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When to replace valves?

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I'm wondering when you should replace the intake and exhaust valves on a dirt bike. I have a 2005 kx250f that just dropped a valve. It had 64 hours on it. I had adjusted the valves and put them back into spec and the bike ran amazing for 4 hours but then one of the exhaust valves dropped. This has damaged everything and now am in the process of rebuilding this motor. The manual usually has a service limit on things in the motor but I don't see anything about when to replace the valves. Is it after you use a certain size shim to go back to spec? or after a certain number of hours?

 

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Once you have to adjust the valves, you have be prepared to replace them. Some can adjust twice but when the third adjustment is needed, it is rebuild the head or suffer. Pretty much, once they go out of adjustment once, the valves are done for. Delaying is a risk/reward proposition. I'd never take the risk.

Valve life is dependent on how well you take care of the bike.

How can an 2005 (12 year old bike) only have 64 hours on it????

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Thank you for the info! Now do you just replace the valves or also the guides and seats?

 

It's 64 hours after a full motor rebuild.

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New valves won't seal on worn seats, seats need to be rec-cut (not lapped).

With all those dropped valve failures you've had, check for a bent connecting rod.

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This is the first time a valve has dropped. I will make sure to get the seats re-cut before installing the new valve kit. Thank you guys for the input!

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