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Pete, I'm still thinking the crumbs are the remnants of either a gone bad timing chain tensioner shoe or the timing chain guide or both. The OEM parts that contact the double-row timing chain were of a black plastic material with a metal substrate.

A PO may have changed the shattered tensioner/guide bits some time in the past and was not industrious enough to pull the oil pan and check for the debris. Oil changes probably did not generate enough current to wash a few of the bits toward the drain plug.

Having changed a terribly worn OEM guide system in an '83 Airhead, I'm aware how easily small bits can find their way into the sump. It may still remain a mystery for you but I believe any engine damage would have shown itself much earlier.

Hello !
Before the reed valve type ventilation system, of what material was the valve made ?
If it was destructed for one reason or another, as it breathe the motor's internal it could be easy for a bit to drop into the sump.
I'm, too, puzzled.

I'm still voting square section oil filter o ring.
I think there is enough room between the cannister and the casing for an o ring to get through.
If it's been in the sump for ten years, it's gonna be hard.

ME 109 wrote:I'm still voting square section oil filter o ring.
I think there is enough room between the cannister and the casing for an o ring to get through.
If it's been in the sump for ten years, it's gonna be hard.

That would probably be my #2 guesstimate. Having just yesterday changed the oil filter in the R100S and inspecting for any foreign objects, I did notice there's more than adequate room for trash to slip into the sump.

Maybe the previous owner had a "helpful" 3yr old in the vicinity when he was doing an oil change . . . "What goes in this hole, Dad/Grandpa?"

georgesgiralt wrote:Hello !
Before the reed valve type ventilation system, of what material was the valve made ?
If it was destructed for one reason or another, as it breathe the motor's internal it could be easy for a bit to drop into the sump.
I'm, too, puzzled.

Before the reed was a disk (metal) that floated on a metal post with a spring above the disk to keep it in place (at rest). I don't think it had any plastic bits.

DucatiPete wrote: An old stem seal? hmm... we did find at least one still on a valve. I'm not sure if there was another. I'll have to ask Glyn.

Wow! My theory is looking good. You know this was an aftermarket thing, right. The fact that you have one on at least one stem is good hard evidence. These bikes didn't come with stem seals. The idea being, that on horizontal engines gravity isn't working to move much oil down the guides. When stem seals fail on other applications they become hard like plastic and will eventually break apart. They could then easily go down the pushrod tubes and end up in the sump. The thing that would worry me is what happened to the little spring collars that went around them.

But: the bits are a hard PLASTIC they are definitely NOT RUBBER.

Like I said. They become hard and brittle.

justoneoftheguys wrote: Before the reed was a disk (metal) that floated on a metal post with a spring above the disk to keep it in place (at rest). I don't think it had any plastic bits.

No plastic bits as such. The breather disc was made of Tufnol which is phenolic resin reinforced with woven glass fabric and doesn't look at all like the debris. Tufnol was once commonly used as an electrical insulator.

Thanks heaps for the thoughts and considered opinions.
I thought it may have been a past disintergrated chain tensioner, but the none of the bits don't seem to correspond and the rest of the timing gear set up is quite OK (and likely original).
They're definitely not old rubber bits gone hard, so that pretty much rules out the o-ring theory...
... I guess they pretty much remain a mystery... for now...

DucatiPete wrote: so that pretty much rules out the o-ring theory...

Well, I'm going fishing then.