I have a 3/8" drive 10mm allen that I use on the AC tensioner pulley bolt for a W126 M117 engine. That is one tight bolt and the 10mm ball head handles it just fine (so far).

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FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered