Author Topic: M21 oil leak.  (Read 454 times)

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13Brads

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M21 oil leak.
« on: 08 February, 2020, 16:42:09 »
Hello.
I fired up the M21 for the first time in about nine months today. I had to drain the crankcase first to release nearly a pint of oil which had found its way there from the tank. It started first kick and seemed to be fine ,but after a short while, I noticed oil pouring out from behind the primary chaincase as if it was coming from the area of the crankshaft. I haven't investigated this yet, but thought I would ask the group for opinions before I start stripping things down. There was no visible smoke from the exhaust for the shortish period it was running. I didn't have time to check if oil was returning to the tank, but feel sure it was otherwise the build up in the crankcase would have caused it to smoke.                                                                                                                               Any advice/suggestions gratefully received,

Grahame

chaz

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Re: M21 oil leak.
« Reply #1 on: 08 February, 2020, 17:10:53 »
firstly as it had wet sumped, any history of if spring/ball has been changed.
look at this first as it will happen again.
as for the leak, sidestand or centre stand? how was it parked.
normally if side stand then would seep through bushing. but in either case weight of shaft may have slightly distorted oil seal. may settle back if used.

13Brads

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Re: M21 oil leak.
« Reply #2 on: 08 February, 2020, 17:41:40 »
Hi Chaz, thanks for you thoughts. The bike has a rear stand so is always upright. When it first came under my guardianship some years ago having been in storage for a long time, it failed to return oil to the tank and flooded the crankcase causing lots of smoke. The fault was traced to the spring loaded ball above the sump plate being stuck. After it was freed off all was fine and this has been the case up until now. It seems now almost as if oil was coming out under some pressure with no evidence of smoke from the exhaust so I presume the crankcase isn't flooding. I will put a container under the leak tomorrow, and start it up to actually check if oil is returning to the tank.

JulianS

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Re: M21 oil leak.
« Reply #3 on: 08 February, 2020, 18:15:01 »
Oil from breather caused by non return to tank?

Wet sumping does not always mean burning oil.

The earlier M21 had breather on the primary side and the later models had breather on the timing cover, change came after engine YM21 602 accoedig to the 1948 parts book.

If yours is the earlier type this would explain the oil coming from behind the primary case.

13Brads

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Re: M21 oil leak.
« Reply #4 on: 08 February, 2020, 18:54:33 »
Hello JulianS.  Thanks for your thoughts. My breather is on the timing side and nothing seems to be coming from there.  My connection to wet sumping and burning oil was only when the return to the tank had failed and the sump was consequently filling with oil while it was running, hence masses of smoke.

13Brads

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Re: M21 oil leak.
« Reply #5 on: 10 February, 2020, 15:10:23 »
Ok so I started her up this morning and verified that oil was returning to tank nicely. I decided to remove the primary chain case cover to get a better view of where the oil leak was coming from. With the cover off I started her up again and hey presto no oil leak. So it seems that when the engine crankcase filled with oil, over a period of time, some of that oil escaped along the crankshaft into the primary drive case and it was escaping from the rear of there when running. From what I can see in the manual there is no oil seal as such on the crank shaft, just a thrower ring, so I will just have to be wary of furure wet sumping, or start her up and get out on her a bit more.