I'd hate to try drilling by hand because you only have one chance to do it. A machine shop would do a superior job of drilling and tapping for a helicoil.

How many miles on the head? Has the head ever been off before? It may not be such a terrible expense if the head is due for reconditioning anyhow to have a machine shop do the repair.

One more thought - how much longer of a bolt can go in there? Job 05-2190 suggests that you may have some good thread further down the hole to bite into for a patch repair. Though I wouldn't expect a lot of strength from just a few intact threads.

What's the worst that can happen if the patch repair wouldn't hold and the assembly started working loose?... incomplete valve opening?

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95 E320 Cabriolet, 169K