How's this for irony? I ran across this old thread while searching for something else.

I replaced the expansion valve in my '93 124 about 2 years ago. So far that has been the only a/c repair required on this 15 year old car. One of the allen head bolts used to retain the expansion valve was stuck, and stripped. I ended up following my own advice and drilled the head off the bolt. The process was tedious but straightforward. No special tools required. Once the valve was removed there was no difficulty unscrewing the remains of the bolt.
Guess I can say BTDT now.

- JimY