CB350 Spark Plug Thread Repair Question - What size thread?

So I stripped the threads on the engine head on my cb350 for the spark plugs. I went and bought a M12 x 1.25 helicoil kit, and installed it. But the tap hardly drilled any material out, and the helicoil installed won't take thw spark plug.

Did I get the size wrong? Should it have been a M14 x 1.25?

yes

Ah that's what I suspected then. How do you go about removing a helicoil?

The gunk you see inside is shaving cream btw

I would just hit the very end with a nice sharp awl tip bend it inwards,get under it, whatever it takes to be able to grab with needle nose then yank it out