Timing chain adjustment

billirwinnz

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VOC Member
I'm about to overhaul the magdyno on my A twin. I'm surprised to find a rather slack timing chain which is tensioned by two strips of rubber bonded to the inner case. Is this a bodge for a worn chain or should there be some more sophisticated form of adjustment?
 

A_HRD

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VOC Member
Bill,

Coincidentally, I've been struggling with the Twin mag-mounting for a few weeks now. After looking at endless photos and sketches, rubbings and samples, I made my own. Great, progress at last. NOT. Finally, I came to the only possible conclusion that the 1/2" slot is wrongly positioned on the original brackets. This limits any movement of the mag-mounting, (which needs to be in line with the two sprockets) and actually makes it cant over at an angle. So adjustment is severely limited. (I accept though that it could be an issue with the jigging of my frame at the front!).

Finally, I made a mock-up with the offending slot re-positioned and it works like a dream. So then I bought a piece of 1/4" alloy channel, 4 inches across, and marked it up as per the photo. I've since got it roughed out and hope to have it all fitted-up soon.....

I'd be interested to know if you've got full range of movement (in line with the sprockets) on your original mag-mounting bracket?

Peter B

P1080520.JPG
 

A_HRD

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VOC Member
Bill,

Not wishing to hi-jack your thread, but if you still have the chain cover off, would you mind measuring the thickness of the sprockets please? And also the the length of a roller in the chain (ie the inside dimension of the chain where it goes over the sprocket).

Reason is, folklore has it that the chain is 1/2 x 5/32". But I am not even sure that size ever existed; it's certainly not been available for decades. It appears that the sprockets are nominally 1/8". Therefore a 1/2 x 1/8" chain (available everywhere for bicycles) would be appropriate.

Peter B
 

vibrac

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VOC Member
I think by memory thats the same as the Scott magneto chain and they recommended a certain type of superior chain when I built my Scott I cant remeber what it was but could look it up if required certainly it has never slackened in use and its a long long way from central sprocket to mag sprocket on a Scott and its lubricated by air and road dust.....
 

A_HRD

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VOC Member
Tim,

Thanks, that would be useful if you can find the info reasonably easily. Certainly, if it is ordinary bicycle chain, there are all sorts of price/strength/manufacturer variants on the web.

The A Twin between-centres dimension is a nominal 6" plus scope for a further 1/4" of adjustment either side of that (sprockets are 12T). Lube is via whatever gets by the rear camshaft outer bushing - then it seeps out of a union and pipe at the bottom of the housing. Some people simply grease it liberally every so often and do their best to seal the rear camshaft oil source... Of course, if your name was Neal Videan, you would make the whole thing into a silent belt-drive. :) And if you owned the first production A Twin, you would be using the housing as a route for the one-off 'light spring and washer valve' breather - discontinued by the factory for the rest of the production for reasons unknown.

Peter B
 

vibrac

Well Known and Active Forum User
VOC Member
This is what I got from Roger Moss via the Scott Forum (2016) it looks like 1/2 pitch but its cold and the Scott is wrapped up (NB Roger uses a mag belt drive on his racer)

However much later "The Chain Man" suggested I recommend a 3/16" wide chain by IWIS that was intended for use on the final drive of a 125cc Honda racer. He assured me that it would work fine and the extra width would give no problems in operation. I have recommended it to customers over a few years now and nobody has come back and said that this does not work. I can only pass on this information in good faith, as I have not used a mag chain for over 30 years, but believe this is very likely to be a quantum advance on rebadged Chinese bicycle chain. (I have never seem a bicycle with pedals revolving at 4500 rpm or more)
So to information
The Chain Man
Andy Forsdick
11 Gloucester Way
Bewdley
Worcs. DY12 1QF
 

billirwinnz

Well Known and Active Forum User
VOC Member
Bill,

Not wishing to hi-jack your thread, but if you still have the chain cover off, would you mind measuring the thickness of the sprockets please? And also the the length of a roller in the chain (ie the inside dimension of the chain where it goes over the sprocket).

Reason is, folklore has it that the chain is 1/2 x 5/32". But I am not even sure that size ever existed; it's certainly not been available for decades. It appears that the sprockets are nominally 1/8". Therefore a 1/2 x 1/8" chain (available everywhere for bicycles) would be appropriate.

Peter B
Hi Peter

The chain on my bike has been changed at some stage with new sprocket rings welded to the old centres.

I see that you have received far more useful answers to your question than I could provide. Useful to me too as the extra rubbers were to compensate for the chain being at it's tightest adjustment yet still loose. I need a new chain.

Cheers Bill
 
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