Help with stripped spark plug hole?

Okay this wasn't on my car but on my son's 150cc go kart. The spark plug hole was stripped, so I tapped it and inserted a 10mm heli-coil. A test fit of the coil shows it screws onto the spark plug just fine when not inserted into the hole, but after I installed it in the spark plug hole, I can't get a spark plug in without stripping the plug. Any suggestions on what I did wrong and how to fix it?