Hi there,
Take a look at this post:
http://www.rcuniverse.com/forum/m_73...tm.htm#7421279

I did a little more than what was suggested there, and dremeled around the bottom as well, as I found it was not enough if a bigger rock gets in. The problem with the sprint is that its easy for a rock to get stuck between the belt and rear tower, or between the belt and rear diff, tearing the belt.
This happened to three of our sprints here while running on a very dirty parking lot so we had to replace belts and diffs as well (since the outside diff gear is plastic, it can get damaged too)
So far the mod has worked great, almost a year from that... and all 3 sprints are still on the same belts.

The spring is a nice car... not really a race car but for the money its ok. We have a blast running them and they're tough, still holding.
The only thing I dislike is that it has too much slop in the steering, so when you put a powerful motor in there its hard to drive straight.

cheers