just as a reminder... may be useful to get back to an early stage of the discussion that we've maybe strayed from..
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Quote from Stu:
Brad,
Its a misconception that the head becomes "Too thin, or weak" and also that its about the altered "Compression Ratio"
The issue is the area around the valve seat becomes dangerously thin after 138.68mm.
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the compression change isn't so much the issue as the machined face breaking into the valve seats... (compression increases to about 8.2:1 for a 0.5mm material loss).

spacer plates and chamber machining won't change this at all..