how do i do the math?


4 replies to this topic
  • ridinredneck

Posted March 14, 2008 - 10:23 PM

#1

i know there is some math involved in shimming your valves. but how can i do it when i cant even get a .0015mm ( smallest I have) gauge under two of the intake valves. how can i tell what size shim i need? all three has a #8 in them right now.

  • usmcpaul

Posted March 15, 2008 - 05:08 AM

#2

Are you sure it's at Top Dead Center?

The math is simple. Take a reading for all the valves before you disassemble it and then subtract that from the factory spec. (call it X) and then as you pull out each shim, measure it and subtract X from it (call it Y). Now Y is the size shim you need.

Example. If the factory spec. is .005 Measure and record all the valves.

If the first valve you measured is .002, you know that you are .003 Out of specs. So when you get it apart you know that you have to subtract .003from whatever that particular shim is.
So lets say you removed a .050 shim from that valve.( .050-.003=.047)

If you are sure its at TDC,then I have in the past just estimated, In your case Iwould use .001 as a starting point then recheck when you are done.

I don't know of another way. If you need practice buy a CRF450, you will get all the practice you need.

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  • ridinredneck

Posted March 15, 2008 - 07:21 AM

#3

Are you sure it's at Top Dead Center?

The math is simple. Take a reading for all the valves before you disassemble it and then subtract that from the factory spec. (call it X) and then as you pull out each shim, measure it and subtract X from it (call it Y). Now Y is the size shim you need.

Example. If the factory spec. is .005 Measure and record all the valves.

If the first valve you measured is .002, you know that you are .003 Out of specs. So when you get it apart you know that you have to subtract .003from whatever that particular shim is.
So lets say you removed a .050 shim from that valve.( .050-.003=.047)

If you are sure its at TDC,then I have in the past just estimated, In your case Iwould use .001 as a starting point then recheck when you are done.

I don't know of another way. If you need practice buy a CRF450, you will get all the practice you need.

no thanks on the honda crf. lol. i know how to figure the shim size, but on two of them, i cant even get the smallest gauge i have under them. so i dont really have a starting point. thanks for the reply though.

  • maddogyz250f

Posted March 15, 2008 - 07:35 AM

#4

If your valves are so tight that you can not measure the clearance, you are going to have to replace the shims before measuring. You need to have thinner shims available and replace your existing shims until you can get a measurement. Then, adjust with thicker or thinner shims using the process usmcpaul gave you.

  • grayracer513

Posted March 15, 2008 - 08:41 AM

#5

If the clearance is zero or less (and it could be less), that means that the valve has at least .10mm wear on it if it's an intake. Assuming zero, the valve would be OK at .15mm, so start by using a shim that much thinner than what you currently have and recheck.

The bad news is that this almost certainly means you need a valve replacement. If you shim this valve, ride it one day and then check the clearance again. If it's closed up any in that short time, the hard coating is gone, and you need a new one. Running an engine with valves in that condition runs the risk of breaking one off and costing yourself as much as $2000.





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