Oil leak in vicinity of dynamo/crankcase

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

Postby Groily » Wed Nov 27, 2019 3:19 pm

I'v never rated the cork gasket much Oggers either. I've used all sorts in the past, including a 'laminate' of layers of klingerite-type stuff, thick enough to more than take up the depth of the recess, and O rings too. Silicon is horrible stuff, but I use it here as I haven't found anything else that works.
Following the advice of people with dry joints in this area (there are quite a few!!) I assemble initially semi-tight, let the silicon set overnight say, and then tighten up the draw screw (not very though, the thread will rip out of the dynamo in a heartbeat if you're aggressive) and lastly the strap. Sometimes I get thousands of dry miles. Sometimes, er, as far as the end of the lane. Wear on the crankcase can make it hard; wear on the dynamo end plate can also cause a bad fit. But I've had the same hassle for over 40 years, with 2 different engines (and 3 different sets of crankcases) and several dynamos. That's a 500.

My 650 is a bit 'better' in that it usually lasts longer between leaks - but is still not perfect. I find that how long things last is revs-related too. Low speed and all looks great for ages . . . then thrash it and things can change quite rapidly.

The fact that so many folk experience the same headache speaks to poor design - but on Triumphs and Nortons with not dissimilar timing side arrangements, there doesn't seem to be the same constant problem. BSA were ahead of the game using a dry compartment for the dynamo drive. The thing ought to seal with just 'average' fettling skills, not require a Doctorate in Sealing Technology. What is really needed is TWO drawstuds - but then you'd have to get the timing case off to get the dynamo on and off, which would be BAD for a few more reasons, not least of which is the use of those 2BA screws on the case. Which is, however good the alloy casting might be, a very weird choice and deters (me anyway) from taking the cover off unless I have to. (The reason for one crankcase change years ago was the death of several 2BA threads . . . . fixing them all by welding and drilling & tapping etc was beyond me, and more costly at that time than another pair of cases.)
That said, the fact I've fought this good fight for 2/3rds of my entire life says I like the machines, for all that, and the Main Offender has probably done 400K miles in its life.

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Mick D » Wed Nov 27, 2019 4:54 pm

Hi

Could someone direct me to an illustration, or post a photo of the draw stud please, I think this could be a contributing factor to the problem.

Regards Mick

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Groily » Wed Nov 27, 2019 5:40 pm

Haven't got a pic Mick but it's just a plain 2BA stud which comes from a tapped thread in the monkey-metal dynamo drive end plate, through the bearing retainer plate (very close to one of the wee screws that hold that on, then goes through the timing cover from the rear and has a deep nut with blind head and round shoulder under the head to allow it to seat on a spot-faced recess in the timing cover.
It's off-centre - obviously - so it will tend to pull the dynamo into the case on the angle.
Designed to allow the dyn to be removed without removing the timing cover. Nice in theory, less brilliant in practice as the thread strips in the dynamo if one is heavy-handed, the hole in the crankase can get enlarged over time, and the land between the stud and the mating surface where the seal goes is inadequate. Might have been good when all was new and perfick - but not the case after half a century of on-and-off.
I think it's the one thing that has caused me more irritation than any other (well, maybe the speedo drive gearbox thing can be almost as annoying sometimes . . ) . . But hey, compared to getting the pushrods into a Norton or BSA twin . . . small beer!

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Oggers » Thu Nov 28, 2019 9:49 am

Cannot say I care for the cork either. It seems far too incompliant for this job, upon being compressed, all too keen to stay compressed rather than seal as it should. Think I may try a combination of cork gasket, close fitting O-ring and silicone next time around - or perhaps just a suitably thick O-ring and slicone - removing the cork altogether.

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Re: Oil leak in vicinity of dynamo/crankcase

Postby MalcW » Thu Nov 28, 2019 11:08 am

Mine has the cork gasket and a liberal helping of Wellseal. That, and shoving the dynamo firmly into position when doing everything up seems to do the job.

Malc

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Re: Oil leak in vicinity of dynamo/crankcase

Postby les ward » Thu Nov 28, 2019 2:38 pm

Hopefully here is a photo of the stud in the dynamo and on its own.
I went through the same problem. The new cork gaskets didn't work, but I found one in an old gasket set and that worked perfectly. The cork was much finer than in the modern versions.
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Re: Oil leak in vicinity of dynamo/crankcase

Postby les ward » Thu Nov 28, 2019 3:05 pm

Stud in the dynamo
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Re: Oil leak in vicinity of dynamo/crankcase

Postby clive » Sun Dec 01, 2019 1:19 pm

Oggers wrote: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.

The draw bolt should be a stud as illustrated by Les. The gasket can be compressed into place between the fitted stud and the ridge on the Dynamo. With the stud in place there is little place there is little problem locating the Dynamo. Ditch your bolt and get proper stud and nut. However don't use it to exert the pressure needed do that by pushing on the Dynamo from the other side then do up the centre strap Use the stud only to stop it all moving once you are under way and you won't strip the 2BA into the monkey metal. I had to helicoil mine.
clive
if it ain't broke don't fix

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Re: Oil leak in vicinity of dynamo/crankcase

Postby Mick D » Sun Dec 01, 2019 2:23 pm

Hi

Thanks for the photos Les - not too sure what the function of the stud actually is. The clamp will position the dynamo and provide even pressure to the gasket if fitted in the manner described earlier, tightening up the nut on the stud will only upset the even pressure distribution. If it's possible I'd be tempted to just 'snug' the sleeve nut up finger tight and add a half nut to the outer side to lock it in place.

Regards Mick

Edited: I've just seen that the stud passes through the timing cover and loss of the sleeve nut poses no foreign object issue inside the engine. In this case I'd use a fibre washer beneath it, snug it up finger tight and 'glue' the thread with Wellseal.

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Re: Oil leak in vicinity of dynamo/crankcase

Postby g5wqian » Mon Dec 02, 2019 6:37 am

my 56 model 30 AJS was leaking at the dynamo a while ago .
i took the opportunity to rebuild the dynamo at same time .

when i took the dynamo off [which is an easy task ] i found an o-ring and silicone sealant had been put in there by someone previously before i bought it a few years ago .

i took the o-ring out and cleaned it all off , i bought a new seal from AMOC spares .

i put the new cork seal onto the dynamo and sealed it using welseal , on mine i had a short amount of thread sticking out of the dynamo from one of the casing bolts which required me taking more care of fitting the cork gasket although im not sure if all are the same .

the welseal is on both faces of the cork gasket and i applied some to both faces of the alloy casings it comes into contact with .

after re assembly i used a piece of wood to pry the dynamo against the timing case hole and then nipped up the strap bolt and the long through bolt that retains it all .

i left the bike overnight and for the next day so the welseal would "cure" .

so far its all good and no leak from the dynamo .

i do however get a tiny weep from the oil holes in the cylinder barrel to cylinder head joints , i have not quite solved it all yet but solved the base gasket side by using the thick base gaskets and welseal , and although the upper end is still bugging me i am thinking of what i may do to solve this .
i dont have the head gaskets using viton o-ring seals at the oil holes but i may go that route .

with the tin primary chaincase i use the club modified seal and NO gasket sealant except for where the join of the ends of the seal are which i put at the top in the middle , i have no chaincase leaks .

burman box drips a small amount out of the kickstart shaft hole in the casing but im not bothered about that too much as i tend to keep the box topped right up all the time and give the undenreath of engine and box a clast of brake cleaner every so often to keep it tidy .

i did tend to find that any engine oil leaks were much less if i kept the speed down below 60mph but of course that would point to oil pressure being lower at lower revs no doubt .
i do have very good oil pressure though even at tickover .

hope you get something sorted as i know how much these things make anyone think about what to do about it and how many hours we might spend trying to "solve" it .

cheers
ian

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