VW TDI Diesels are "interference" engines which means the valves and the pistons occupy the same space at different times. The timing belt keeps them in time. Don't bet your engine on a cheap timing belt, especially if it is a TDI. Make sure you only use high quality Continental/Flennor timing belts. VW recommended changing interval is 90K miles using this long life belt, we suggest changing it every 75K miles.
We recommend purchasing a timing belt kit, rather than buying individual items. The kit includes, timing belt, timing belt tensioner, all the rollers, and the water pump. Our tensioner tool allows for precision adjustment. Also, we have cam locking plates (part #18723) and pump locking pins (part #18724) to make replacing the belt easier.