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Putting holes in a part of the frame that is subject to stress is dumb. You will create a stress riser, i.e., a place where stresses will be focused, and where the piece will be most likely to crack. A common example of a stress riser is a nick on a aluminum connecting rod.
The thing is.. they rust, and that rust eventually ends up in the pan. Occasionally, they should be cleaned, a total PITA. No.
The factory put a tube in the hole and then welded completely around the tube. That would take care of the stress riser problem. I don’t see that construction in your photo.
My 87LM utilizes the frame in the breather system, which I'm told, tends to rust from condensation. Not the best solution!
You'd never be able to get a sufficiently even coat in the tubes for that to work Kiwi...a new breather box either in plastic like some of the late bikes or stainless would be ideal I think. There is a nice but expensive aftermarket one available in Europe that's stainless and fits between the tank and frame rails on a tonti.You could quite easily fabricate something like that in copper tube or plastic waste pipe.
but the factory put holes in the frame so they must not be worried about stress? the wall of that pipe are pretty thick so I wouldn't think stress is a factor. ever heard of a frame cracking in that area?
I think that if the bike is ridden and not stored for long periods, the chance of corrosion inside the frame / breather section is minimal as the expelled oil will maintain a protective film on the steel. If you are getting rust in your sump from a frame/ breather it's because it's not getting enough use.