Motorcycle Repair: Broken Spark plug, honda cb125, paris streets

Question
Hello Mark,

I own a Honda CB125 TII 1980, in good condition. One morning,
I tried to start it and I realized somebody broke one of the two
sparkplugs just above the threading (I'd like to know how he did
this by the way). That's how safe Paris streets are these days...
The porcelain is all gone ; all what's left is the thread within the
motor block with no grip (it's flush to the surface) that would
allow me to unscrew it out.
My plan is to saw the spark plug thread in its thickness at two
opposite points, so I can "plug" a screw driver in the hollow parts
thus created, and try to unscrew very carefully. Then I would
remove the exhaust and inspect the cylinder with a small mirror
to check if there's some porcelain bits in it. If so, I would
probably blow some compressed air (or should I hoover in there)
to remove all traces of porcelain and dusts. Then replug
eveything and give it a go.
If I can't remove the thread this way, I will have to remove the
breech, meaning find some new gaskets, which for this model
begin to be hard to find.

In short, my question would be : how do you remove a thread
that is flush ?

Thanks a lot for your time and expertise.
Read you soon,
Thibaut

Answer
Hi Thibaut,

You described a very good approach to extract the plug.

There are other means if your approach should fail. Can you send me an image of the broken plug?

I return tech information files by email. Include your email address with reply. I'll return the files I have on extracting broken fasteners.

Respectfully,
Mark Shively
Dayton, Ohio, USA