Rattle from sprocket area, Model D

[Ancient Rower]

I've got a 10 year old Model D with ~4 million meters. In the last year I've noticed a minor but increasingly annoying rattle coming from the sprocket area only during drive. Not there on return or flywheel coasting. The harder the drive, the louder the rattle. The frequency of the rattle is roughly the rpm of the sprocket. The machine has been well maintained including regular oiling of the chain. Otherwise the machine is performing really well, the noise is not associated with any roughness in the feel of the machine. At first I convinced myself it was coming from the idler pulley so I replaced it but the rattle is still there. The sprocket has no visible damage and the chain shouldn't be worn out in 4 million meters should it? I'm definitely not rubbing the chain guide (well OK sometimes I do but that's a different sound). Maybe the bearing? Any advice would be appreciated. At this point I'm ready to replace the axle, sprocket, bearings and chain, but boy will I feel dumb if the rattle is still there after that.

[Tsnor]

Bumping this for experts.

First remove the flywheel housing. Anything lose in it?

Then remove the flywheel. Sometimes the screws behind the flywheel loosen and stick out enough to hit the flywheel or get completely loose and rattle behind the flywheel. You can's see them until you remove the flywheel. Example: https://www.youtube.com/shorts/QMKY6_6TpDA

There is not much there in the flywheel/sprocket area once you pull the flywheel. The cause of the rattle should be clear. When you find it please post what caused it.

Google to find videos on flywheel removal.

Once you find/fix the rattle while it's disassembled it's a good time to clean the flywheel cage and oil the chain.

[Ancient Rower]

I took it apart again yesterday. No loose screws, but when I remove the R8 bearing and shake it, it rattles. So I guess the bearing is shot. I can't take the 17mm bearing off the shaft without removing the sprocket which I don't want to do unless I have to, but it doesn't rattle. I've got a new R8 bearing on the way, and I'll report back whether or not that was indeed the problem. Another thing I noticed was that both rubber o rings on the "quiet sprocket" are in place but are split. Do they come that way?

[Ombrax]

Another thing I noticed was that both rubber o rings on the "quiet sprocket" are in place but are split. Do they come that way?

I don't have any answer to this, but I can think of at least three additional ways to find out:

1) Call C2 and ask them.

2) Send C2 an image and ask them.

3) Look up the parts on the C2 web site ( https://shop-uk.concept2.com/53-parts ) and if you're lucky the images of the spare parts will allow you to see what the ones they're currently selling look like.

Good Luck

[Ancient Rower]

The rattle on my Model D was indeed the R8 bearing, as I suspected when I removed the old one and it rattled in my hand. The replacement arrived today along with a new rubber bearing cup. No more rattle - yay. There is some chain-on-sprocket noise but I can't remember what it sounded like when new so I don't know if that is a sign of chain trouble or not. It certainly functions smoothly so that noise will need to get worse before I replace the chain and sprocket. But the bearing rattle was driving me crazy, glad it was such an easy fix.

[Ombrax]

Glad that fixed the problem. : )