Wow people - I feel guilty with all the posts - and time taken. Thanks so much. I will try to address all questions asked.
Well, I got the wheel off (Remarkably easily in the end) and I am ecstatic to report that the drive rubber is in perfect condition. Phew. No grease was on the rubber nor in the enclosure for it.
The sprocket carrier is in decent shape too...
However, I am an idiot for forgetting I instructed the Ducati service guy to flip the sprocket to gain the extra 2mm clearance for the D250. Because... this morning I went out to see my extremely experienced Motorcycle engineer Max. It seems I made several basic mistakes in this small modification.
1) I was not aware of the countersunk recess for the sprocket bolts.
2) I was not aware of the type of bolt which should be used there.
3) I did not use Locktite.
4) I did not measure the torque I used to tighten the sprocket bolts
Here's what Max had to say, not necessarily in order.
1) Why are you using a cap bolt with no shank?
Bolts are prone to failure at the point where the thread begins, especially when torque is applied directly to that area. (And the thickness of my washers meant the thread started at exactly the outboard surface of the sprocket. This style of bolt was what Duane's Bikes installed when I asked him to flip the sprocket, so when I replaced them I thought I was doing the right thing by getting identical items. Ugh.)
2) Why are you using a high tensile bolt?
If it were my bike, I'd be inclined to use a non-high tensile bolt as by the evidence on the threads of the snapped items, there is significant work-hardening occurring at the point where the thread/wear starts, further enhancing the likelihood of a failure.
3) Why did you not take up any slack in the connection and then tighten them to the correct torque?
That can really affect the connection strength and endurance of it. (I'm an idiot who doesn't own a torque wrench, never torqued anything in my life, and just assumed that "as hard as I can do them up by hand" would be sufficient.)
4) The recess in the flipped sprocket concentrates the torque in half the space, higher up the bolt, with a gap between the point loading and the sprocket carrier.
(I had already thought of this, and had even thought to fill the recesses and redrill the holes...)
Max is smarter than me:
So, we'll just drive 5 new holes for you...
So, the solution seems to be:
Max said: "it'll be all right now, I reckon."
What do you think?
An interesting question has been raised: was it my idea to use the never-undo washers? Err, yes. Yes it was. Because the original bolts came undone. I thought that made some kind of sense...
.. but maybe not!
I am now wondering if I should use any washers at all, given that I'll be using a lock nut + locktite.
Oh, finally, how many pounds of torque SHOULD I use on the sprocket bolts? I purchased a torque wrench today.
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Claude, (My Grandfather's name!) thanks. I think you are correct due to the issues mentioned above. It would seem my attempt was pure fail.
I've only flown planes four times, but I have completed over 5,000 paragliding flights and once held a distance record here. No solid parts in those bad boys though, except carabiners and the like.