I finally had the Electra running this morning - I've been struggling but think the problem has been under the tank - maybe a short somewhere. However.....
Started 2 or 3 times and was getting ready to attach the strobe. All connected up, and what's this? Starter motor just spinning. I walk over to the left hand side only to discover the taper holding the sprocket to the epicyclic gear has sheared!
So does any kind soul have a replacement? Happy to pay the going rate.....
Sheared starter shaft
Hi Ian,
That's an unusual failure. Sheared woodruff key is much more common.
Let me guess.
The woodruff key was missing or already sheared off and previous owner tried to compensate by wringing the nut as hard as possible.
Glad you are sorted. Lap the sprocket taper to the new shaft taper.
Peter
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Not only...
... as hard as possible it appears the nut has been welded (or maybe brazed) to the sprocket! There's no sign of a key. NOC has these as well so ordered. I have made a start in dremelling the nut off - I tried drilling the shaft remains in the lathe but it seems to be hardened. I hope the sprocket bore is OK......
Yes, will lap all together.
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The taper...
... in the sprocket is in a bad way and no sign of a keyway. What seems to have happened is that the taper hadn't been able to transmit the drive so was spinning. A previous owner had decided - foolishly in my view - to "rectify" this by doing up the nut then brazing it to the sprocket. Of course this meant all the driving force went through the reduced diameter threaded portion rather than substantial taper. And now that has sheared. I don't know whether the taper in the sprocket is retrievable without losing the alignment of the sprocket with the driven sprocket.
Having realised the above I intend to turn off the nut and remains of braze in the lathe to release the threaded stub then investigate further. But I'd really like a new sprocket if possible - they're not listed ny NOC spares. Does anyone have one?
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Sprocket size?
There is a huge range of pilot bored sprockets available, might be that a suitable one is available? What pitch is the chain? I seem to recall that the chain might be an unusual (obsolete?) size. It would not be too difficult to make a sprocket from scratch.
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New chain
New chain is available from NOC. I have a split link in mine which is helpful from time to time.
Not sure if the NOC offering is endless.
The hole in the starter sprocket is for the tab washer that goes behind the nut. The tab washer you haven't got.... But you can probably guess the shape. Very similar to crankshaft end bolt tab washer.
Peter
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Thanks both.
I don't think my machining skills are up to modifying a pre-bored sprocket, still less making one from scratch. I have found that Norvil (fairspares) are listing a sprocket but at a hefty £60 +vat + carriage. I may have to bite the bullet and go for that.
I did successfully turn off the remains of the nut and brazed "repair". The key in my existing sprocket has been totally destroyed and smeared over everything. I've been trying to clear out the keyway in the sprocket with a ground down screwdriver acting as a slim cold chisel but feel this may be a hiding to nothing.
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Key removal
Ian, you said you turned the debris off the gear. Can you not set-up the gear in your chuck, keyway aligned to the toolpost, and grind up a boring bar to cut lengthwise. With the spindle locked (or in the lowest gear), use the saddle and cross feed manually as a horizontal slotter to clear the keyway? Sorry my description is a bit long winded, probably easier to do than describe!
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Thanks Peter..
... I've sourced a new sprocket (thanks Mr Holland!) otherwise I may have tried your approach. The taper surface in the sprocket is quite chewed up and I think I'd always be rather suspicious of whether it would hold.
As an aside, I'm sure I've read that when you have good tapers a key is actually superfluous as it's the interference fit on the taper that provides the power transmission. Have I just imagined this?
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The key
I'm sure this is obvious to some, but.....
Make sure after fitting the key, and before fitting the sprocket, the height of exposed key must be less than the depth of the keyway in the sprocket. Some of the keys on the lightweight are standard keys but reduced in either height or length. Standard keys are often supplied and can catch the unwary.
Tightening the nut and having the sprocket sitting on the key, rather than the taper, will surely lead to a sheared key and another disaster. The taper on the starter sprocket is quite steep, which makes this check more important, as a slight snag on the key top will not be obvious.
Peter
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Yes indeed.
In fact, I'm not sure what the purpose of the key is as if it is in fact subject to any rotational load it's likely to fail. I will be lapping the sprocket and shaft together with fine grinding paste and will carefully check clearance as you suggest. Oddly enough the NOC shop have the keys in stock although they're described as dynamo keys - correct part number however.
I can however see the point of a key where a parallel shaft is used.
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Progress....
Okay.
Thanks to the good offices of Peter Holland and the NOC I seemed to have everything I need.
I couldn’t work out why the main part of the spindle wouldn’t go through the bush in the end casting. It turns out my new spindle has a diameter of 0.625”. The one that came out is 0.588” and the bush has obviously been made to suit. Unless Norton changed the size at some point which seems most unlikely. Lacking a suitable reamer I’ve bored the bush out the required .037” in the lathe.
I looked at the key supplied by NOC spares – correct part number – and it measured 0.104” thick and 0.199” deep. The width of the keyway is .095” and the combined max depth of shaft keyway and the one in the sprocket is 0.146. Added to this, the new key is 0.477” long and the length of the taper on the shaft is only 0.385”. So the key is plainly incorrect.
Wanting to crack on, I thought I’d try assembling without the key – I’ve previously lapped the sprocket and shaft together. This seems to have worked very well and the starter now spins the engine over very satisfactorily.
It’s reluctant to start – I have strobed the ignition using both a mark on the alternator rotor and on my timing side device – both identical but of course the latter is much easier to see! Until I am in a position to use 5,000 rpm I’ve set it so that it’s around 10 degrees at a fast idle. Boyer suggest the advance range is 20 degrees crankshaft (obviously 10 on my device).
The engine runs well at higher revs but is very uneven at idle speeds so there’s obviously a carburation problem which I will get to the bottom of.
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Just realised the NOC shop has them in stock - 31 of them - so hopefully all is not lost.