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Engine Sprocket, Crank Gear & Sprag Clutch

Submitted by jack_blakemore on Wed, 23/06/2010
Forums
Commando

Folks - I have a decision to make, and I would apoppreciate the views of anyone who has had similar experiences of their bike.

I was somewhat emotively told this morning "If you don't replace all (...three components above...), then you'll end up with a streak of molten copper down you leg...when it seizes when the bike is revving at 5,000 rev's, then the starter motor will be trying to do 60,000 revs and it will melt the field coils and seize (and so on)...".

Now I like positive sensible technical advice/critiscism, but this is the kind of emotive language that would be used by a replacement window salesman!

Which I sort of resent really.

So what I need to do is establish the real probability of the sprag failing, followed by it picking up on the crank gear and/or seizing to the engine sprocket and driving the starter motor permanently.

I have changed the sprag, and the crank gear (sprag u/s, crank gear quite worn and out of tolerance), however the Crank Gear is worn but seems to measure within tolerance. It is not rebuilt yet as the clutch is shafted and the alternator is similarly b******d.

I am minded to just bite the bullet and change it, but I would appreciate any observations or ideas or comments that anyone may have?

Has anyone experienced the catatstrophic sprag failure described by our expert, and if so how many of you and how many times?

Thanks.

Submitted by tony_ripley on Wed, 23/06/2010

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Hi,

I have not had the problem you mention but I have had two sprags fail both due to the boyer not firing correctly on low voltage, in case you are not aware, a Boyer will work down to 9 volts or so but any less and the sparc could be up to 30 degrees out, in either direction, randomly, on any stroke !

Both times when the sprag failed it just knocked one of the teeth round to face the wrong way which then prevents it from biting but will not drive the starter motor. I think you would need all teeth to swap direction.

By the way, in another thread someone mentioned that the sprag was broken on the outer ring in two places. This is how they are built and is not a fault (unless anyone can tell me different). Both my new ones had the same 'break'.

I have never heard first hand of the exploding starter motor story.

Hope this helps.

Regards

Tony

Submitted by peter_evens on Wed, 23/06/2010

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Hi Jack

I have heard of theproblems with the sprag clutch but not quite as catastrophic as yours, I have left the sprag out of mine and until someone sorts out the problem I start mine on the kick start.

Peter

Submitted by hans_taylor on Thu, 24/06/2010

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Hi Jack,

I have had more than my fair share of sprag failures, and boy does it get expensive.

The latest, a couple of years ago, was after having the whole set-up rebuilt by Norman White and was working fine for a couple of months. At the time he renewed the sprag and both gears as apparently they all need to be perfect to work freely. Also important is the setting of the backfire device, but that is after the sprag, so who knows. As said, it had worked fine, I stopped at a fuel station, filled up and then very casually pressed the button. BANG crunch, game over. RAC truck home. ( some may ask why I didnt disconnect everything.I tried taking off the starter, but all the gears still turn when the engine runs.)

Previously the sprag went when coming off a slip road up to a roundabout, blip throttle, change down, BANG, whirr crunch. Luckily we were on a tour so had a plethara of tools. The only way to carry on was to lay the bike over ( save as much oil as poss) remove chaincase and remove the sprag or any of the connecting gears. A starter motor in your pocket is a bit much, and wont help, but a couple of gears doesnt matter and you could carry on the journey and leave the problem till you get home.

This is the only time I have heard of a sprag going on the move, and there really isnt much chance of it happening at 5000rpm, as its mostly caused by the low voltage BOYER problem, when you hit the start button. I have discussed this with Nick at Nortons and he wouldnt move on that theory, consequently no refund ! SoI now use the kicker....

The only way around it that i can see is to ensure that your battery is A1 all the time, or dont fit BOYER to a Mk111. I have BOYER on all my Commando engines and a TR6 and cant fault them, but on Mk111, I dont think so.........

It may have been mentioned before, but if you insist on using the starter, fit thicker leads to and from battery, solonoid, starter, and a Leyland Mini solonoid,plus the biggest battery you can fit in the hole.

Cheers and happy kicking.

HT

Submitted by jack_blakemore on Fri, 25/06/2010

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Gentlem, thanks for the advice.

Which shall all now be keenly borne in mind!

I wish to put forward a motion - from now on the elctronic ignition is now referred to as "Del Boy(er)" ignition...show of hands please!?

I have my new Pazon ignition ready to fit.

I also found that the stator was completely fubar, as it had contacted the roor - at speed seemingly - at some point; so I have a new 3 phase alternator and Wassell power box ready to fit, the rotor has passed the "test" and seems still to be magnetic enough where it should be; and of course the new sprag and both gears will be fitted.

The only things left to check are primary chain run alignment, backfire device - and myself in for rehabilitation for the alcoholism I have recently taken up for soem strange reason.

Battery is new and should be OK. It was the charging cct that was u/s.

Hans - you didn't say whther yours had Del Boy on it when it went bang last time?

Anybody want to buy a lightly used Del Boy - for a non-e/s Commando?

 


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