Do shimmed exhaust valves wear out at a faster rate than untouched ones??


6 replies to this topic
  • x2468

Posted April 24, 2008 - 08:16 AM

#1

My question is, do valves wear out faster once they are shimmed as compared to the rate they wear out from 0 hours to their first needed shimming? not sure if my question is clear.

  • KJ790

Posted April 24, 2008 - 08:19 AM

#2

Basically the answer to your question is yes for titanium valves. Titanium have a very hard coating on them from the factory. Yamaha seems to have mastered this coating, hense why they normally last a long time before the first adjustment is needed. Once the coating wears through the valve itself will begin to wear. You If shimming is needed once, odds are the coating has worn through. It will be a shorter period of time before the next adjustment is needed after the first one.

  • USED YZ426F

Posted April 24, 2008 - 08:22 AM

#3

For Ti, the short answer is yes. Just as KJ stated. For the SS valves, those things seem to last forever.

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  • Atwater

Posted April 24, 2008 - 08:30 AM

#4

i've had great luck with yamaha valve life, yes I'm older and slower, but they hold up great. I have a 2003 yz250f that has never been shimmed.
I just redid the top end of a friends 05 yz450, 140 hours on the motor,
it was in great shape. I thought it would be rough. The cams, cam chains,
chain guids, rings, piston, etc were excellent. It did not need to be changed. no leak thru on the valves and one valve is 0.001" from the minimum. Yamaha has something figured out.
change the oil often and keep it off the reve limiter,
Once they are shimmed, I would check more often.

  • x2468

Posted April 24, 2008 - 08:36 AM

#5

okay, so it's like diminishing rate of return. each successive shim delays the need for new valves, but by less and less time each shim.

  • grayracer513

Posted April 24, 2008 - 08:37 AM

#6

The complete answer is more like "it depends". For the most part, what's posted above is right. But the valve face isn't the only thing that wears, and the seats are not hard coated; they're iron, and iron wears uniformly through the part.

IF the amount of adjustment required is small, and a followup clearance check after 2-3 hours and again at say, 4-8 finds that the clearance is still where you set it, then the adjustment was probably made necessary by wear at the seat, and not from a worn valve face. However, if the clearance has already moved at the second or third hour, the hard coating on the valve is worn through, as described above, and it's time for a new set.

It's not uncommon for a new engine to have one or two valves settle in a little, but this should happen early in the engine's life, and not continue past the first few hours.

Also,it bears mentioning that when restoring heads, the finish cut on the valve seat itself should always be done with a grinder, and not with a cutter. Cutters, no matter how good, can't leave teh surface as smooth as a good stone does. The better the finish on the seat, the longer the valve will live.

  • x2468

Posted April 24, 2008 - 08:57 AM

#7

The complete answer is more like "it depends". For the most part, what's posted above is right. But the valve face isn't the only thing that wears, and the seats are not hard coated; they're iron, and iron wears uniformly through the part.

IF the amount of adjustment required is small, and a followup clearance check after 2-3 hours and again at say, 4-8 finds that the clearance is still where you set it, then the adjustment was probably made necessary by wear at the seat, and not from a worn valve face. However, if the clearance has already moved at the second or third hour, the hard coating on the valve is worn through, as described above, and it's time for a new set.

It's not uncommon for a new engine to have one or two valves settle in a little, but this should happen early in the engine's life, and not continue past the first few hours.

Also,it bears mentioning that when restoring heads, the finish cut on the valve seat itself should always be done with a grinder, and not with a cutter. Cutters, no matter how good, can't leave teh surface as smooth as a good stone does. The better the finish on the seat, the longer the valve will live.


hmm. interesting.... i see what your saying. that makes sense.





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