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Oil Leak Into Primary Drive Chaincase

Submitted by nick_ohare on Sat, 23/07/2022
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Commando

Hi All

My Commando is in France so I only get to ride her from time to time. In March I had not run her for 14 months so did an oil change using Motul 20/50. She started third kick and ran well.

On returning in June I( noticed a large puddle of oil underneath. Suspecting the drain plug seal I removed it annealed the copper washer replaced and topped up with oil. She started first kick but after a short way down the road the clutch would slip under acceleration. Past experience indicated overfill of the primary and oil on the clutch plates. BUT I have a Hemming belt drive so it should be dry.

I undid the overflow screw and sure enough a lot of oil came gushing out. I have run it with the chaincase off but cannot see any obvious leak.

I suspect the crankshaft seal but is there any other element I should consider as the culprit? I will need to take all tools and spares with me when I go back in September. If there is any kind of tutorial material for doing this that you know of it would be handy

The last consideration is that the engine case number is 312288 which I believe is a MK3 850 case (PO had engine rebuilt by Mick Hemmings when a rod broke). Is the seal going to be the same as a 750? 

I hope the collective wisdom of NOC can help

Cheers

Nick

 

 

Submitted by john_holmes on Sat, 23/07/2022

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312288 is MK2 850, MK3 starts 325000,so seal is same as 750 for your crankcase and is where the oil from an overfilled sump is coming from. But as you have no oil showing in the primary when running that suggests its only leaking very slowly when the sump is full and the engine off, a new seal may be no better as possibly the crank journal is scored. So leave the seal as is and drain both the sump and primary when the bike has been stood. When the day comes that there is a leak when running then change the seal but also inspect and repair any crank journal scoring using say an SKF speedi sleeve.

Submitted by ian_soady on Sat, 23/07/2022

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... the main problem as implied by John is probably that it's wet sumping. The oil which should stay in the oil tank when not running is draining down via the pump gears - a common Norton issue when the bike is left standing due to the gear pump. The crankshaft oil seal is not really intended to stop a crankcase full of oil getting past it - it's normally only dealing with oil mist.

If it has wet sumped and you try to start it, you'll also be causing high crankcase pressure which will find any weak points such as the oil seal.

As John says if it was mine I would leave it as it is for the time being especially if you don't get many opportunities to ride. But before starting the bike when you return to it, drain the crankcase. You'll probably find a substantial amount in there. You can just return that to the tank. Another option would be to drain the tank after your last ride so that there isn't much to drain down.

I won't mention anti-drain valves and taps here!

Submitted by stan_thomas on Sat, 23/07/2022

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I suppose #1 is leave the primary chaincase drain bolt off - permanently.

-- oops : no drain plug pre-Mk3, so leave the rubber band out instead.

 

Then given, as others have said, that the crankcase is half-full of oil, check the bolts that hold the inner primary cover are tight or, better still, remove, clean and refit with sealant on the threads.

Crankshaft seal is same regardless, Mk3 cases have a circlip to retain it.

 

[edit]

While I think about it ... if your belt has been soaking in oil you'll want a new one of those plus strip the clutch and degrease everything in there.

 


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