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I often find the need to have a depth gauge on a hand held drill.
I just measure how much drill bit I don't want to use and cut a bit of wood to that length.
Then just drill through the bit of wood. (preferably on the pillar drill to get it square and central.) Then put it over the bit up to the chuck jaws and you've got a precision depth gauge.
Refining that for this application, I reckon a bit of broom handle might fit in the access hole?
So long as it was drilled dead centrally it would be a guide mandrel and depth gauge as you describe.
A little touch of the bit on the ally shouldn't be too bad though, especially if a washer is used under the replacement bolt head.
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