Hello... After placing the original post / question way back on the 2nd, I checked every day for replies that would bring help and boost my confidence that I could do this somewhat intricate job. When I did not get any for days, I gave up checking. However, I just finished the job 2 hours ago and thought to check once more for replies; very surprised & gratified to see many worthwhile suggestions.
I'll be brief... After calling 3 or 4 places and being told it would cost $$$ (HP = $1066 !!!!!!) and require this and that, I decided to call LPS Computer in California and bought their "kit". Took a few minutes to look at the manual and realized this was going to be quite involved. One practically takes apart half the printer simply to get to / change a belt. What a lousy design !! Might be purposely done this way, to be able to charge big $$, as most people would not attempt changing belts themselves on their printers. The actual belt changing is about 10% of the job.
Imagine the serious daily 'stress / strain' that car belts operate under and, yet, they last YEARS through heat, cold and high speed !! In comparison, how much stress is a printer belt under, really ? HP could never convince me that they could not produce or manufacture belts that could last at least 9-10 years through thousands of prints. The belt used in this printer is simply too thin and not wide enough. Examining the damaged belt I removed from the printer convinced me of its low quality... I'll keep that in mind when buying my next printer, even if years from now. HP shoud think of a better design.
By the way, this belt changing CAN be done by a reasonably handy & PATIENT person, if willing to take time to follow the steps. This aside, this HP 3100 is an excellent printer, producing consistently great prints. I just made 2 large prints to check my work and all seems OK.
Simple suggestion = might want to consider less tension on the belt (adjust the tensioner...). The printer came with the tensioner all "OUT" to its tightest position; I think that might have been too much 'pulling stress' for such a thin belt. I adjusted the new one with about 30-40% less tension (estimating...) and still seems to work well, although i'll keep an eye.