Snapped Primary chain.

[MrB]

1956 G80 S. Started bike up yesterday, crunch then dead stop. Looks like the dynamo sprocket had undone and the chain had caught the primary chain and snapped a split link apart. The odd thing is, the dynamo has a bolt in it to secure the sprocket not a nut. The bolt was loose and the tab washer was spinning. I also didn`t find a woodruff key and the keyway in the dynamo shaft was full of grease so I assume it didn`t have one. Luckily no damage other than a pair of new chains required.

[clive]

sounds like you have a twin dynamo rather than a single one. That would account for the different securing method. Without a woodruff key the tab washer would serve no function. The correct nut has a circlip to retain the locking washer. I am not sure how you are going to make a twin one work other than locktite.

[Groily]

There be quite a few dynamos whose threaded ends have been damaged and cut off, or sheared off, and whose spindles have been drilled and tapped for a bolt. Could be that this is what has been done as it's not an uncommon bodge - if the dynamo is a 'short' one marked E3N or AR.

If it's a long one marked 'E3L', then it would be one for a BSA A7/A10 as they have the requisite taper for a sprocket, whereas the Twins have parallel shafts and two keys for the pinion like on Triumphs etc - a sprocket couldn't be fitted without some work to sleeve it with a parallel bore.

I believe long ones won't go on (some) singles owing to their length fouling on the gearbox on the timing side - not sure if that's a Burman or AMC 'box thing.

[clive]

Long dynamos won't fit CP boxes but you can get them in on the GB box (B52)

[MrB]

I took another look at the dynamo to see if I could figure out which type it is. Other than Lucas on the body and 200798 stamped on the end plate where the center shaft is I can`t see any other marks. I will whip it off and investigate further. Either way I don`t think its safe to use as is, unless there is a better way of securing the bolt. How do you post photos ?

[SPRIDDLER]

How do you post photos ?

This is how I do it. See this link:


viewtopic.php?f=19&t=27956&p=237049&hil ... os#p237049

[g80csp11]

most chain failures are caused by the dynamo ( on a single) being out of alignment and the chain catching on the primary chain ,.
The dynamo chain should be endless

[MrB]

So after removing the dyno, and as Groily suggested it looks like someone has cut the end off it and drilled and tapped the centre to take a bolt. This was the cause of the clashing chains, the bolt was undone.

[MrB]

Looks like replacing the whole Armature is the only solution, along with a new tab washer ,nut and circlip. Club shop lists them. Is this an easy job to do ?

[Groily]

Other than Lucas on the body and 200798 stamped on the end plate where the center shaft is I can`t see any other marks.

Can't find a part # 200798 in the Lucas lists (although it's easy to miss something, they're not a great read!) BUT 200799 is the bearing retainer end-plate, drive end, of an E3N, which is what I would have expected you to have. That plate can be fitted on various E3 versions though, and as the main body or yoke is common on all the short 3 inch dynamos, you could have anything. (The long ones also have their own body in common, irrespective of version.) A clue would be whether you have 2 threaded holes and an adjustable bent tin strip screwed to the outside of the dynamo. This stops the thing sliding into the primary case - and is unique to the E3N and earlier AR.

If there is only one large screw (to retain the field coil pole piece inside) visible on the topside of the dynamo at mid-point, it's a short one, and therefore it is, or it has been made to be, either E3N or AR. If there are 2 screws up top, it's a long one with larger field coil, and the only tapered armature option for them is the BSA style as I mentioned before.

Thinking about your situation, I reckon your armature has simply been 'fixed' after a previous mechanical disaster. So I agree with you 100% about what you should do.
The 'drill, tap and shove a bolt up' approach is OK, sort of, but even with loctite I'd worry on these particular bikes as the risk is ever-present of a chain clash and quite expensive consequences.
There is a fair bit on this forum about the very specific parts used on AMC singles to try to prevent calamities. Although you will see that there are those who have successfully contrived their own methods of avoiding self-destruction, the balance of opinion seems to be that you need:
* the right armature in unmolested shape,
* the correct fasteners (they are unique to these bikes)
* the locating strip on the dynamo yoke.
* Also, the endless chain referred to.

I know nothing about owning and running the singles as I don't have any - but I have repaired a good few dynamos from them in the wake of problems such as yours. For peace of mind - mine and the owners'! - have followed the book closely. I don't think that was me being a nervous-ninny.