I would guess piston skirt they drop alot more often then the rods fail.

 

yes the pistons dropping skirts is alought more common then rods braking

 

go hpd-inc.com, they have a lot of aftermarket kits for the 400. you might also try hot seat performance, i'm doing there 440 exterminator kit on my explorer 400 i just got from a rancher. ( broken piston skirt) polaris pistons are junk in the 2 strokes. but i'm going from 35 hp to 60 or 70 hp according to hot seats dyno numbers. it's a little pricey, but i have a couple of the things for it already that they recommend.

 

It is the probably the counterbalancer.
Check the dipstick behind the carb and see if there is water in there.
The water pump seal leaks then antifreeze gets into the counterbalancer and the bearings fall apart and sieze.