Combat Breathing

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Dec 24, 2003
It was recently suggested that an extra "Breather" from the rear of the inlet rocker cover would help my 1972 Combat Engine "Breathe" a bit better.

Well, I installed one about 6 weeks back now & must say it seems to do a good job. The only place my oil was still trying to "force" its' way out is via the rear engine case stud & the tacho drive(to be "cured").

As most would know on this forum, the "Combat" type breather was not as good as it could have been. That is y some people drill their timing case & run the 850 style breather out of the rear of the timing case.

Having a spare inlet cover, I drilled & tapped for a 1/4" bsp fitting & ran the hose back to the oil tank via a "T-Piece". Cost was $10 AUS. It was simple to do & I would recommend trying it.
venting into oil tank

Hello Nortonfan,
Two years ago after reading an extensive thread of letters regarding engine breather modifications posted on the NOC technical page, I installed a PCV (actually a $4 automotive power brake vacuum check valve) into the breather line of my 850. From the valve to the oil tank, and from the oil tank to ground I used clear plastic hose. After running this way for about a thousand miles or so I became concerned about the water condensation visably forming in the clear hose, not to mention whatever other nasty combustion products entering the oil with it. So I rerouted and left the oil tank out of the system, now its check valve straight to ground. which gives me alittle more peace of mind.

engine breathing for the combat

I did the 850 style breather mod to my combat engine seven years ago during the restoration. Reading the NOC Commando Service Notes as well as a web posted article it made sense at the time to pre-empt this "poor combat breather". Honestly I don't see any difference after the mod and with PCV.

"Don't be tempted to replace the rubbery hose pipe with plastic on 200,000 series motors - I have seen a nylon reinforeced pipe melted by the hot oil going up it!" quoted from the Commando Service Notes.
I did the same modification with the rear rocker cover several years ago and this worked good on my bike also. :)
extra breather

I didn't have any problems with the original combat breather in the past and don't notice any difference with the 850 type breather modification.
What do you guys see improved or what was wrong in the first place?
Re: extra breather

illf8ed said:
I didn't have any problems with the original combat breather in the past and don't notice any difference with the 850 type breather modification.
What do you guys see improved or what was wrong in the first place?

My Combat blew oil out the head gasket on the left side. Replacing the head gasket didn't completely solve the problem. One day I noticed the engine was trying to breath via the sealing washer under the nut holding the rear rocker cover. This gave me the idea of adding a breather line to the rocker cover which solved the problem.
What exactly is this modification that you make to the intake valve covers?

My Combat engine shows seepage (sp?) on the top of the engine cases just below the carb.

I hope this mod will solve the problem.
oil leak


The leak at the top of the crankcase just below the carbs is described in the NOC Service Notes written by T.R. Stevens, former Quality Engineer and then Service Engineer for Norton Villiers from 1970-'74.

"That oil leak between the top rear boss and the base of the barrels. Any 750 owner will show you the one I mean." This is caused by the cases moving against each other during operation. "The only type of gasket compound I know, which will stand this sort of movement is the silicon rubber type." I use Yamabond which also resists being dissolved by the gas dripping from the Amals.

I don't think a better breather is going to help with this case.

I am curious as to why you did the 850 mod to your "Combat" engine if you were not having a problem with the "standard" setup ??

My "Combat" is ridden 5 days out of 7 & apart from the "normal" oil leaks that need to be addressed like the "tacho" o-ring, my crankshaft seal into the primary started letting oil through from the engine.

In hindsight, I reckon it was because it had not been ridden for some time as we had torrential rain for a week or more. Now, I reckon that there was a buildup of oil in the "standard" combat type breather. When I started my bike after sitting for this period of time, the oil tried getting out through the crankcase seal as the "breather was blocked with excess oil" due to sitting idle & "wetsumping".

So after discussing my problem with others, I thort :idea: one of the ideas that seemed to make sense was the breather at the inlet rocker cover. It was easy to do, you have to "look" for it & now any "weak" points in my engine "gaskets/sealants" will not be the first point of exit due to pressure....It will travel out the new "breather" & back into my oil tank. If your engine does wetsump & block the original breather, any pressure now has an easy escape route.

I like it & recommend any "combat" owners trying it. Then they can discard it or keep it. Mine is staying as I can see an improvement.

A bit like the Autolite/Platinum tipped plug argument, I never have to pull my plugs these days..they work GREAT !! And I think it was your David that clued me onto them ...thank you.


Since I had the engine completely apart it made sense at the time to follow the advise of several articles to modify the combat breather. Drilling the 3/8" holes inside the timing case really needs to be done with
the engine cases apart, so the answer to "why" is opportunity.

Regarding blowing drive side engine seal, mine is on it's second replacement in the last year. I agree the wetsumping is likely the cause when starting up the engine. My 850 breather didn't stop the seal from going, so I'm not sure any other type of additional breather will stop this either. It's best just to drain the crankcase before starting if the bike has been sitting or drain the oil tank after each ride.

My riding is about 3000 miles a year. I'm lucky to get in 2 rides a month. Then there's the winter down time I spend doing maintenance. You can see mine does sit for long periods.
spark plugs


It wasn't me that recommended Autolite plugs. I worked for a division of NGK from '84 to '94 and have used their platinum BP7EV plug in a past '73 750 Commando. This is another, I really didn't see any difference when firing the plugs with a RITA.
Have read your responses David.

So, even though you were having no problems with your breather, you did the 850 type mod because you had the opportunity, ok. That would not be a good enough reason for myself.

Lots of other people have had a problem, hence the reason for making a modification. I am very happy & recommend the type of mod I have just done, the reason for this post.

I would suggest that if an owner is not having a nothing.

I have seen the 850 breather mod done to "pre 1972" engines also for no apparent reason other than, "they" said it would be a good thing. They may have "discovered" a simple mechanical problem with the rotating disc at the end of the earlier cases after having done the mod, but instead of telling people that, they will say it is an "improvement"........... There may also have been nothing wrong with the original setup, but they do it anyway...cause "they" said it should be done.

Every bike is different in the way they are put together. So in my book you have look at each engine individually.

The change in spark plug use was great for me also, even though it may not have done anything for you. I recommend owners try them for themselves & use them or discard them.
I think the Autolite plugs are great! No more fouling when I start up. The bike seems to run better and idles better with them too. And they're dirt cheap. I get them at NAPA and they cost about half what the bike shops charge me for non-platinum NGKs.



There's nothing wrong with trying the many modifications out there. How else can you know if they work or not. My post was to comment these two didn't show me a significant difference for "my" '72 combat.

Dynodave recommended the RITA over the Boyer ignition for combats. I tried that and no longer have detonation using pump gas...was using 108 octane Sportgas blended with pump gas.

I agree, there is nothing wrong with trying different mods,"if you have a problem with some aspect". Otherwise it is usually wise to leave well enough alone.

The RITA ignition is used by mates also, but after riding their bikes with the RITA & then riding mine, I notice no difference.
Granted, they may make a difference with the fuel, but I have yet to experience a problem with my boyer setup. I know others that race & use the rita & they claim they are better for racing. It is good that the rita works for you.

I have to mention that I went to the "British Singles Club" meeting about a fortnight ago. The funniest comment I heard was "Oh, did you actually "ride" that Norton to the meeting ? We never ride to meetings !"
Well one bloke apart from myself rode a beautiful Norton International that night out of about 15 that showed up.

People have asked me to form a NOC here in brisbane as opposed to a Norton Riders Social Club. The main reason I won't do it is because of the "mindset" people get about originality, mods you can do & can't do, etc etc. We don't care what you do to your bike, as long as you enjoy riding & it is reliable. To many people in the "owners clubs" just seem to "own em" & get a kick out of telling people what they have instead of just enjoying what they were built for.

Having said that, sure there would be people in these clubs that just don't have the time to ride em often because of work, families etc.

David, we had one bloke coming for a ride with us one day but his Norton wouldn't start. On the phone, I asked him if he had spark? "How do I check that", he replied. It was his first norton, but surely a 50 year old man would know how to check for spark in any engine ! Anyway we went around and got it going for him. Next week he rolls up with a leather vest, along with the "attitude". "Guess what ?" he says, "I went in to buy a vest & brought a Harley Fatboy also." He wasn't invited anymore.

My point is, there must be a lot of people out there without the "this is not original" attitude & the Vest with the Fatboy attitude with no clue as to how their bikes run. They are type our club is going to end being, I hope.

Anyway David, I hope your bike runs sweet as & you enjoy many more miles on it.

That's pretty much my excuse for so few miles per year... family, job and it's been drizzly all this weekend. I've put in my time in younger years riding a Commando in all types of weather before family, so I don't appologize for not doing it now. Nevertheless I do love a good romp in the Sierras on a five hundred mile Sunday in the warm months. When you don't get to ride that much, pick the best days! Alpine firs at 9,000 feet and a winding, well paved narrow Hwy 4 on Ebbetts Pass. And you can run any grade of gas at that altitude, there's not enough oxygen to detonate.
It would be nice to visit your country one of these days before my wife & I croak it. We are in our late 40s also & know that time is "precious".

One of the best things we all seem to enjoy is firing up our nortons & heading off on our favourite piece of highway/backroad. All the time we spend getting them sorted doesn't matter when our bikes are running well & doing what they r sposed to do.

It would be a good thing to meet all forum members one day for the ride of all rides. What a party that would be :!: Imagine a few hundred/thousand Nortons on a run 8) All the insignifigent things would be totally forgotten.

Stay uprite and enjoy those nortons all of you :D
I guess the reason for my last addittion to this thread is the fact that one of my work mates will not be returning from Thailand after his "holiday".

Another bloke that rides with us "Murray", his wife & daughter are still over there, but "safe & alive".

It does make one realise that life may be taken from us at a time when we may least expect it.

Enjoy what you have while you are able to, petty disagreements mean nothing. If you wake up, have a job & are healthy, you should be thankful.
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