Is it stock or modified, I have had a kibblewhite valve fail and killed a motor, never any problems with Polaris valves. The tip of valve broke where keeper rides and dropped into cylinder. KABOOM

 

Completely stock.

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Can't say anything about the 800 ,but I did use kibblewhite valves in my 500HO sportsman when I rebuilt it. 4 years and still holding together.

 

Kibblewhite are supposed to be the best just saying that even they can fail, for stock I would use Polaris.

 

Ask your engine builder, whatever they are confident with.. run that.