[RayBerggren]

Manual for my '02 says 16mm or 5/8". Anything I can put in there just spins. Nothing bigger than 16mm will fit. I've tried all the way down to 13mm. Same with 5/8", nothing bigger than I have will fit in the opening and 9/16 and 1/2 grab nothing.

I'm wondering what Honda managed to do to make it necessary to go to the stealership? Nah, there's gotta be something I can do?

 

[2011.2017.odyssey]

5/8" sounds right...
you need a plug socket btw...

is this issue on all the plugs? get a good look down the front plug holes see wth is going on in there.

maybe your extension is pushing out too far stopping you from getting past the insulator/terminal

 

[Ted]

Suggestion: Assuming you're replacing plugs, take one of the new ones and see what size fits. I have three "standard" spark plug sockets, and one of 'em works on my '02....the 5/8. I assume you are not the original owner or you would have changed them before. Consider that the person before you may have used weird plugs.