Never ceases to amaze me. Always someone who makes a boat or submarine out of an ATV.
I work on them for a living. There's your disclaimer.
Something a lot don't consider-and I learned this the hard way with my own stuff when I was younger and dumber. You run the ATV off into a hole, fill the engine with water. Drain the water, fire it back up, then do 3-4-5 oil changes until the oil comes out clean (not milky). And you're done, right? Wrong! Read on....
When the ATV goes under, it gets water into the engine obviously. Through an open exhuast valve. Or an open intake valve. Or more commonly the crankcase vent system. The crankcase has to be vented, and most are vented to the air box. Some early units were vented to the atmosphere via open ended hose. Water gets in-it will find a way guaranteed. But it's not just "water". That water is also silt filled-that's why it's muddy. Where's the silt go? Into the crankcase. The crankcase on most ATV's is not like an oil pan on a car, where it's fairly smooth shaped. There's a hundred different nooks and crannies and the silt settles in them. You change the oil a bunch of times to get the oil to come out clean, but the silt stays in those nooks and crannies. Over time, sometimes a long time, the silt loosens and then is circulated with the engine oil, which is basically sandpaper on machined moving parts. Destroys them, usually over time (unless it's a kawasaki V-twin.....). The ONLY way to get all of it out is to remove the engine and totally disassemble it, then clean every inch of the insides. Everything that the engine oil touches.
My own experience speaking, you can do top end overhauls until you're blue in the face...replace the head, jug, piston & rings, cam(s), etc. But until you split the cases and clean it, you'll keep pouring money into it. I went through this with an old 400 Bayou that I beat the ever living poo out of, until a seasoned tech told me about the cases and how they hold the dirt/silt. Once it was cleaned, I never once had another problem in the 11 years I kept it. And I learned that if I can go around, it's financially a better decision. Sometimes you can't. Most of the time you can.
I mentioned the Kawi V-twins because almost every other ATV engine uses needle bearings on the crank & rods. The 650-800 V-twin does not. They're "plain bearings" like on an automotive style engine....which also means they're EXTREMELY intolerant of ANY dirt. Little dirt in the engine and it's not long until it spins or stacks rod bearings. Oil pressure drops and the engine then eats the case bearing. $5500 to $8000 (if you pay the dealer) later, you're fixed back up again
. Moral of the story, dont' sink a Teryx or Brute Force
Just sharing my experiences...for what they're worth. Probably not much.