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@gbj_tester
The screws are 7075T6, not cheese. I've never had any issue with these screws (from ProBolt) when using decent tools and appropriate tightening torque. You shouldn't need to apply more than about 2Nm to the headset preload bolt when adjusting, and I usually tighten mine to 4-5Nm for extra security after securing the stem clamp and have never had this issue. It looks like some combination of insufficient tool insertion, over-torque or use of a ball-ended driver has exceeded the strength of the bolt.
Because the bolt is aluminium (7075T6 is strong as aluminium goes, but still only about 1/2 of the strength of 8.8 steel or A4 stainless steel fasteners) and uses a smaller driver than the cap-head used on other headset preload bolts, it is imperative that you use good quality tools and a decent torque wrench to avoid a repetition of this failure.
I'll drop a replacement screw in next time I'm in the Chancery Lane area, try not to break it.
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