Tossing my 2 cents in here.  I have two X300s.  One of them was breaking a belt every 4th or 5th usage.  The second mower has never, bought new in 2008 ever had a belt break.

My guess is is the materials used for OEM belts are not the same as the replacement belts (all factory).

Thus, I thought is it possible that during a mowing session the belt gets hot.  You turn machine off, the belt "molds" itself into the shape of the pulleys as it cools.

The next time you go to mow the cold belt has 5 or 6 kinks in it and in my case immediately was breaking when the deck was engaged.

I started loosening the belt after each usage, haven't broken a belt since.

Great diagnostic and repair. To technical for a John Deere tech to figure out at what $ 100.00 dollars an hour. That should be a factory recall for sure.. Great job buddy.

Gee you took the time to look at it and think about it.   THANK YOU! GREAT DETECTIVE WORK!  Take a hint John Deere repair.

Did exactly this. Worked perfectly and haven’t broken a belt since. Thanks for showing

I've had more trouble with John deere belt breakage than any other brands.

Good to know and what to be on the look out for. Thanks! (I have welders too)

I wonder if something like a piece of plastic PEX pipe slid over the bolt threads would prevent this.

My neighbor has an x300..I hear the blade clutch constantly engaging/disengaging... drives me nuts to hear.
  He also breaks belts. Blades on/off, every time he backs up. Maybe a dumb safety feature. I mow 3 acres once a week, engage blade clutch, nothing stops until I am done.
  Start and stops are abusive- minimize cycles.
  I listen carefully for Blades slowing without a corresponding engine rpm drop. If you hear that, belt is slipping and will die quickly.  I run minimum tension I can without excessive slipping occurring,  too, to maximize life of idler and tensioner bearings.
   All idlers and tensioner modified to allow bearing replacement.  Bearings are cheap, pulleys are not.
   Belts I match to standard industrial belts, I use gates or d&d powerdrive.
  OEM's don't make belts, they just vastly overcharge for a repackaged belt with a part number that makes matching standard belts a hassle. Well worth it to get good belts for around $20.
  I have also replaced some idlers with larger diameters, to reduce belt flexing. Especially helpful when pulley runs on the back of the belt, flexing it "the wrong way". Changing that involves experimenting with belt length.

Geeeeeeeeeezzzzzzzzzzzzzeeeeeeeeeeee  Saw off 1" of the bolt and put on a solid acorn nut.  DONE!!!!!!!!!!!!!!

I have to say for me this has to be the best video save me a lot of money I want to thank the producer of this many thanks

Keep in mind when that belt is in operation it is vibrating back and forth in that running path.

My deck with the OEM belt engages really hard even at full rpm.  Larger belts over a 1/2" will shred, overheat and vibrate bad. The OEM belt runs smooth in the pullies.

I now own one of these very badly designed mowers. At least one belt a year, if it’s not a factory belt, order two. A 146” belt is absolutely ridiculous. Have never owned a mower that required as many belts, especially at the cost of this mower. Will be my only one. One more factory belt, and I’m selling this thing. I have had much less expensive mowers that never needed a belt.

Wonder if you could have put 2 30 degree bends in the hanger and gotten to the outside far enough.

Any belt after market will not last! I went thru 5 belts until I finally got the OEM from JD. Solved the problem...for now.

Talk to Deere tech told me a lot of people also scallop the bottom of deck by mowing to close to ground on uneven lawns.Just 42 in deck stripped 6 hundred

Thank You !!!!!!!!!!!! I have also had numerous belts break on my x300.....just installed a new belt and thought I would check the belt orientation before running it and noticed the exact same "bolt problem"....with vibration and deck movement side to side, it's no wonder this has been an issue.
I don't have a welder, but I do have a grinder 🙂

I just placed the bracket to the outside rather than welding anything, and it worked fine.  Thanks for the idea though!

Just lost a belt on my X300 at 132 hours. I had a a JD GT242 I bought new in 1994 and the first replacement belt was at 20 years, over 2100 hours.

I will try changing the orientation of the bolt but it seems on mine the end of the U channel is rubbing it as well.