My troy-bilt lawn tractor keeps throwing or breaking the smaller upper drive belt.
I have replaced the pulley and several belts.

Where did you get the belt? Is it an OEM belt?

You can find the belt you need at Dayco - MarkIV - Automotive Worldwide Website and then take that number and go to a local NAPA Auto-parts store.

The replacement belts came for Lowe's and a Tractor supply warehouse.

Need a model and a serial number to get the manual online.

Out of curiosity do you have the belt numbers and manufacturer's of the belts you got?

Alignment, belt tension and the wrong belt are the biggest causes.

Was the belt green?

The manufacture is MDT and the part number is correct with the original owners manual. The 3rd belt is greenish. The 1st 2 are black.