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Shim sizing help?

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hey guys my bike is in need of re-shimming but im very confused about how the math works to calculate the sizes needed...i would like to learn how to reshim myself as the nearest mechanic is very far away, to my understanding you can only shim a bike a number of times before you have to replace the valves etc?....but im very confused on the sizes needed ive worked out i need a 78mm (2007 CRF250) but there are a number of different sizes, could someone kindly explain how you work it out using feeler gauges etc....ive watched youtube videos so i have an idea of to assemble/dissemble the pieces 

 

thanks for your time

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umm kinda, im looking in my manual and it says this 

 

Calculate the new shim thickness using the
equation below.
A = (B – C) + D
A: New shim thickness
B: Recorded valve clearance
C: Specified valve clearance
D: Old shim thickness
• Make sure of the correct shim thickness by
measuring the shim with a micrometer.
• Reface the intake valve seat if carbon deposits
result in a calculated dimension of over 2.450
mm.
Reface the exhaust valve seat if carbon
deposits result in a calculated dimension of
over 2.900 mm.
 
but i don't really get it i mean the spec says
 
Valve Clearances:
IN: 0.005 ± 0.001 in (0.12 ± 0.03 mm)
EX: 0.011 ± 0.001 in (0.28 ± 0.03 mm)
 
but i don't get how to calculate it when i use the feeler gauage how do i do the math to get the correct size shim i need?
 
 
sorry if its all a little confusing had time wording it

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Not really sure what is tripping you up.

Your recorded valve clearance is what is measured with the feeler gauge, if it is larger than spec, you need to increase the shim size to decrease the gap till it is within spec.

So, take your set of feeler gauges and figure out which one fits with a slight grip, that is your recorded clearance, if the recorded clearance is the same as your specified clearance, job done. If not, use the above formula to figure out what size shim you need to make the recorded clearance equal the specified clearance. You will likely need a larger shim, the difference in size between the old shim and the new shim will be the same as the difference between your recorded clearance and your specified clearance.

To work the formula just substitute A B C and D for shim size/clearance and type it into a calculator.

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Yea i think your thinking way too much into it

 

Just stick in a 0.005 feeler gauge and if it wont go in then go however many gauges it takes till it goes in snugly but not forcefully  , the shims (in a HotCams Shim Kit) go in consistent increments so it will be a 2.40 , or a 2.45 or 2.35 , there are no 2.42 like OEM would be , so you measure your current shim (i would guess it would be tighter than the 0.005 it calls for) now remove the shim and take that measurement (say its a 2.45 for a example) , lets say you could only get a 0.003 gauge in , so now you know you need to go to a THINNER shim  , so you try the 2.40 and check your clearance  , if its within 1,000 plus or minus from the 0.005 you need your fine  , so if a 0.004 or 0.006 feeler gauge fits , your fine , the calculator i showed you will do all this  , all you need to do is enter the sizes and it tells you what shim size you need

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Yea i think your thinking way too much into it

 

Just stick in a 0.005 feeler gauge and if it wont go in then go however many gauges it takes till it goes in snugly but not forcefully  , the shims (in a HotCams Shim Kit) go in consistent increments so it will be a 2.40 , or a 2.45 or 2.35 , there are no 2.42 like OEM would be , so you measure your current shim (i would guess it would be tighter than the 0.005 it calls for) now remove the shim and take that measurement (say its a 2.45 for a example) , lets say you could only get a 0.003 gauge in , so now you know you need to go to a THINNER shim  , so you try the 2.40 and check your clearance  , if its within 1,000 plus or minus from the 0.005 you need your fine  , so if a 0.004 or 0.006 feeler gauge fits , your fine , the calculator i showed you will do all this  , all you need to do is enter the sizes and it tells you what shim size you need

 

right that makes a little bit more sense to me, i think i have enough information to try it now

 

thanks

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Great advice guys. Question. I'm doing my first shim job and wondered how you know where to begin if valves are so tight you can't fit any size gauge.

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Depending on the bike, that may automatically mean that you need a valve job.  With any engine using titanium valves, you can generally figure that any valve that has worn enough to need a shim 0.10mm smaller than what it originally had needs to be replaced.  Titanium valves depend on an extremely thin hard coating of titanium nitride or a derivative; much less than .001" thick.  Once this is worn through, the valve wears very rapidly, and can cause seat damage and/or break off in operation.   Proof of this is a valve that will not stay adjusted beyond 2 or 3 hours.

 

To your question, you need to pick a shim on that valve that is 2 to 4 sizes smaller and see what kind of clearance you get with that, then go from there.

 

What kind of engine are you working on?

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I will add one more complexity to valve shims; metric vs inches.  It would be a lot easier if we had metric feeler gauges because the shims are metric.   

If you use inch feeler gauges you will need to convert the results to metric at some point before selecting a new shim.  I wait until I'm selecting a shim because I have way to many years of working with inches and all of my mics are in inches, but there are other places during the process to convert.  Caveat is to always remember what system you are working with.

 

This is a table of Honda shims but it may be helpful:

 

MM      Inches

1.200    0.0472

1.225    0.0482

1.250    0.0492

1.275    0.0502

1.300    0.0512

1.325    0.0522

1.350    0.0531

1.375    0.0541

1.400    0.0551

1.425    0.0561

1.450    0.0571

1.475    0.0581

1.500    0.0591

1.525    0.0600

1.550    0.0610

1.575    0.0620

1.600    0.0630

1.625    0.0640

1.650    0.0650

1.675    0.0659

1.700    0.0669

1.725    0.0679

1.750    0.0689

1.775    0.0699

1.800    0.0709

1.825    0.0719

1.850    0.0728

1.875    0.0738

1.900    0.0748

1.925    0.0758

1.950    0.0768

1.975    0.0778

2.000    0.0787

2.025    0.0797

2.050    0.0807

2.075    0.0817

2.100    0.0827

2.125    0.0837

2.150    0.0846

2.175    0.0856

2.200    0.0866

2.225    0.0876

2.250    0.0886

2.275    0.0896

2.300    0.0906

2.325    0.0915

2.350    0.0925

2.375    0.0935

2.400    0.0945

2.425    0.0955

2.450    0.0965

2.475    0.0974

2.500    0.0984

2.525    0.0994

2.550    0.1004

2.575    0.1014

2.600    0.1024

2.625    0.1033

2.650    0.1043

2.675    0.1053

2.700    0.1063

2.725    0.1073

2.750    0.1083

2.775    0.1093

2.800    0.1102

2.825    0.1112

2.850    0.1122

2.875    0.1132

2.900    0.1142

3.000    0.1181

3.050    0.1201

3.100    0.1220

3.150    0.1240

3.200    0.1260

3.250    0.1280

3.300    0.1299

3.350    0.1319

3.400    0.1339

3.450    0.1358

3.500    0.1378

Edited by Chuck.

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