Oil leak in vicinity of dynamo/crankcase

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Oil leak in vicinity of dynamo/crankcase

Postby Oggers » Tue Nov 26, 2019 9:55 am

A favourite topic perhaps, but advice much appreciated. My 55 G9 leaks oil from under the dynamo. I have tied 3 times now to seal up the dynamo joint with the timing casing - taking all advice here, but last time I was out and about, there was still quite a drip from under the dynamo. Of course, it could still be the dynamo joint, but I guess it could also be the crankcase joint. To eliminate the latter, I guess a strip, flatten off the mating surface and Wellseal is required - not forgetting the paper gasket- but this is a stack of work. hence, easy things first perhaps?

Viweing the attachment - the lower nearside of the bike - are bolts A and B and studs C and D the clamping bolts for the crankcase halves? My thinking was to nip these up. A and B were tight but tooak a wee bit more, C and D took perhaps a turn more. It may help matters.....???

Also, I read somewhere one could try and insert some thin gasket between the crancase halves in the area of the oil tunnel, by loosening bolts and (gently) prising tha halves apart in situ and inserting some thin gasket material and Wellseal. That or some Belzona! Tempoary fix only naturally.

Thoughts most welcome as always.
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Re: Oil leak in vicinity of dynamo/crankcase

Postby MalcW » Tue Nov 26, 2019 10:50 am

Yes, as far as the bolts are concerned, you are correct. But I would say it's far more likely to be the dynamo seal to the rear of the timing case that's the culprit, with oil seeping down and making its way along under the dynamo. If you do a search of the forum you'll find a number of posts about sealing that joint,including some by me. Pressing the dynamo firmly against the hole whilst doing everything up is important. I use a suitable length of wood between the free end of the dynamo and the garage, and lean the bike against the wood, pushing the dynamo up against the timing case (my prop stand is set up so that it stays down, so I leave the stand down just in case...). Do up the dynamo strap and the stud that passes through the timing case while the wood is in place. Mybike gets a bit of an oil mist from the joint, but I can live with that.

I'm not sure I like the idea of removing the bolts and the front engine mount and then trying to lever the crankcase halves apart; I don't know how successful that would be, and you risk damaging the mating surfaces. I suppose you could clean the dynamo cutout thoroughly and then put a smear of clear silicone sealant on there, but I've never tried it.

Good luck.

Malc

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Oggers » Tue Nov 26, 2019 10:57 am

Malc

Yes, I agree to the likelihood of it being the dynamo seal, but as I have done it 3 times already, observing all recommendations yet still getting quite a number of drips, I am beginning to look elsewhere....Oil mist would be fine by me also!

Incidentally why does it leak from the dynamo seal? I cannot envisage much pressure in the timing case and there is surely not much oil sloshing about in there?

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Re: Oil leak in vicinity of dynamo/crankcase

Postby MalcW » Tue Nov 26, 2019 12:41 pm

The oil that is in there is picked up by the gear train and thrown around to lubricate the gears. So although there isn't a great deal of pressure in there, the oil is being flung around the dynamo pinion area,and elsewhere, at some speed.

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Mick D » Tue Nov 26, 2019 4:43 pm

Hi

Just thinking aloud as it were, and I'm no twins expert but could this be a breathing issue?

Over to the twins experts :)

Regards Mick

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Oggers » Wed Nov 27, 2019 10:02 am

Mick

I checked out the breather valve hidden within the chaincase. Maltese cross shaped spade with a weak spring basically. All looked well and I even compressed the sring a little to reduce relief pressure. Can't think it would have made much difference anyways, the spring is so weak.....

The only other thing I though of is that the dynamo seal sits very close to the oil PRV in the timing case. I also adjusted that, so oil does no splurge towards the seal when pressure is reached.

Part of the problem is that it is difficult to understand whether it is the dynamo seal or the crankcase joint just underneath the dynamo. Blanking off the dynamo orifice in the timing case seems to be the only solution there......

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Groily » Wed Nov 27, 2019 11:17 am

Oggers wrote: Blanking off the dynamo orifice in the timing case seems to be the only solution there......

Good plan. When I did that on one of mine (a persistent offender), it proved it wasn't the crankcase joint, which was good. A bit of alloy plate is what I used, attached by the draw stud hole, gasket paper and Wellseal. Even if it leaks a little, you can still see if it's NOT coming out at the crankcase joint, which is what you need to know.
My bet is it's 'all dynamo joint' as the oil seeps all over the place quite fast when it starts.
Fingers crossed.

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Oggers » Wed Nov 27, 2019 1:20 pm

Groily

Yes - I think the need to establish root cause is pressing! I have nipped up the crankcase fasteners a little, so I will check everything again after the next run. If it still leaks, then blanking plate/bung it shall be!

As an aside, that dynamo joint is a swine. Not overly impresssed with the cork gasket and the nick you must make for the drawbolt, which introduces another potential leak path. Copious quanties of silcone this last time after using wellseal beforehand. Still thinking about using a suitable large and thick O-ring in lieu but difficult to source.....

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Re: Oil leak in vicinity of dynamo/crankcase

Postby clive » Wed Nov 27, 2019 2:04 pm

You should not need to nick the cork gasket where the draw stud is, it will compress to fit there quite satisfactorily. Possible source of your leak?
clive
if it ain't broke don't fix

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Oggers » Wed Nov 27, 2019 2:21 pm

Clive

That's interesting. I could not see how the drawbolt passed through its timing case hole otherwise, as part of that hole is covered by the cork gasket. Unless you force the gasket on the dynamo stub with the drawbolt in place - which seemed a tricky oepration when locating the dynamo in its recess. Perhaps "nick" is also the wrong word. Create a very small recess on the outer edge may be more accurate, and there was still quite a bit of meat on the cork gasket remaining.

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