So I got new spark plugs for my 305 and noticed that the length between the threads and the nut on the new ones are shorter than the ones I pulled by about a centimeter. Would that cause any issues or should I take them back and exchange for similar spark plugs?

 

Depends.

1 cm is ALOT. A YYYYYYYYYUUUUUUUJJJJJJJJJJE LOT in case I wasn't clear about that. Since the RIGHT plugs for stock heads have about 1½ cm of threads, sounds to me like maybe you had the wrong ones in there to begin with.

What heads are on it? What part # plugs did you take out? What part # did you put back in?

 

So I figured out what went wrong. I can still use these spark plugs, the threads are exactly the same in pitch and length. It's the bit of shank between the hex nut and the thread that's different. The new plugs will just put the cable boots closer to the heads, which in theory won't cause any issues.