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The chain stretch has to be heat related. Short chain, small sprockets, sealed chain case cover has to be an oven. Spray lube can only do so much. A sled can go years on a chain in oil bath. I think exposing the chain to the cold air and snow will keep it cooler than sealed in the oven it's in.My first set lasted aprox 700 miles and once they started wearing, I was suprised how fast they were trashed.
The chain is a major problem, once it wears to a point it goes totaly bad in one hard ride. This has now happend to me twice (sprockets OK before ride and toast after).
And like Randy stated the smaller sprockets are the first to go because of the smaller load surface.
My second set has at least 400 miles. With a quality chain the sprocket teeth will not be dished out as fast and should last longer.
Sprocket retail is $75 X 3 and my Tusabki Omega 140 link chain was $180.