Routine tear down

[CRF119]

Mossy is onto it, its the sparkplug tip. Look at the indentations, rectangular like the end of a tip. This can happen & some bikes specify an expensive plug to lessen chance mid 90s RMs, not sure of later.

This was my first thought but because the bike never had a problem and this was just an inspection i did not mention it. Also bike would have stopped plugged removed and problem found. You might have gotten a few dents not as many as seen here.

[ktm84mxc]

Yep I'd go for a spark plug tip as it's square , you can file down the high points and just fit a new top end with out an noticeable power loss, it all depends on your budget.
My 84 KTM has a similar pattern in its head from a roller bearing pin coming loose never been an issue , a mates PE175x tossed a piston circlip did the same thing still ran sweet.

[pete-blen]

Looks like the ring lowcating dow has come out of the piston..
Yer do realise it could have migrated though the transfur ports
to the bottom end... whats it like in the crank case...?

[barty5]

seen this a number of times and it has always been the big end its small bits of the needle rollers doing the damage until it completely fails. this is is as long as others stated the end of your s/plug hasn't fallen off.

[theforeman]

Was back in the shed today checked over the bottom end , no play in bearings or any debris came out when I flushed it, tidied up the head looks good, checked over the Carb and reeds nothing missing, I did replace a plug a few months ago on a ride didn't take much notice of it but that's what I'm guessing it was, new piston here tomorrow assemble and away we go, thanks guys for your input

[F5 Dave]

if you got paranoid one could use a good torch & rotate the thrust washers on the big end with a thin object. You would see some damage to the washers if anything had passed between them & the rod from the BE bearing.