SPORTSMITH Response: The space between a belt and deck at the rear roller is actually created when there is a height difference between the rear roller and the deck. The rear roller height cannot be adjusted on these. On some models you have a rubber deck cushion that supports the deck. If your unit is equipped with this you may need to replace it as well. If not then you shouldn’t have any more space than normal between the belt and deck.

The length of these belts is meant to be somewhat tight when you first install the belt. This is because after some use the belt will begin to stretch. If the belt is at the precise length needed for the treadmill, you would not have enough adjustment left after the initial stretching of the belt. To install the belt you may need to pull back on the rear roller by hand while starting the bolts.
In some cases with a brand new belt you may need to adjust the finger guards to make them fit correctly. This can be done with the finger guard bolted in to the unit, by bending the finger guard away from the belt and or deck just enough to alleviate the problem.
Please review our video on this subject to answer any questions you have.
You can find this video at: http://www.sportsmith.net/video-player.aspx?Category=f6e251d7-5d62-47fc-8e39-5420aedabc6e