Magneto/auto advance sprocket coming loose

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Magneto/auto advance sprocket coming loose

Postby Jdraper » Sun Jun 30, 2019 9:14 pm

As I was pulling onto a main road and accelerating hard to around 60, the engine popped, backfired and died. Fortunately only a few miles and roadside examination showed that the auto advance unit had come loose from the magneto shaft - second time this has happened. It seems to happen when I give it some beans. Is it possible I have mismatched units - mag or AA unit?

I just can't seem to get the sprocket to seat tightly. No sign of wear on the mag or AA. I managed to get things running smoothly enough to get home by using a strip of foil wrapped round the shaft & re-timing the ignition.

I note that the mag shaft has a keyway, but the AA unit doesn't have a corresponding slot. Should there be one? It would make sense, especially as tightening the AA bolt is difficult due to the mag shaft turning when you turn the bolt.


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Re: Magneto/auto advance sprocket coming loose

Postby clive » Sun Jun 30, 2019 11:06 pm

No key way slot in the auto advance so you cannot use a woodruff key. I would be inclined to use some bearing fit or locative to get the thing to lock onto the shaft. The auto advance is fiddly because of the weird way the bolt acts both to tighten and as removal tool. I always have to have spend time remembering how it works. Once you have got it properly tightened leave it alone and always do the timing by popping the sprocket on the camshaft loose.
if it ain't broke don't fix

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Re: Magneto/auto advance sprocket coming loose

Postby Allen_F » Thu Jul 11, 2019 6:34 am

Slipping/worn advance/retard mechanism.
Put your thinking cap on.
When tightened the retaining nut for the advance mechanism the nut bottoms out on the end of the tapered shaft.
I took the nut out of the mechanism completely, measured the end of the tapered shaft with Vernier callipers then used this measurement to drill/concave the nut slightly. approx. 1/8"
The now concaved nut allows it to further compress the mechanism onto the tapered shaft.
You can't correct the worn mechanism but you can modify/concave the nut.
I also used blue Loctite on the taper.
these old things need to have a piss on the side of the road while you tinker about.
Last edited by Allen_F on Thu Jul 11, 2019 6:47 pm, edited 1 time in total.

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