So I pulled the plugs today and noticed that the front plug came out easily until the last few turns, then it got a bit difficult. I figured maybe some carbon had built up on the end of the plug and was causing the issue. Once I got the plug out, I notice a sliver of heli-coil sticking out of the plug hole. And surprise surprise, the rear head has a heli-coil too.

Looking at the threads in the head, I'm thinking I have the wrong spark plug as it has longer threads than those in the head. And that could be a problem when removing due to build-up on the exposed threads.

My machine is a '75. Do I have earlier shallow-thread heads, or do the heli-coils just make it look that way?

 

75 had the shallow plug hole still, that coil looks right. Plug on the other hand looks like a long reach, be real close to the piston top I'd suspect and yeah carbon build up will walk a helicoil out on you if it binds up.

 

twist is right you have the wrong length plugs a complete moron replaced a short reach plug with a long reach plug