Valves, Better loose or tight?


6 replies to this topic
  • luckyguy19

Posted December 03, 2015 - 08:53 AM

#1

I'm installing a new piston on my 14 YZ450F since its at about 110 hours.  Just preventative maintenance. 

 

So I checked the valves for the first time while I was in there.  The exhausts were perfect at .23mm, spec is .20mm to .25mm.

The intakes were right at .13mm, maybe just a little looser.  My .15mm gauge would not fit at all. 

Spec is .13mm to .20mm, so they were just at the tight limit. 

Both had 200 shims installed.  Next step is 195, which should make the clearance around .18mm to .19mm.

So it will still be in spec but at the loose end.  

Should I leave them as is or shim them to the loose end?  

 

Since this was my first time checking them I have no idea if they have moved, or if they were set on the 

tight side from the factory.  The bike still starts the same as new and I feel no power drop off.  


Edited by luckyguy19, December 03, 2015 - 09:34 AM.


  • UncleLuke

Posted December 03, 2015 - 09:07 AM

#2

I'm installing a new piston on my 14 YZ450F since its at about 110 hours.  Just preventative maintenance. 

 

So I checked the valves for the first time while I was in there.  The exhausts were perfect at .23mm, spec is .20mm to .25mm.

The intakes were right at .13mm, maybe just a little looser.  My .15mm gauge would not fit at all. 

Spec is .13mm to .20mm, so they were just at the tight limit. 

Both had 200 shims installed.  Next step is 195, which should make the clearance around .18mm to .19mm.

So it will still be in spec but at the loose end.  

Should I leave them as is or shim them to the loose end?  

 

Since this was my first time checking them I have no idea if they have moved, or if they were set on the 

tight side from the factory.  The bike still starts the same as new and I feel now power drop off.  

 

Short answer - loose end 

 

Long answer - Depends on which part is wearing. 98% of the time the wear is on the valve seat, slowly making the clearance tighter. Then the motor, at operating temperature, the metal expands and the clearances become even tighter to the point where the wear becomes so bad the valve never fully "closes".

 

In rare cases, the hard coating on the top of the valve stem wears off allowing rapid wear on the top of the stem, this puts the clearances looser over time and you have to shim accordingly until you replace the valve.

 

Make sense?



  • luckyguy19

Posted December 03, 2015 - 09:34 AM

#3

I know exactly why valves get tighter.  I wish I'd had checked earlier to see if they have moved at all.  From searching and searching it appears Yamaha will set valves on the tight end from the factory.  This helps with overall valve train life.  I'm super anal about my air filters, they get changed out at first sight of dirt.  Oil is done with full synthetic every 2-3 hours.  Considering this, I'm thinking my valves have not moved.  I'm going to reinstall the original shims and check again in 20 hours or so.  My 2010 YZ450F went 275 hours on the stock intake valves, I sold it at that point with the stock valves in spec. These Yamaha's are bullet proof if you take care of them.  I probably didn't need to do a piston but I felt guilty and want to take care of my machine. 



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  • billygildark5007

Posted December 03, 2015 - 10:59 AM

#4

Actually the valves are better on the tighter end of spec, there is less strain on the valves, seats and lifters.
The manufacture sets them as tight as possible when they built the motor. Aftermarket kits only have shim sizes every 0.5mm, the factory has them in every 0.1 sizes which you could find odd ones in your bike that your kit doesn't have.
Keep them where they are and recheck clearances in about 5-10 hours. Then repeat another 5-10 hours. If nothing moves your fine.

  • grayracer513

Posted December 03, 2015 - 02:10 PM

#5

Short answer - loose end


Wrong answer.  :naughty:
 

Actually the valves are better on the tighter end of spec, there is less strain on the valves, seats and lifters.
The manufacture sets them as tight as possible when they built the motor. Aftermarket kits only have shim sizes every 0.5mm, the factory has them in every 0.1 sizes which you could find odd ones in your bike that your kit doesn't have.
Keep them where they are and recheck clearances in about 5-10 hours. Then repeat another 5-10 hours. If nothing moves your fine.

 
Truth :prof:
 
If the valves are in spec, just leave them, but if you have a choice when resetting, keep to the tight side.  Usually, you won't have a choice unless you own a couple of odd size factory shims. 
 
While you're agonizing over the matter, remind yourself that the entire specified clearance range is only .002" from tight to loose.  Not a big deal either way.



  • billygildark5007

Posted December 03, 2015 - 07:22 PM

#6

May want to do timing chain and tensioner since your in there... Cheap insurance

  • luckyguy19

Posted December 07, 2015 - 09:51 AM

#7

I did replace the timing chain too, I always do when I replace the piston. 

 

I left the valves as is. I'll be doing pistons more often from now on, every 80-90 hours.  I'll just plan on replacing all the valves, springs, and keepers at the next piston change at 200 hours. 

 

The bike ran so great this weekend.  The snap on the bottom end is back and the bike runs quieter. 


Edited by luckyguy19, December 07, 2015 - 09:54 AM.






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