E-Revo/Maxx transmission

[Sammus]

Howdy. I have a 3905 emaxx tranny kicking about, and I just bought an erevo chassis + a few parts to convert my 3.3 erevo.

The motor plate is the only difference I can see in the exploded views for the erevo and newest emaxx tranny, and it fits perfectly on the erevo chassis with the emaxx motor plate.

My question: Both of them come with a 68T 32P spur gear, which apparently (according to tower and briang's gear calculator) has a diameter of about 55mm. I want to run mod1, so I go to install my 50T mod1 spur of choice, which is 52mm diameter, and it rubs on the chassis. What gives? Can anyone tell me what might be up here?