Ok, a little long winded probably but may clear up some confusion. The reverse gear assembly bolt is just for that,"assembly" and maybe a slight preventativ measure in case other things fail in the system. If you have tried to put the gear case cover on without the bolt it is near impossible. That is the reason the bolt is in there, it compresses the spring inside the collar that holds the reverse gear assembly together on the drive shaft. The reason it does not cause damage when it breaks is two fold. First when it does break there is nowhere for the pieces to go, they are contained. Second, the space between the collared retainer and the gear case is very very small so when the bolt does break and the spring releases the collar laterally into the chain case it does not puncture the case. You might ask if the collar is rubbing against the case why does it not rub through it eventually. It's because the collar is not spinning with the shaft. This is why when the bolt breaks it does not cause damage, it just rests in place up against the case. I know 2 guys that run without them and have for 2 years without any damage. What is interesting to me is WHY the bolt is snaping off in the first place. Everyone I have seen snapped in half did so internally and left about 1/2-3/4 of an inch of the bolt attched to the head. This happens because you are putting a bolt that is 9.7mm in diameter through the hole in the collar that is 10.7mm wide. There is 1mm in play. There is also a gap bewteen the end of the collar and the driveshaft about 7.5mm wide. So when you tighten down the assembly you have the bolt head tight against the collar which is pressing the two reverse gears against the bearing housing with side to side play. What happens is the collar has a little play in it side to side and this (under load of the track turning over) causes stress on that bolt that is not fully supported inside because of the 1mm gap between the bolt and the wall of the collar and the 7.5mm gap from the collar end to the drive shaft. The easiest fix is to insert a collar that has a wall thickness of .5mm, a diameter of 10.7mm and is 22.5mm long (the length of the exposed bolt inside the collar). This will support the bolt and should stop it from snaping off because you will eliminate almost all of the side to side play. The best fix would be for Yamaha to send out updated collars that have a narrower bolt hole and are extended 7.5mm and butt up against the drive shaft.
As for holding the drive shaft in place that bolt has nothing to do with it. On the drive shaft on the chain case side there is a permanant bearing guide fixed to the shaft that turns with the shaft. This fit into the bearing housing that is fixed to the chain case side bulkhead piece with three bolts. The shaft cannot move laterally towards the chain case unless that housing were to fail and the bolts gave out.
On the clutch side the drive shaft is stopped from running out laterally towards the clutch by the bearing housing on the outside of the bulk head. There are 6 bolts on this side for a reason. They needed extra strength on this side. As for the set screw assembly on this side it is not the only thing or even the primary system to keep the shaft in place. It would be impossible because the plastic piece fits inside the bearing housing piece that bolts to the bulkhead. The plastic piece sits against the inside of the actual bearing not the housing or the bulkhead. The job of this little plastic and metal piece with set screws in it is singular. Once you have your drivers on and both sides of the shaft bolted up into the bulkhead and all is snugged down you simply reach under and and pull that set screw piece laterally against the bearing then tighten it down. It's job is to be there as a back up to prevent the bearing from walking back and forth in it's little spot between the two bearing housings and causing them to fail. Your clutch side bearing is encased and cannot move in either direction more than an 1/4 of an inch even if the pressed on bearing were to move and the set screw assembly were to move. This is because on the outside is your speedo sensor gear and it is slightly bigger than the inside diameter of the bearing so the bearing cannot slip over it. On the inside your shaft tapers out within a 1/4 inch or so on the backside of the retainer with set screws. There are alot of things in place to prevent such a catastophic thing as loosing you drive shaft out the side of your sled from happening.
