Center Caps don't fit...need advice

[sj007]

I just switched from 18" to a set 17" with snow tires that I picked up cheap (but without center caps). However, although the hole sizes are about the same (within a mm from rough measurment) the center caps are loose in the 17s. The 18s do seem to have a slightly deeper hole as the hole tapers away inside the wheel

outside wheel inside wheel
|_/
center cap tab/tooth --> plastic tab on center cap hooks on this side


versus |_|


Hopefully I'm making myself clear...if not sorry..let me know I'll draw a proper picture.

In any case, aren't these center caps supposed to be the standard? And if not what do I look for when trying to pick these up online? Most sites / private sellers don't have the measurements listed.

Any help is appreciated!