Pieces are about 10-5 mm. They are soft in the edges so they seem to have been in the housing for a little while, but then it wont take much time for a sharp object to get round when rewing the engine.

Here's another pic.

I don't think they are from the previous ring. And also not from any bearing at the crank, since they go around pretty smooth. But to be sure I will of course take a closer look at the crank case.

Any one think the three problems are linked together? (metal object, detonation (?) and piston wear at exhaust side).

The metal object and the detonation was both on the top cylinder, and also the loose wires to the power valve was on this cylinder.
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)