Oil Tank Vent Reservoir

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Dec 1, 2006
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I'm looking for the ultimate solution to the oil tank vent reservoir problem.
I am building a Mk3. The bike has a single Mikuni and previous owner had a hose from the tank breather jammed into a flattenned beer can.
I have been scanning the supermarket etc shelves looking for a suitable container without any luck.
I was thinking of making a rectangular (say 1.5" deep) tank from clear acrylic and then double sided tape plus cable tie it to the front surface of the Odyssey battery. Acrylic is strong and easy to fabricate.
Any comments? Has anyone found anything off the shelf that works well?
I have done a similar thing with mine.

I have used a plastic container (that was originally a cod liver oil capsule container), I have painted it black so that it is nearly impossible to see i.e. it doesn't catch your eye, although I have an early air filter on mine blocking direct vision to the battery. I have then glued a small piece of black foam to the battery carrier , then put two tiny holes three quarters of the way up the bottle, and cabled tied the bottle to the carrier using the two small holes. Then the breather pipe from the oil tank runs directly into this.

Result is a very small amount of oil residue, and the amounts being so small, I cannot envisage the emptying of it being more than once or twice a year, and what's better, a completely oil/drip free Commando.

I have seen this done using the cap for the container fitted as well, and fitting two elbows to the cap. The "feed" pipe into the container is left long, and the other elbow vents (less emulsified oil) to the air cleaner if fitted.
Clever solution to the problem, but don't you run a serious risk disabling the "British Anti-Rust Chassis Mist Lubrication System" used on Brit bikes and cars since the dawn of time? :lol:

Kansas, America
kanlimey said:
Clever solution to the problem, but don't you run a serious risk disabling the "British Anti-Rust Chassis Mist Lubrication System" used on Brit bikes and cars since the dawn of time? :lol:

Kansas, America

Not a very good system then because they rusted anyway :D Perhaps they needed a breather down both sides (and higher up and nearer the front :shock: )
Thanks Reggie, I'll look in that aisle when I'm next shopping! As for the famous anti rust system - isn't that a secondary function of the tacho drive? - or have I got an optional extra.
I put one of those CNW type ones on mine. So far, doesn't seem to have absorbed any oil, so maybe I wasted the money, but it was pretty short bucks. - BrianK
I also use a single Mikuni on my MK lll . I drilled a hole in the back of the air breather and installed rubber grommet . Then i took the breather hose from the tank and slipped it through the grommet. It has been running that way for two years with no problem what so ever.
thanks for the replies guys, maybe I'm over complicating things. for the bucks I guess it's worth going the filter way, then if nothing seems to get trapped just feeding into the filter as per Ron would seem to be the tidiest sol'n.
This forum is amazing! Right from the jig to paint the frame I have constantly found great solutions to problems and good advice from the collective wisdom!!
Breathe in and breathe out

Had a similar experience on the 850 recently. Fitted a Mikuni for all the good reasons and had to find somewhere to put the oil tank breather. I used a 500ml BP plastic oil bottle with a 1/2 right angle plastic retic joiner fitted into the screw cap. This catch bottle sits in front of the battery.

I fitted a clear plastic hose to the top cap joiner and ran a hose under the battery tray down between gearbox plates and exiting alongside the oil filter. I used clear because I wanted to see what vapour was coming out. The inlet from the oil tank is the same original hose, but this connects to a hollow brass tube slighly less shorter than the bottle height. The brass tube is extensively cross drilled to allow gas pressure out if the catch bottle starts to fill up. The brass tube is inserted into the upper side of the catch bottle and gives a good anchor to the inlet hose.

After about 1,200Kms, I checked the contents of the catch bottle. Not much but typical black slime. The clear hose hardly had any water vapour or oil mist in it so it seems a fair solution. But, while this was a OK substitute for the original airfilter connection, it also raised the questions of a) increased blow by pressure from worn piston rings and b) blow by contamination of the oil in the tank.

It appeared that after changing with fresh oil, it did not take long for it to get fairly black. My thought was that the piston ring blow by is pumped up there by the big engine breather hose, this narrows down to 7mm internal bore at the oil tank and blasts dirty vapour into the oil. Next, it seems while Norton may have accounted for the volumetric displacement of crankcase air when new, the increase in pressure and contamination when the piston rings are worn in has not been adequately catered for.

At the same time I had a persistent small undetectable leak from the cyclinder head area under hard riding. A good mate, a real expert on Commandos advised that porous heads and barrels do occur.

2 things seemed obvious - divert the engine breather from the oil tank to prevent contamination and add a second relief breather to the engine, logically the cylinder head. I added the 2nd breather a 1/4 hose to an air line angle brass fitting to the inlet rocker box cover. Again I used a clear hose to see what was coming out of it. Low and behold, you wanna see the shitty goo coming outta there! Grey slimey crud (oil water vapour emulsified)

This head breather hose now runs over the top of the Mikuni, into the rubber ring on the frame junction bracket, around the left of the battery and sneaks down through the left footpeg plate to exit in front of the rear hub sprocket giving the chain a little extra moisture.

Having removed the original breather to the oil tank, the diversion now runs down to exit alongside the oil filter, I sealed off the existing tank junctions and drilled a very small hole in the underside of the tank cap and in the top to increase atmospheric compensation. This works fine, no vapour traces at all on the cap itself.

Results so far - Oil in the tank is much cleaner, demonstrable pressure from both crankcase and head breathers and remarkably NO OIL SPLATTER from the head / barrel anymore. A clean dry Commando.

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