combat oil return

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Dec 2, 2004
Just read Kieth 1069's reply in breather post, and now thinking too much!
I also relocated the breather as per 850 years ago, running out to atmosphere at rear but kept old breather location running to oil tank. Last time I really rode it hard up in the mountains my oil pressure gauge went to zero and there was no oil in the tank. Put a couple of quarts in and went home. I thought to much pressure was getting past the rings, honed and new rings. Did not ride it for years after that, as I was busy with bevel drive Duck. Now have both breathers joined together into alloy oil tank w/valve. Am I facing disaster as I prepare to blast to Birmingham next week? I never had any oil problems with my original Combat 35 years ago. It seems that I should now strip it out and do the crankcase mod, is that correct?
I think you know the answer already, you have the classic syptoms and I would not run it over 4K revs until its fixed.
Sorry........I lived with this problem for several years and had to limit my revs. When I did the mod I ran a really unscientific test and put the empty cases in the sink and ran hot water down the front inside and watched the flow. As it increased the water stopped flowing from the pump pickup port in the timing case and started to exit the Combat breather hole faster. So I figured there was void appearing at the front pickup. OK, water isn't oil but I convinced myself the subjective proof was there. The flywheel acts like a scoop and oil circulates so fast it misses the pickup completely at high revs.
I still can't completely get my head around why the Combat system worked from the factory unless the breather really did do a good job of returning oil. My engine later blew up after the mod but I think that was partly due to ignorance of what I know now and often running no oil pressure while I had the original system. A newish (2000m) rod bolt failed, wrecking the cases and bending the frame! On several occasions my tank emptied and only after a period of 40-50mph running would it refill.
I read in a Commando book recently that a Combat owner had reduced the pump feed to stop the cases filling with oil. That was an 80's article and I think his comments were that the pressure side flow was too high.
The Commando service notes also state that "the pump can hardly keep up with the rate the stuff is flying out of the bigends" so any on the limit clearances will only make it worse.
commando, I know you are correct, however, I am 60 now and I can still take things apart but it seems that it takes forever to put them back right, and I don't want to leave my 15 year old with to many unfinished projects. I am tired, I just want to ride. I keep other's rides going but I have trouble ridding my Norton.
Kieth, how in the world could the Combat pump a sufficient amount of oil via the breather with that Hugh piece of foam stuck in line. Nothing makes sense to me anymore.
I have no idea. My bet would be the foam would restrict any serious oil flow but the story is often quoted that the breather did a better job than the pump return. Quote from Commando service notes "at high rpm more oil went up the breather than the scavenge pipe".
All we now know for sure is that 1972 was a crap redesign, addressed the following year on 850's. It's fairly unbelieveable that even the British bike industry could make such errors but they did.
To get a 1972/3 750 commando engine to work properly you have to do the modifcations to the crankcase, stopping the oil cavitating and returning back through the breather.

I have modified mine and the engine works fine at all revs with the oil returning back through the oil return pipe and not the breather
Peteuk001 said:
To get a 1972/3 750 commando engine to work properly you have to do the modifcations to the crancase, stopping the oil cavitating and returning back through the breather.

I have modified mine and the engine works fine at all revs with the oil returning back through the oil return pipe and not the breather

This got me to thinking about an oil pressure issue I'm having. 72 Combat (no crankase mods) with low oil pressure at idle, 2-3 psi when the oil is hot. At startup I have about 50 psi at idle. I was assuming it was all about viscosity. Maybe not. It does take some serious running to create the low pressure condition. Maybe it has more to do with cavitation than viscosity. Looks like I'll soon be splitting the cases, huh?


How much oil pressure do you get with hot oil at idle?
Just ride it.

I've put several thousand miles on a '72 Combat, and many of those were above 4K rpm. A long time friend bought a '72 Combat new and put over 50K miles on the bike before the lower end was rebuilt.

I think this breather problem is blown out of proportion, and not really necessary unless your racing or high speed touring.
I've put several thousand miles on a '72 Combat, and many of those were above 4K rpm. A long time friend bought a '72 Combat new and put over 50K miles on the bike before the lower end was rebuilt.

I'm glad some one else weighed in here. My '73 MkV spent a lot of time above 4K (and 5K) when I was younger. Now, 34 years later and at least 50K miles (speedometer replaced several years ago), it still is on the original bottom end. On the other hand, I have never experienced the empty oil tank after a run.

My other MkV looked good when I split the cases. but since I was in there, I had the case milled.

I would always make this mod when the cases are apart, but would not necessarily pull a motor and split the cases to do this unless I was running the bike extremely hard. Of course if I found the oil tank empty after a run I would either change my riding habits or do the mod.

VA, Ron
I did the mod because I had the empty tank problem but then I didn't have a rear breather as the PO had blocked this and added the 850 style which did nothing really. For sure I would have left the cases alone if all had been OK. Good to hear that there are runners out there without any issues. Must just be us unlucky ones who get all the problems!! Mine had all the good (costly)faults when I bought it.
Kieth, what is your reasoning behind your statement that the 850 breather didn't do any thing anyway?
The funny thing is I have been advised by a member of this forum since I had already modified the crankcase to 850 design, to remove the Combat breather, which I left in place, and block it off, which is what the PO of yours did, and your tank ran dry, correct?
Thanks to everyone for their much appreciated advise. One would assume that after 45 years of working on these beast that running them reliably wouldn't be such a chore.
katescottageiom said:
One would assume that after 45 years of working on these beast that running them reliably wouldn't be such a chore.

45 year old Nortons are all sorted. The problems come from all the changes that came afterwards. The newer the model, the less development time. The ultimate example is the Mk111 850 - They solved 10 problems and introduced 15 new ones :D
Kieth, what is your reasoning behind your statement that the 850 breather didn't do any thing anyway?
I believe he meant the breather mod didn't do anything for the cavitation issue. The scraper in the casting was still keeping oil away from the pickup.
Ron is correct. Putting the breather in the 850 pos'n and blanking the Combat outlet made mine worse. The PO had done the work before he sold me the bike after a minor blowup. So he had not tested the mod. That was my job! It took me a while to find the information but the rule seems to be, 850 breather = rear pickup and no scraper. Doing just the breather mod alone is not advisable.
I am still amazed why when the cases were full of oil (2 litres or more) and under some pressure that so little escaped up my '850' breather. You'd figure that with the whole bottom end flooded and under pressure, including the timing chest, that a 1/2" bore hose would form the least resistance and some oil would vent through here. And yes I did have the 3 x 3/8" timing side holes. It preferred to escape via the barrel case joint of course by which time smoke was visible in my mirrors and it was time to slow down.
A good oil pump and worn (though not knocking) bigends probably assisted lots of oilflow into the cases.
72...rear bottom breather, 120,000 miles, cases not ever opened, 20-50 Castrol, one way valve in breather tube, not for long periods brought above 4 grand...only up through the gears as it is so much fun... :wink:

Going to ask a dumb question here...

Fail to picture in my mind that at some point the oil tank can get The cases would have to have 4 quarts of oil in them, and that would go up the breather, and into the tank. Can kind of think the oil might get swept away from the oil pickup at high revs and cause temporary loss of oil pressure, that could be...on an empty set of cases...but an empty oil tank too at the same time? Where is that oil? Gotta be somewhere. Can't have both symtoms at the same time, or? If the cases are's that oil pickup going to be dry then? It should be under a couple of inches of Castrol. How's that pump not going to be pumping it to the places it should be, bearings and....tank too? How's that tank going to be dry?

Either I'm not seeing the things you are all seeing, or not reading here what is being written...someone enlighten me please...
First, if you have four quarts of oil in the system you have at least a quart too much. Three quarts is what the manual calls for and it is usually a little less to take it to the midpoint on the (short) dipstick. If you have been running the engine hard, there is probably half a quart in the top end of the engine, leaving about two quarts plus in the crankcase.

Visualize the crankshaft spinning rapidly (5-6K). The spinning crank and inertia force the oil to the back of the crankcase. The cast-in "scraper" actually prevents the oil from dropping down the back wall of the case to the pickup. Combine this with frothing from the flywheel and you are not returning as much oil to the tank as the feed side of the pump is sending to the crank. After a period of time, the tank goes dry, the delivery of oil to the crank stops, and disaster.

If you allow the engine to slow, the air will release from the oil, the oil will migrate down the "shelf" (scraper casting) and into the small sump area to be returned back to the tank.

I believe I have not had a problem because excursions into the rpm stratosphere is usually in short bursts and the tank never runs dry before the pump is able to recover. I certainly recognize the problem, just deal with it differently. With the broad torque band of the Norton I don't find it necessary to keep it at high revs.

Oil doesn't get sucked into the oil return side of the pump because it is busy keeping the inside of the sump covered with frothy deposits, which like a beer, have to sit a bit before they turn back into something worth slurping up...but how much of this oil can be possibly hanging onto the walls? Amount of oil for an oil change is perhaps here a point. Know some of you have a bit less oil in the tank than the stick tells you to put in, I always have filled to the max on the stick but never add oil between oil changes...air filter full of oil and wet puddle in front of rear wheel teaches some of us not to do that... :wink:

So maybe I have a bit more oil in there...could be. But picturing the entire oil amount all clinging to the walls of the sump and not enough going up the breather to land in the tank and keep the pump primed...uh....hard to imagine a dry tank at any time and any speed...unless the owner is over cautious and puts only half the oil in the tank that it calls for and then thrashes the heck out of the the poor machine. My point much can possibly be sticking to the walls of the sump...or filling the head? A whole tank full? I can see what the beef is with the oil pickup in the sump, that's not the point. I have even, to be truthful, reduced my time in the above 5 grand zone due to this interesting point brought to my attention by the kind folks here in the forum...but I just can't see where all the oil could be kept busy covering sump walls or filling heads up and not leave enough to keep the pump busy. I know there is some obscure reason why it is said that you shouldn't fill the tank up to paar with the stick, but it fails me at the moment. Maybe my blessing is that I have no oil pressure gauge and that I don't thrash the bike except for blasting up through the gears every chance I get... :lol:
HEWHO, That was my first assumption, that all the oil had blown out the original breather as I ran both the 850 and original to the rear tag area, and I was out of oil! Thanks to the oil pressure gauge I saved the motor.
RON, I have always spun the Nortons up from the very my first Atlas, this is the first Norton that has really let me down. I enjoy keeping the local Ducks in sight on a Norton, although, obviously, not recently.
KIETH, I was referring to my years of involvement with these machines, and you are correct the earlier the better, at least for the 750's.

There was another thread on those nasty tubes that puke the oil out on the street....don't like em.
But mine has the later type that returns it to the tank so it can prime the pump, etc. I would have thought as you did, had I had one of those things though...logical assumption, but if it is indeed going back into the tank like mine question much can possibly be on the sump walls and in the head? All of it? Unless there is too little to start out with.

I question the empty tank theories set here for discussion...not saying they aren't true, I've been wrong before, just wondering how they can be true. Perhaps for those of us that wish to thrash the old girl, it is time to just build an increased volume oil tank...cooler oil and more for the motor to play with... or do what they says to do, and machine off this offending thing in the sump...:wink:

What is the advantage of running the breather into atmosphere (tag area), as opposed to venting it back to the oil tank per original design? The tank is vented as is. No pressure build up , as far as I can see. I can well understand the conditions where the Norton starts pushing oil out the breather. If it's dumped into the tank, then no oil loss. I've heard of some running a catch can. Isn't the oil tank already a catch can?
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