Sheared woodruff key

[ChrisB]

Just started a rebuilt motor, engine span on the starter motor a few times as a quick check before trying to start it. Third prod of the starter resulted in unwelcome whirring from generator side. Stripped to find the woodruff key had sheared.
Before rebuilding, i'm not sure whether my dished damper is too big. it measures 7.3 mm. As far as i can measure, the gap is approx 7mm so i'm thinking it'd squash a little and should be ok to re use?
My question is, what does this damper do and can i run the motor without it?
Any ideas on what would cause the woodruff to shear?
Thanks. Chris.

[chrisNI]

Starter clutch could be worn - if it is that means the woodruff key is trying to crank the engine over and it’s not that strong...

[zed1015]

The sheared key could be down to a worn taper or loose rotor bolt.
The damper must be used as it controls the starter gear end float.
When fitted you need to make sure it isn't too wide and binding on the gear which will over ride the starter clutch and cause the gear to drive the starter motor when the engine is running.( this could also contribute to the sheared key ).
You can check this with the alt cover removed.
If it is binding you can lap the dished rubber edge of the damper on some fine wet 'n' dry to reduce it's width until the gear can freewheel ( remain stationary ) when the crank is rotated in the direction of normal running .

[Julian_Boolean]

The sheared key could be down to a worn taper or loose rotor bolt.

This, I had a KE125 that sheared woodruff keys because the taper was worn on the flywheel, even two strokes don't like random variable ignition timing

[chrisNI]

When my Z1000 was doing it it was a worn starter clutch boss, I got a good second hand one and it hasn't done it since. The wear on the boss was visible.

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[ChrisB]

Thanks for that. I've put it back together without the dished rubber and with it and it's not binding. There seems to be next to no end float on the large gear wheel when built without the dished rubber. Cleaned the tapers with brake cleaner and torqued up above the recommended to about 24 ftlb but with no woodruff key (new one in the post), it turned the motor 3-4 revolutions, the next prod of the starter had the rotor spinning again.
With no noticeable end float on the gear i'm suspecting a problem with the taper. I have had SEP at Kegworth straighten this crank and remember lapping the tapers three years ago when i first rebuilt the motor. Back then, before the strip, the rotor was fouling on the stator. This may have caused unseen damage to the woodruff key which i didn't notice when putting it back together. It used to start ok three years ago but has been languishing in the back of the garage this long.
If the taper was good, i'd have thought the rotor would grip well enough for a while without the key in place?
How would i check the condition of the tapers on both parts?

[ChrisB]

Also thanks ChrisNI. How would the worn flywheel affect things? Is there a recommended measurement for the diameter on this? Doesn't seem to have much 'wobble' but it does have some. I could also whip off the flywheel from my good bike to measure any difference.

[zed1015]

With the 8mm bolt on the keyed crank you will always spin the rotor without the key in place as you can't get enough torque on the 8mm bolt.
This is why they went to a 10mm then 12mm bolt with the associated higher torque on the later non keyed rotors .
It's no surprise you spun the rotor with no key in place.
To check the taper I would give it a light lap or use some engineers blue or marker pen etc and you will see where it's contacting and when nipped up check for runout to make sure it's running true.
Also check that the nose of the crank isn't hitting the extractor threads in the rotor before it actually seats on the taper .
This will give the impression of the rotor being tight when it isn't.
EDIT... Just seen your last reply and any "wobble" isn't a good thing and can quickly loosen the rotor bolt and shear keys.

[chrisNI]

Also thanks ChrisNI. How would the worn flywheel affect things? Is there a recommended measurement for the diameter on this? Doesn't seem to have much 'wobble' but it does have some. I could also whip off the flywheel from my good bike to measure any difference.

When the gear boss is worn it seems to jam when the barrel things spin out to grip it when starting and that puts more the stress on the taper.

[ChrisB]

great advice, as always.
I don't have any engineers blue but will give it a lap again and check for contact with a permanent marker. I borrowed a DTI when i built the engine to check runout on the taper, which was good. I remember not finding any smooth surface on the rotor to do a similar check when assembled. I'll also try to see whether the crank end is touching up to the rotor threads.

[ChrisB]

Each time i've dis and re assembled the rotor assembly i've been looking for a chamfer on the big thrust washer, but only looking at the outside edge. I even got my magnifying glass on it. Imagine my surprise when, for maybe the 10th refit, I noticed the (massive) chamfer on the inside diameter!
Pretty sure torquing up the thrust washer the wrong way round onto the radius on the crankshaft will have contributed to the rotor not being seated on the taper. Sometimes I even surprise myself!
Anyhoo. Lapped the taper again (rotor taper doesn't look too clean) installed the new woodruff key, torqued it to 19lbft and it seems to be holding fine. Regarding the dished rubber, when assembled, I can turn the gear wheel clockwise by hand. How much force required is a hard thing to describe. Firm but not excessive? Bearing in mind i'm also turning the starter motor.
So, thanks for your input again and I hope my idiocy won't prevent further advices as will inevitably be required.

[zed1015]

If you need to check that the damper rubber isn't binding just turn the engine on the kick starter with the alt cover removed.
The starter clutch gear should remain stationary.
If it's too tight it will rotate with the crank and drive the starter motor .

[ChrisB]

would love to but the engine came without a kickstarter, just a blanking plate. Could i fire it up with the gen cover off momentarily if i held the idler gear pinion in with my finger?

[Pigford]

Put it in gear & turn rear wheel.

[zed1015]

would love to but the engine came without a kickstarter, just a blanking plate. Could i fire it up with the gen cover off momentarily if i held the idler gear pinion in with my finger?

That's a bit dodgy, it would risk cracking the pinion boss on the cases and possibly lose the end of your finger in the process.
It will be easier and safer to put a 17mm spanner on the ignition side and turn the crank with that.