How to determine which belt you need for your Cummins mechanical 6BT/A 5.9 engine:
The majority of Cummins 5.9 engines out there will take the GREENbelt. However, there are a few circumstances where you may need the YELLOWbelt. Look at the images below and focus on the alternator mounting configuration and match the pictures to your engine to determine which belt you need. Engines with LARGE frame alternators (long lower swing arm) will need the added length of the YELLOW belt. In addition, older engines that DO NOT have an idler pulley will also need the YELLOW belt… everything else uses the GREEN belt.