Time for a new head?

3 replies to this topic
  • Crzy Joe Devola

Posted March 08, 2010 - 05:47 PM


I have a 99 YZ400F. Adjusted the valves maybe August of last year. they were very tight then. Checked them 2 days ago in anticipation of spring. Found the following:

Exhaust clearances are .13 with a 145 shim in it and .10 with a 140.

Intake are .13 with a 145, .12 with a 155 and .12 with a 145.

Obviously very tight. How many more adjustments do you think I can get if any? Is it time for a new head? Any other options?

  • BBrown626

Posted March 08, 2010 - 06:15 PM


Not enough information to know. You should pull it apart and look at the condition of your valves and the seats. I believe that the seats can be replaced. Seals and valves are very easy. Depending on your skill level and tools you may be able to do it on your own. Replace the valves, seals, seats and springs. You shouldn't need a new head.
I had to rebuild a head last year and found a few great deals on OEM parts kits at NCYamaha's e-bay store and a couple other e-bay sites.

While you have it apart replace your timing chain no matter what the condition of it is.

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  • grayracer513

Posted March 08, 2010 - 07:55 PM


Time for a rebuild, certainly. Let the machinist decide if the seats need replacement or not; they probably don't. Likewise the guides.

  • Crzy Joe Devola

Posted March 08, 2010 - 08:07 PM


Thanks guys. I didn't know you could replace the seats. I will finish tearing down and send in the head and go from there.

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