Counter bores are |_| shaped as oppossed to counter sinks \ / shape right?. the ' Vee' shape is acting as a wedge causing the failure is'nt it?...
I'd say it's best to use a normal allen bolt and forget about 'sinking' it at all. You can get 'smooth topped' allen bolts that have a rounded off top that's nicely low profile (Maze 3 second revision(?) use them). I believe they had problems sinking the bolts as well (which rev2 countered)...
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