Shim bucket expanded?

KZ750H3 750-four.

The number one exhaust shim bucket was hard to get out. Won't go back in, at least not easily. Measurements compaired to the other shim buckets show that it has somehow expanded by prehaps .002 or .003mm near the opening!! I'll replace it of course, but how could this have happened? Overheating? Wadaya think?

Funny, my number two exhaust did the same thing on my 750. I attributed it to it being the one valve that actually had no clearance when I checked it and must have overheated. Bought a new one from Z1 and it worked fine.