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Long story short, I married into a 2007 Brute Force 650 Solid Rear Axle, Father-in-Law had recently bought it used and I think the previous owner abused it significantly. From what I can tell, thermostat went bad and engine overheated enough times that it caused a crack. FIL took it to one mechanic who took it apart enough to determine that the engine was cracked and it should either be scrapped or get a new engine. We ended up getting the Brute back in several pieces along with a box of parts, some of them missing. Finally ended up buying a rebuilt engine (08 650 Prairie) from a guy in Missouri on eBay (turned out to be a good deal, as he took our old engine and gave a "core fee" discount, engine looks and runs great.)

We had a friend of a friend who is a part-time Kawi mechanic install the new engine and everything seemed fine. It was still overheating as the thermostat was still bad. I replaced the thermostat with a hard-wired fan switch and added a temp gauge where the thermostat went, as other have suggested on this site, and I think its a tremendous improvement.

So here's the problem: There is fluid leaking out of the rear diff, and after some research, I realize I'm missing the boot that connects the rear of the engine output shaft to the swing arm tube. There are no rear brakes, as all the hydraulic fluid has drained out of the swing arm. However, the rear differential is still full and not milky, as I recently drained it and it appeared to be something in the 80-90 weight gear oil range. As I understand, there's a seal inside the swing arm that separates the light viscosity wet brake fluid from the high viscosity gear oil. I don't believe the seal on that end is the problem.

Anyways, I have bought the boot and the clamps, but it seems to me that there should be a corresponding seal at the mouth of the swing arm tube that would keep the wet brake fluid inside the swing arm and not allow it to flow forward and basically fill up the boot and push against the engine output shaft seal. I would think there would be something similar to Part# 92049-0095 that goes on the swing arm side, but the driveshaft isn't necessarily centered in the swing arm opening just prior to the u-joint, and the shaft is a different diameter than what is on the engine side, so that part wont fit. Looking at the part schematics, it doesn't appear to show a seal there. I bought what could have been the right part on partzilla but it didn't fit. I went to a dealership and the parts guy convinced me to order some parts that weren't in stock. I paid for them and a few weeks later when they arrived, I knew they weren't the correct parts.

I want to get this right before I go through the trouble of dropping the rear axle and swing arm. I was only riding on roads as I knew I didn't want to get dirt or water on the rear shaft without the boot, but now that I've drained the rear diff as well, its stuck in the garage awaiting repair. Maybe the boot is all that separates the outside environment from the swing arm fluid, but that seems like a bad design to me.

As a side note, I realize I have only "officially" joined today, but I have been visiting this site for at least 2 years and I have learned a good bit about operation, maintenance, and upgrades from this site. Thanks in advance for any advice that's out there.
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