450f top end rebuild

I am getting ready to replace the top end on my 450 this winter during the off season. Do I need to replace the crank and bearings or just stick with the top end? Valves,piston, rings etc. It is an 05 I bought used in 07 it had appx. 20 hours on it when I got it, and now probably in the neighborhood of 45-50.

It still has great compression, I am new to the 4 stroke scene, how long do the 4 strokes last before needing rebuilt? I am very meticulos on oil changes and air filter cleaning after every ride etc, check the valves at 30 hours and they where still spot on. When I tear it down I want to do it right if I am going to spend the time and money to rebuild it, I am a diesel mechanic so the expierience thing is not an issue just looking for some feedback on intervalves on rebuilds for a 4 sroke.

My '03 went from '03 to January of '08 without ever needing a valve adjustment, or getting a top end, or anything other than two timing chains and oil and filter changes.

I'm thinking of dropping a set of rings in my oldest '06, which now has perhaps 100-120 hours on it, this fall, but that's mostly because I'm suspicious of the head gasket in that bike, and I figure I might as well, while I'm at it. It's never been apart before, either, except to replace a timing chain last spring.

I change the piston, rings, wrist pin, and c-clips every 50 hours on my hour meter on my bikes. This is really just to be safe, you can go much longer than this. As far as valves go, if they haven't needed shimming, I wouldn't worry about them. Once you have had to shim them a couple times, I would replace the valves, as they will be getting thin enough that you could have problems down the road. Yamaha's are known for their great valve life, so unless you sucked dirt or damaged the valves in some other way, they should be fine with 50 hours on them. Most people get well over 100 hours out of Yamaha valves. At 50 hours your crank should be fine, again assuming that you have changed the oil regularly and kept foreign debris out of your bottom end.

How many hours between timing chains? Is this a common issue with the YZ's? I was reading anothe r thread where some I valves where bent after 90 hours on a rebuild, he must not have changed the cam, gear or chain on that one. I will probably do the piston and rings and then at 90 - 100 do a complete.

The timing chain should be a roughly annual thing. When you check the valve clearances, back the chain tension off and check the chain condition. It should move freely at each link and not have a tendency to kink.

DO NOT attempt to turn the engine over with the tensioner loose. The engine WILL jump time.

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