2mm overbore & porting --> cracked exhaust port?

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Dec 30, 2002
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#1
I just purchased a used CR250 that had been bored 2mm over and had port work done by Eric Gorr. The bike seemed to run fine until the engine got loud and stalled. After pulling off the exhaust I discovered that the exhaust ports had literally cracked on both sides, leaving the jug as junk. The fuel was 50/50 race gas with 32:1 pre-mix. The radiators were full.

Has anyone else experienced such a failure? What did you learn about possible causes?