I did my 6635, which is a 1999 with the Duetz engine. I bought the timing belt kit that came with the gauge needed for tension all for around $125 if I recall. It may have been Ebay...don't recall. Mine had 2600 hr's on it when I did the change. Alright...plan to get dirty. The biggest mess was when I removed the radiator, which requires unhooking all the oil/hyd hoses (oil cooled engine & hyd oil cooled by radiator). I used silicone plugs, zip-ties & baggies to keep the hose ends plugged clean & sealed...it works! Once the radiator & battery is removed, air-cleaner, move alternator out of the way, remove fan blade etc, it was all pretty accessible. There is info telling how to re-confirm the timing with a bolt/hole in the side of the block that aligns with a hole in the crank lobe. Once you find the sweet spot, just a matter of replacing the belt & tensioning till the needle reads in the green on the gauge. Wasn't hard just dirty...assuming you've done things / mechanical repairs before. This is my first diesel engine, but no difference was noticed doing the timing belt other than the their procedure for timing reference of the crank & cam. I also changed the oil, fuel-filter, hyd filter at the same time, since all stuff accessible at that time. Work of advise, remove the lower skid-plate, so when you drop bolts, nuts & sockets, they have a sporting chance of getting to the floor...else the skidsteer acquires them! I found driving my skid up onto rail-road ties gets it up high enough to easily get under and the engine is at a better height for working on. Picked up 2 of them at a local salvage yard and have them behind the barn for just this need.
