English French German Italian Spanish

Tragic oil loss

Submitted by andy_bone on Wed, 07/04/2021
Forums
Commando

OK, so I am bumbling home on my 1972 750, and shock-horror, notice that my oil pressure guage shows an oil pressure of about 10 psi, as opposed to the usual 30 to 40. As I watch it the pressure continues to fall, down to nothing. (Why do I have an oil-pressure guage? Stops you forgetting to turn on the anti-sump tap).
With my wife on the back, (combined age 158) this was not the time to stop and consider pushing, so I press on, hoping it was a guage problem. There was no visible sign of oil looking down at the engine.
Getting home OK was a relief, as you might imagine, but the problem was not the guage, in that the whole underside and back of the bike was covered in oil, and the oil-tank was empty.
Having refilled the oil-tank I got my agile son to get under the bike, not something I can do these days, and in fact I have just come out of hospital with a new knee.
He said there was a stream of oil coming out from the back of the primary chain inner case, although it was nigh on impossible to say from exactly where. The oil pressure was only 10 psi, or less, despite having refilled the tank.
I did however notice a seepage of oil from both the outer primary chain-case inspection cap, and from the bolt that holds it in, so I assume the primary chaincase is pretty full of oil.
As I was due in hospital the next day for the knee job, the bike is as I parked it.
So what is the problem?
I doubt it is the crank-shaft seal, as having recently dismantled the primary drive there was no sign of any leakage at this point, and anyway, unless it has blown out completely, I can't see it allowing the contents of the oil-tank to escape over just  few miles.
Any ideas, anyone? If you are local to Clare in Suffolk you are welcome to come and have a look, as for sure I am not able to wield spanners at ground level!
I wonder if the low oil pressure that may be a clue to the answer?
Over to the combined experience of the NOC...

Commandos, don't you love them?

Andy Bone

Submitted by ian_soady on Wed, 07/04/2021

Permalink

... but I lost all the oil from my 850 when one of the pipes came off the filter (not fitted by me) and covered the back end of the bike. I managed to whip the clutch out just before it locked solid, bending a conrod and taking a lump out of the barrel. This was 2 weeks before I was due to go to Germany on it so some heroic spannering was necessary.....

It happened when I was doing the Roman Ride in the early 1980s - anybody here done that? - and I was 50 miles from home with no breakdown cover. The story of how I got it back to Brum on the train sounds so incredible that I sometimes wonder if I dreamt it.

Thanks for the suggestion, but first thing we looked at, and thankfully not the problem. I think the oil seal has popped out.

Submitted by john_holmes on Wed, 07/04/2021

Permalink

As its a 72, my guess is the breather mounted on the back of the crankcase came loose and the oil left the engine out the gap and never got back to the tank, now you have worn big ends giving you 10 psi and the breather is still loose so oil pouring out back of cases just next to the primary.

 

I thought the same thing too, and hoped the pipe had sprung  a leak, something nice and simple. But all was OK in that department.

Submitted by mikael_ridderstad on Wed, 07/04/2021

Permalink

If the crankshaft seal stops to seal, engine oil will go into the primary cover. Seen that happen to a 6650SS. Thus emptying the oil tank. With no oil coming, big end bearings will wear quickly, so pressure with filled oil tank will be low. So first thing to check is if the primary is filled with oil. If so, the oil seal needs replacing. And also crankshaft regrinded.

Of course breather could be the problem too.

Thanks for the reply, which attunes to what I am thinking as well. I don't think there will be any crank wear over such a short distance, there is usually plenty of oil slopping around in the crankcases. I had a full seize up 20 or so years ago, needing new pistons and a rebore, but the bottom end was unaffected.

I will find out once I can bend down!

Submitted by john_holmes on Thu, 08/04/2021

Permalink

The seal can pop out, normally when a wet sumped bike is ran without the sump first being drained, but you have an anti sump valve. So unless that was leaking or not turned on when last sat in the garage it seems odd it did. It was cured at the MK3 by it being retained by a circlip but pre MK3 there is not enough room to add a circlip. 

Submitted by allan_walker on Sun, 11/04/2021

Permalink

If the bike had been running normally beforehand (you refer to being on your way home,) then even a blown crankshaft seal should not allow all the oil to pass into the chaincase and then be blown out again.

The scavenge (return) side of the oil pump is far greater in capacity than the feed. Are you sure that the oil is returning to the oil tank? Could there be a blockage in the oil pick-up in the crankcase?

Good point. I refilled the tank, but was more concerned to see where the oil was coming from, so I didn't check the return flow. I am repairing fast from the op, so might tackle the job later this week.

 



© 2024 Norton Owners Club Website by 2Toucans