when to rebuild a top end

3 replies to this topic
  • 116CJ

Posted April 30, 2007 - 07:53 AM


just curious as to when you guys are rebuilding your 450's. I have an 06 YZ450 and i have only had to shim once at 35 hours. i currently have 40 hours on it. i change the oil ever ride and filter ever other ride. at how many hours do you guys usually replace the piston and ring? how long before the head needs to be rebuilt? im just looking at what to expect for upcoming maintenece.

  • grayracer513

Posted April 30, 2007 - 08:58 AM


I put a new cam chain in mine every year, and except for oil and filters, that's been it. No valve adjustment on either bike ever.

Frankly, most of them that get reshimmed early on are done because the owner misreads the feeler gauges, mistakenly believes that there is something wrong with being at the tight edge of the specified clearance range. In fact, Yamaha deliberately builds them like that, and more than 95% of them will stay that way for a very long time.

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  • 116CJ

Posted April 30, 2007 - 10:45 AM


so your saying its good for a while? id hate to see the thing all the sudden go downhill because i didn't replace something when i should have. it was only one intake that needed a shim, the other 2 were right on the low spec so i set all 3 in the middle. the exhaust havent moved since i got the bike.

  • grayracer513

Posted April 30, 2007 - 11:49 AM


Keep an eye on the clearances for a while. You'll know soon enough if there's a wear trend or not.

Ti valves are coated with an extremely hard compound to make them hard enough to serve as a valve face. This coating is also unbelievably thin. because it cannot be machined after it's applied. So, what typically happens with Ti valves is that they will wear so slowly that their clearance will not change perceptibly at all for an often incredible amount of time, until the coating finally wears through. Once this happens, the clearance will change with almost every ride, and once the clearance has changed enough to require a shim .10-.15 mm smaller than the original, it's time for valves.

On a brand new bike where there is one tight valve, it is usually the result of the valve "settling" due to a a slightly rough valve seat, or rarely, a valve incorrectly set at the factory. In this case, the change in clearance should be over with in a relative few hours time, and the valves should then hold their current clearance for many hours.

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