Flywheel to crankshaft - balancing?

[R6_kid:Me]

Does the flywheel need to be balanced to the crank shaft, or just balanced to itself?

It would seem that the untimely 'death' of my RX125 motor was not infact a poked bearing or anything that sinister, but infact the flywheel had ripped off its rivets and was no longer providing spark to the engine.

Prior to the failure of the rivets there was a vibration that grew from about halfway through the rev range and got worse as revs went up.

From what I cant tell only four rivets were used, when there should have been 7... i'm guessing this is why the rivets that were there failed in the first place.

I will post pics of what I have found soon, but in the meantime can anyone shed some light on the initial question? Its an RX125, two stroke, single cylinder.

[R6_kid:Me]

Here are the pics of the flywheel, and the end of the crank where it sheered away from the mounting plate.
Is it just a case of getting 6 new rivets and re-attaching it?

[xwhatsit]

Does the flywheel need to be balanced to the crank shaft, or just balanced to itself?

Well one would think, logically speaking, that if the flywheel is perfectly balanced to itself, it won't add anything to any imbalance that the crank already has. In fact the extra weight of the flywheel will help dampen that out anyway, won't it?

This thread is giving me the shits because my bike has suddenly developed a mid-range vibe that wasn't there before

[speedpro]

Had this exact problem on my bucket sidecar with a CR125 ignition. As long as the rotor still fits snugly over the slightly protruding boss it will just be a matter of re-riveting it. My ignition had six holes as well but only 4 holes were used. The other 2 were used to attach a small flywheel to the outside of the rotor. I drilled those out and removed the flywheel to improve engine response in the sidecar. It probably allowed a bit of flex in the rotor side which resulted in shagged rivets which all ended when it came apart. Your rotor looks like it might be susceptible to similar flexing. I had to get the rivets machined up and then pressed in a humungous press as i didn't want them coming apart again.

Check, check, and then check again that you have the rotor in the right position "before" pressing any rivets.

[R6_kid:Me]

Hmm yeh, makes enough sense. I'll have to get to an engineering shop and see what they recommed, I sure as hell am not going to be using hollow rivets like the ones in their that have sheared off.

[F5 Dave]

When i swapped a rotor over i got some rivets turned up & tried to press them but press was not man enough for the job.

It was fairly easy once i found a nice bit of seriously thick steel tube that rested on the rivets to smear them over with a large ball peen hammer. Looked just peachy after. The steel was soft enough that I don't see them being more prone to fracturing. Worked fine, never broke.