In uk.rec.gardening Chris J Dixon wrote:

If you have [access to] a mill and the part is sufficiently robust you can
mill the top off the bolt.

I haven't, but this would also take off the surrounding recess,
and I was rather hoping to fasten it together again afterwards.


You say it's about 6mm, plunge mill with a 1/4 flat-bottomed cutter.

(For extra points hand-grind a left-handed one.)