here is an extractor specifically designed for this: https://www.google.com/url?sa=t&rct=j&q ... vhwy-0T3pg

There are a 11% of 1* reviews:

Don't bother, particularly if you've already tried "regular" Allen tools and the head is already damaged. They don't bite enough to do anything useful- and the largest one (6mm), the drive end snapped off first time I tried to use it.

If you absolutely must, you will need a 1/4 AF deep-reach socket as the shaft is an IMPERIAL size, NOT metric, and DON'T use a short socket, as the tool will just snap off at that weird narrow shoulder a little way down...

Also hammering an impact rated torx driver bit into the bolt head is a great extractor option. You can get a set of impact rated bits at harbor freight for not a lot of money (they're the all-black one-piece ones). Find one that is just a little too big to fit in the hole as is and then Hammer it in. The hammering action helps loosen it and then you can get some torque applied to it with a ratchet handle.

And yes.

Throw the plate away when it comes out. Even a new boiler, with integrated heating elements is only like $30-50 for the machine...

Cheers!

-Jake

Thanks everyone for your help here. I successfully dismantled the machine down to parts, soaked the grouphead/holding plate/stripped screw in cafiza for 2 hours, and tried in vain to get it out. It was really stuck in there and I couldn't get anything to bite.

I invited my buddy over and we spent a few hours trying different methods, finally realized that since we had a new holding plate/screws on hand we could just drill the screw out as long as we didn't mess up the group head. Around midnight we finally drilled deep enough that I was able to pop the holding plate out with a screw driver!

The machine is a mess inside, so I still have a lot of cleaning to do before I re-assemble.. But I'm on my way so thanks again everyone for the help!