The stock aluminum VES valves will be flat both sides. If they have been upgraded to SS, there will be one flat side and one side with a cavity. The aluminum ones don't hold up because there's no cooling around the valve in the 800 cylinder, so heat builds up to failure.
The 800 crankcase is the weak spot on those engines. The 800 has a bunch of torque, and the case can't handle it. As the torque pounds on the PTO end of the case, it creates more and more clearance between the bearing and the case until ultimately, the crank is no longer supported and the crank fails. Sometimes it's pretty interesting, I had one buddy that broke his, the engine ran fine but the break was just inside the case on the crank, so the PTO couldn't turn the clutch. That was spendy. Fixes include a wide bearing kit on the PTO end and getting the primary balanced. Those two things seem to help a lot in keeping the sled on the snow. You can also add an engine plate, torque stop, and engine stop to keep the engine planted, those are possibly worth it. I did all of 'em when I had my two 800's and never had a crank break.
You can check total runout on the crank end with a dial indicator to give yourself a bit of confidence. Runout should be 0.000" to 0.004". If the crank is in spec, it's not a guarantee that the crank won't fail but if it's out, there's a really good chance it will.
The 800 problem is more prevalent in the mountain sleds, the RMK's have a longer track and are ridden differently than the shorter track sleds. The long track and the on/off throttle all the time really throws a lot of torque at the engine.
If you can find a 700, they are almost as powerful, but hold together a LOT better.
