I'd try and knock the socket in there with a hammer or you can always buy 8.5mm allen keys offline.. it's crucial to hold the head of the ratchet when removing allen bolts to maintain it straight

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k