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You do realize that 0.15 and 0.20 are metric readings right?
In inches, those readings are .006" and .008". Having valves that loose would create a lot of valve slap, and could easily break valves.
It is very odd that it would have done that twice like that.
Buffalo Tools is not what you would call a quality unit at all. It could be about anything from inferior materials, to parts tolerance being out of spec.
If you installed a new head and new valves, and I assume new push rods, then the problem is in the manufacturing. There is no reason that all new parts, on an OHV engine, should do the same thing again.
There is nothing connecting the lower engine to the head, other than the pushrods.
If a valve keeps breaking, it has to be faulty material, or a machining issue with the head/valves.