Wrong size worm gear.

[Yeager]

I recently ordered some parts for a standard F-7.  The worm gear I received does not seem to fit as it is about 1/3 smaller than the original.  I'm wondering if Bachmann intentionally reduced the size of the gear or if a gear from another locomotive got in the parts bin accidentally.  The gear I ordered was 812XX-M3320-0N201.  Anybody know?