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Lars,
Can you post the pictures of the galled pistons and maybe the cylinders? That would tell a lot about what is going on. If the pics above are the heads off your problem motor, I don't see any indication of detonation. What I have seen in the past is that there is a very clear area somewhere on the head void of carbon, almost as if a spot were cleaned off. The piston will also have a very clean spot that matches the spot in location with the head. So I think you're right to rule out detonation as the cause of the piston / cylinder gall.
Also, the information offered by Henry and others would suggest that if indeed your set up uses an aluminum piston without a coating in the Alusil barrel, creating similar metal contact in friction, that galling would be expected. It seems reasonable to assume that in your case the barrel would have deposits left behind from the pistons because the pistons are softer than the surface of the barrels. You could tell if this were true if the barrel galling were raised instead of a gouge or a scratch in the barrels surface. On the other hand, the pistons would have gouges or scratches in the surface of the skirts where the material was lost and deposited on the barrel wall.
One thing I have learned about this site is that although posters may get off subject from time to time, the information they offer is all related somehow. Knowing how the parts work beyond my questions builds my understanding of the big picture. You may already know all this crap but a lot of readers don't, and they benefit from your thread too. We're all on a giant learning curve, some ahead of others.
Thanks for the thread and hope you don't mind some of us using you as part of our "curve".
Lindy
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