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16H magdyno - unable to tighten

Submitted by david_roberts on Sun, 01/02/2015
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This is a really silly question - but I'm stumped. The magdyno chaing on my 16h will not maintain its tension, becoming excessively loose in a few miles.

No matter how I tighten the main magdyno mounting both the magdyno moves pretty easily. The bolt is in god condition with washers each end. The smaller mounting bolt I tighten gently - given its threaded into aluminium.

Has anyone else had this problem - and found an answer ?

Thanks

Dave

Submitted by richard_payne on Sun, 01/02/2015

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It's not an unknown problem and the cure, whilst straightforward, will involve some dismantling. The usual problem is wear on the magneto mounting plate and / or the inside faces of the engine plates themselves.

The problem usually arises if they've been run loose for some time. The previous owner may have crammed a piece of wood between mag and case as a temporary 'fix' but of course it just disguises the problem.

I think that you're going to need to remove mag and platform for measurement. If the plates are not too bad then machining the platform back square and shimming appropriately may be enough. Without a clamping action between the plates, it will never stay tight.

Submitted by david_roberts on Mon, 02/02/2015

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Previously richard_payne wrote:

It's not an unknown problem and the cure, whilst straightforward, will involve some dismantling. The usual problem is wear on the magneto mounting plate and / or the inside faces of the engine plates themselves.

The problem usually arises if they've been run loose for some time. The previous owner may have crammed a piece of wood between mag and case as a temporary 'fix' but of course it just disguises the problem.

I think that you're going to need to remove mag and platform for measurement. If the plates are not too bad then machining the platform back square and shimming appropriately may be enough. Without a clamping action between the plates, it will never stay tight.

Thanks for the pointer Richard. I have already stripped the carb/head/tank/primary drive to get good access, so no real additional work. I have checked the magdyno base with calipers, and it all looks pretty fair, with no more than perhaps 10-20 thou play between the engine plates. I did but shim washers to fit - but thought I'd try "correct" assembly again. Next step is to try with shim washer and see how I progress.

Submitted by richard_payne on Mon, 02/02/2015

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David, It's always difficult to know when answering these posts, just what has been tried already and there's a risk of trying to teach to suck eggs... from memory, the platform ought to be a tight sliding fit between the plates, to the extent that it's not possible to remove it without slackening-off rear engine bolts and, I have an idea, the through bolts on the gearbox lug as well.

If yours has play then I think that is where the problem lies. Can you measure the platform ? I have one here that measures 3.32" (would that be 3 5/16" ?)

Submitted by david_roberts on Mon, 02/02/2015

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Previously richard_payne wrote:

David, It's always difficult to know when answering these posts, just what has been tried already and there's a risk of trying to teach to suck eggs... from memory, the platform ought to be a tight sliding fit between the plates, to the extent that it's not possible to remove it without slackening-off rear engine bolts and, I have an idea, the through bolts on the gearbox lug as well.

If yours has play then I think that is where the problem lies. Can you measure the platform ? I have one here that measures 3.32" (would that be 3 5/16" ?)

Appreciate that info, I would not describe the fit as a tight sliding fit... so this is clearly the root of the problem. I'll measure the base when I get home on Friday. Thanks again. Dave

 


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