather, I don't know why there's enough pressure or a high enough oil level to force oil through the line connecting the oil tank and the air filter. Sorry.
Thanks David

I did not do the engine rebuild when I reconned the bike, and this has always done it at higher revs..

I am of the thought that if the guy fitted or the bike had a high volume oil pump fitted.. the turbulence created.. may be ~ being picked up from the breather to the air filter.. and thus the oil leakage at the air filter. ??????

One suggestion was that I simply put the pipe to the rear chain.. (Hmmm still not ure if he was being humerous or not!)

you stated that it is NOT overfull BUT what mark are you using for your full level? there has ben talk that the stock full mark is TOO HIGH and causes oil to be forced out the vent. I run my oil tank just above the LOW mark and have no trouble with oil out the vent. my bike is a 72 combat with the screwed up vent BUT I changed it to the 850 style when I did the rebuild,

My thought is an over full tank also. I chased all the leaks on my Mark III, This was the last one. I experimented with various levels and concluded just over the low mark was best. I actually do more of a visual of the tank. I like to see a small clearance between the top of the oil and the bulge in the oil tank.
Even with that, it would leave a small froth on the ground from the vent pipe after a ride. I had mine vented under the bike because I was running a K & N.
I finally vented the tank into the Mark III air/oil separater to use as a catch can and then vented to the atmosphere through a crank filter.
No drips, no runs, no froth.
I recently mentioned running another breather from the inlet rocker cover in my 1972 Combat engine. It worked on my engine & is lot simple/easier to do than the later 850 style.

You drill & tap the inlet rocker cover for a 1/8" fitting. I ran a hose from there between my two amals back to a T-Piece into my oil tank.

I have a spare set of 850 cases that need a welding repair to drive side, if anyone needs a set. Asking $150 AUS as they are. I have been quoted $100 AUS for the welding repair.
Interestingly , Reg, some time back I recall some one on (maybe ) the Brit iron site asking why the Commando he had purchased had a fitting as you describe on the inlet cover..

Obviously this what he had on his Norton eh? This sounds like a practical fitting to me too... BUT would not the reciprocating pressure and suction generated from the engine add the "bedlam" already going on?

I have fitted a brake PCV from a Holden Red engine to my crankcase breather pipe and I feel this experiment has been a big success, 8) 8) as the oil vapour film that was typical on the lower front crankcases has dramatically declined.. and I find I only have to consider an engine wash ~ just once in awhile , rather than every 200 kms or so..
You said...
"BUT would not the reciprocating pressure and suction generated from the engine add the "bedlam" already going on?"

Dunno about that mate, I just know it works.

The idea was suggested to me by BP Bob, who is a member of the Hunter Valley NOC & a some kind of mechanical engineer or engineer of some sort that does some thinking & experimenting with all model nortons. I met Bob last October & communicate regulalry now via email. He is a very knowledgeable & helpfull person. All round good bloke.

The pickup on the combats is not the best design apparently, so this lil mod just helps it a bit I reckon. Ignorance is Bliss, that is my motto :D
excess oil consumption and leak

Sorry to barge in , cure for excess oil consumption is to take a sheet metal
screw and plug the vent hole to the air box. as the oil froths at higher temperature especially in higher altitudrs and RPM it gets sucked into the carbs .I beieve it was an attempt by Norton at air pollution technology.
I had a 1970 commando, and I know it sounds low tech but it worked
"as the oil froths at higher temperature especially in higher altitude and RPM ~ "

Interesting thought as I live on top of the Great dividing range that runs the length of the Australian east coast ~ approx at 2,000 ft above seas level.

(You're not 'barging in' 'guest' ).. the oil is not getting sucked into the carbs but does leak from the base of the air filter.. then onto the oil lines and gives the impression that the oil filter is leaking .. just an annoying event and being an annoyance I will always endeavour to 'get it right'..! Thanks
Interesting. Mine drips a bit (I thought) from the oil lines. I wonder if it's really coming out the breather through the filter? I'll have to check! I do have the stock airbox with a K&N.

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