CNW e start conversion, sprag failures

dobba99 said:
With the sprag in the CNW kit operating outside the sprag manufacturers specifications, you have to expect a sprag failure sooner of later.
These sprags are of the disengaging type, but in the cnw kit they don't fully disengage, the heel of the dogbone is constantly rubbing on the engine sprocket snout. Throw in that the primary is run dry (no oil present) and you have almost a perfect storm of errors that will make the sprag fail.
Click to expand...
And fail mine finally has😢
Matt is sending a replacement
 
My spare sprag, from AliExpress, arrived today (link below). In the mean time I might bite the bullet and pull mine apart to put that sprag lube on.
I wonder if any wear on the dogbones will be visible? The wear should be independent of e-start use, and my engine would have done nearly 600 hours by now.
While I'm that far in I might pull the inner primary and install the LH gearbox locking mod (photo below) I made a while ago but have avoided installing.:rolleyes:


IMG_2184.jpeg
 
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So, this thread has certainly got me nervous about the cnw starter kit failing. Mine has only been on since the beginning of this riding season, just a few months now. NO issues yet with starting. I'm wondering now whether I should be applying some different lube to the sprag or buying a spare to keep on hand should this one fail. I know Matt will send a replacement if this one fails, but I'm pretty sure that I wouldn't want to wait on the part assuming he has one in stock. I'm not a mechanic so not really sure how this gear works, but I imagine there is lube between the races and wonder if this where I need to inspect and apply some of the stuff, or similar, that Rob SS was showing...so, what to do...enquiring minds want to know...
 
Starvingphotog said:
So, this thread has certainly got me nervous about the cnw starter kit failing. Mine has only been on since the beginning of this riding season, just a few months now. NO issues yet with starting. I'm wondering now whether I should be applying some different lube to the sprag or buying a spare to keep on hand should this one fail. I know Matt will send a replacement if this one fails, but I'm pretty sure that I wouldn't want to wait on the part assuming he has one in stock. I'm not a mechanic so not really sure how this gear works, but I imagine there is lube between the races and wonder if this where I need to inspect and apply some of the stuff, or similar, that Rob SS was showing...so, what to do...enquiring minds want to know...
Click to expand...
At $25AUD including shipping I figure a chinese spare is very cheap insurance.
Also, if you have a spare on the shelf you are pretty well sure you won't need it! 🤣
 
dobba99 said:
Nigel
This was Trevors answer:-
Not engaging. Tries to hook up but can't. Failure was progressive- the odd attempted start would spin, give it a moment and try again and it'd fire up. Got worse until all it did was spin without turning the engine.
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Yeah, that is what I have heard. I am in the odd (2x) phase. So watching closely before I order a part.

I do know that a few here in NZ have used the "RGM Norton sprag unit. (This is the original sprag that they came out from the factory when new), this being a 14 ‘bone’, this sprag looked a lot stronger. RGM Norton, item 1310145, Sprag cage part number 761316145, 14 rollers, sph Cross and Morse (C.M)."

And our local NOCNZ Prez @MurrayNZ says, "When several customers had sprag trouble with the 6 or so CNW kits I sold, initially CNW replaced the sprags, but they inevitably all failed, so I took it upon myself to find a solution. After trying 18, 16 and 14 bone sprags, the 14 bone sprags were the only reliable replacement, which I replaced free of charge to my customers."

I think these will work in both the Mk2 and Mk3 cNw kits?

 
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Reading all these sprag failure stories only confirms my opinion that they are a rubbish idea for a motorcycle starter. Years ago I ran a few CB750 SOHCs & they suffered the same problems. A few years later my early Hinckley Trident 900 sprag failed, then shortly after my brothers Trident Sprint followed suit. If I ever fit a starter to my Commando it will be the type that Norvil sell.
 
From what I have learned here, I think I would conclude sprague are not a good idea if the dog bones are always rubbing as this would be constant wear on them until one day perhaps they flip. (I guess that is the failure mode?)

Whereas on my Alton, the centrifugal force when engine is running, pulls the dog bones away from the inner race.

Dennis
 
Not really, I have informed Matt that his design uses the sprag incorrectly (the sprag manufacturer would refuse any guarantee with the sprag used this way)
He knows the sprag is a failure waiting to happen, especially used this way.
 
Matchless said:
Reading all these sprag failure stories only confirms my opinion that they are a rubbish idea for a motorcycle starter. Years ago I ran a few CB750 SOHCs & they suffered the same problems. A few years later my early Hinckley Trident 900 sprag failed, then shortly after my brothers Trident Sprint followed suit. If I ever fit a starter to my Commando it will be the type that Norvil sell.
Click to expand...
Yep, my 900 Trophy had the same issue. Unfortunately it was a later engine which meant a complete engine out and strip. I 100% agree with you on the Norvil setup. (That is one thing that they got right on the 961's).
 
The sprag recently failed in the cNw starter assembly installed in my '72 750 Combat and Matt sent me this replacement. It looks quite different than the original one that failed, more robust (sorry no pictures since I just tossed it). It didn't have a lot of use, maybe 50 starts total over a couple of years. I wonder if lack of use has something to do with the lubrication/failure issue? I'm not much of a mechanic, but I was able to dismantle the primary, remove the drive gear/sprag assembly, take it apart by removing various circlips, replace the sprag, and reassemble. It was a little fiddly for me-certainly not something I would want to do on the side of the road, so glad I still have my RGM kickstart lever installed, although the failure fortunately occurred in my garage.

I find it interesting that in all of the accolades that I read about the cNw e-start kit compared to the others available before I decided to go with that one (it won the poll on this website hands down), there was nothing that I recall until now about this sprag dragging on the crankshaft while the engine is running, that this is a misuse of this type of sprag, and therefore prone to failure.
 

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