A few weeks ago I posted a summary of a total oil loss from my Commando, (Tragic Oil Loss), whilst returning home from a ride out. Oil everywhere underneath the bike, with the primary chain case full to the brim with oil. Obvious conclusion: crank-case oil seal failure.
Of course it wasn't, the golden rule being whatever you think it is, it is always something else. In this case one of the three Allen bolts that secure the inner chain case to the crankcase had worked loose, and fallen out. I blame myself for this, in that I have fitted the Alton starter kit , and in securing the chain-case I think I was too hesitant about tightening the bolts too much, being fearful of stripping the threads.
But the hole is tiny, just a few mill, and it seems strange that so much oil would just weep out over a few miles of travel.
Anyway, having reassembled the starter and primary drive, I fired her up, thinking that was the problem solved. No it wasn't. This is the situation as of today, (April 28th).
With the engine running, no oil flows back to the tank via the oil filter return line. I have changed the filter, thinking it might be blocked, but on testing further there is no oil coming through the feed pipe to the filter from the engine. What is happening is that the oil gets back to the tank via the crankcase breather. Presumably the pressure builds up as the oil accumulates in the bottom of the crank case, and having no exit point, after about 20 seconds of the engine running oil is forced up the breather pipe and so returns to the oil tank. (Just as a note relating to the original problem, being under such pressure, I am guessing the oil squirted through the missing bolt hole, thereby filling the chain case).
Before I strip out the pump, has any one any idea why the scavenge is failing to scavenge?
I should add that my oil pressure gauge shows bags of pressure, (70psi on start-up, running at 40psi when the oil is warm), so it is not the pressure side of the pump that is at fault.
I am scratching my head, which is not unusual, even after 25 years of running the old girl.
Andy Bone
On the 72 there is no large…
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That's a neat mod to fit a gauze filter to '72 cases
That's a neat mod to fit a gauze filter to '72 cases, but I do worry about the effects on ground clearance. I'd want to check carefully before riding off any kerbs.
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The sidestand spring is 55mm…
The sidestand spring is 55mm lower than the large sump plug, the sidestand arm lower surface is 45mm lower but the tang on the sidestand for the spring hook is 70mm lower. As the 72 cases are thin at this point the mod adds 30mm if the engine was vertical not canted over so there is a lot of hardware to be ground off before you get to it. Its about 15mm lower than the front of the crankcases taking the forward cant into account.
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Thanks to John Holmes for…
Thanks to John Holmes for setting my mind at rest — did you have those measurements to hand, or have you been out in the shed checking?
;-)
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There are bigger ones for…
There are bigger ones for the large sump plug with reed valve breather already around for some years, and at $300+ then if they grounded there would have been a few complaints around so I knew it was safe to make.
https://nycnorton.com/product/reed-valve-breather-kit/
I made a functional copy of one of these as well but there the likeness ends.
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I had a 750 Commando 1971…
I had a 750 Commando 1971 with the breather on the back of the engine.
I ran it for a long time with all the oil returning via the breather. When I was alerted to the fact that this was a problem.
I stripped the engine to find the drive side oil gallery blocked.
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On the 72 there is no large sump plug with gauze to protect the return side of the pump from large debris, so the teeth are likely damaged and on 72's they do explode more than other years, the breather is in just the right place to return oil to the tank but your new exit hole put paid to that. There is not enough meat on the cases to add the old large sump plug but it can be done by other means.
https://www.accessnorton.com/NortonCommando/72-crankcase-mod-to-add-sump-filter.31727/