Extra tube on oil tank

1974 Norton Commando bought in boxes all apart. Oil tank (stock as all Commando oil tanks, not after market) has 3 vent tubes on top of the tank. Rear one comes from oil fill opening as breather to crankcase. Middle one (same size as breather one) goes to the rear air filter plate to put fumes, etc. back into carb. Third one is at the front top of oil tank and is of much larger diameter then the other two tubes. This tube goes down forward about 3 inches. I have no idea what this extra tube does nor were it goes. Looks like factory made, not added on by somebody. Does this go to another year model of Commando or ? Thanks for the help. Don't know how to post pictures, sorry.

'74 "Oil tank breather" goes to the air filter.

https://www.oldbritts.com/oillines.html

Isn't there a line with a valve that goes to the drive chain for lubrication?

it is teed into the return line from the engine

oldbeezer said:

Isn't there a line with a valve that goes to the drive chain for lubrication?

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Some folks (myself among them) run that oil tank breather hose (more or less) straight down, where it vents to atmosphere.
If you don't have or don't want the original air filter setup, it's a decent choice.

Here's a picture of the original tank. If yours has more tubes coming off the top, it's not stock, I reckon.

I too have the oil feed diagram shown above that is one reason I cannot figure out what i have. But something is coming to light! He bike was all apart in boxes when I bought it. One of the parts was an external oil cooler. I am going to dig that out of my shed and look at the attachments (hoses type) to see if this is the answer to the mystery. This extra tube, if installed by another owner, was done on a first class basis. Small neat welds that look factory. So it is coming together I think. Looks like previous owner added oil cooler and installed the large tube as maybe a return oil line. I am going to take tank off the bike and have a member here post it so you can see why I was wondering where the hell it goes. Thanks for the help, will get back shortly with picture.

Here are some pics of the unidentified tube from Dennis:

Most interesting, I have never seen this before. I will be very interested in hearing what other say about this.

John in Texas

I am convinced that this was added to accommodate the large external oil cooler that was on the bike at one time. Bike was all apart when I bought it. The hoses on the oil cooler are the same size as this tube. So it was most likely set up as a return from the cooler. I think. Weld job was almost perfect and looked like factory made that is why I was searching for a use for this tube, had me fooled. Never know what previous owners have done on a bike.

dennisht said:

I am convinced that this was added to accommodate the large external oil cooler that was on the bike at one time. Bike was all apart when I bought it. The hoses on the oil cooler are the same size as this tube. So it was most likely set up as a return from the cooler. I think. Weld job was almost perfect and looked like factory made that is why I was searching for a use for this tube, had me fooled. Never know what previous owners have done on a bike.

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Since it is there.....a 'good use' for it would be to fit one of Jim Comstock's reed valves to the crankcases and feed the pipe in here.....