2004 yz450 valve service...help?

3 replies to this topic
  • macoroniman2004

Posted June 26, 2014 - 02:31 PM


Ok so im replacing my intake valves..mind that the bike still ran good with old but getting hard to start once warmed...i got the old valves out and the seats in the head look ok but on one i can see a tiny..tiny sliver of light..now do you reface the seats or will the new valves wear in to make a seal the new valves are s.s. black diamond...or can i use lapping to get a perfect seal..really dont wanna buy a head for it

  • grayracer513

Posted June 26, 2014 - 03:03 PM


Any time the valves are replaced, the seats MUST be recut/reground/refinished to square them up with the new valve.  Lapping paste was never intended to accomplish this, but rather to take a finished valve job from a 99% seal to a 100% seal.  You don't have a seal at all. 


Lapping is a 50/50 deal:  an abrasive is put between two components and they are scrubbed against each other.  The idea is to actually wear the two together.  This obviously transfers attributes of one surface to the other.  In a valve/valve seat relationship, both the seat and the valve should have a conical surface cut at a 45 degree angle to, and centered on, the axis of the valve guide.  What you currently have is a properly cut valve, but you have a seat that is tilted off axis, is out of round, and the seat angle surface of which is no longer a "flat" conical 45 degree surface, but rather, one rounded over, top to bottom.  When you lap that, what you'd wish for is the valve to stay perfectly true as is, and all the grinding come from the seat, but the reality is that the valve will try to wear the seat back to square/flat, while the seat wears a rounded groove into your valve.  BAD.


Once the seat is refinished correctly, there is no need to lap the valve, nor should you.  You will need to use slightly smaller shims, but you should be well within the correct range.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • macoroniman2004

Posted June 26, 2014 - 05:46 PM


Cool beans im going to drop it off at my machine shop in morning i figured that would be best...he did my crf450 seats and worked out well ...thanks again for the input..greatly appreciated unfortunately i wont be able to see the finished product for a week as our wedding is saturday

  • macoroniman2004

Posted June 29, 2014 - 07:31 PM


Thank you gray racer for confirming with your knowledge...got my valves done and she runs like new cold AND hot!!..happy trails until the next break down lol
Thank you gray racer for confirming with your knowledge...got my valves done and she runs like new cold AND hot!!..happy trails until the next break down lol

Related Content


Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

Thinking about Yamaha 250... by Arctic Pride

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 replies

Snake pit oct 30th by The Anvil

Dirt Bike   Dirt Bike Regional Discussion   California
  • Hot  293 replies

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride

Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F

Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.