I have been having a look for pics of Tamiya gear covers mounted on the motor mount, and came across some of Lee Martins Vega 201 (
http://events.redrc.net/2013/03/chas...us-lee-martin/). As can be seen particularly in the 6th picture (
http://events.redrc.net/wp-content/u...inTamiya-6.jpg), he is using a spacer between the gear cover and the motor mount, so I guess it's just normal.
I was at my local astro track yesterday doing some practice, with 2mm spacers between the gear cover and motor mount, and 3 times I had to cut short a run as debris had got stuck in the spur gear. I'll have to see what is the the smallest spacer I can use without the spur gear rubbing, or try to make some sort of gasket out of a spare gear cover that will seal all the way around.