VALVE ADJUSTMENT

[RodneyCB750K1979]

I'm trying to adjust valve clearance on a 1979 Honda CB750 Limited Edition DOHC.  The following clearances were measured:

         VALVE  INTAKE EXHAUST
1 Valve 1-1  0.085        0.228
2 Valve 1-2  0.04          0.153
3 Valve 2-1  0.076        0.085
4 Valve 2-2  0.127        0.28
5 Valve 3-1  0.064        0.127
6 Valve 3-2  0.076        0.089
7 Valve 4-1  0.051        0.09
8 Valve 4-2  0.06          0.087
                       
ALLOWABLE: 0.080   (0.06-0.130)
ALL DIMENSIONS mm
April 7, 2013
1979 Honda CB750 Limited Edition

I removed the shim from Intake Valve 2-2 which was showing 0.127mm clearance.  The shim was marked 280.  I installed a shim marked 290.  The clearance became less than .050mm. Out of curiosity, I replaced the 290 shim with a 270 shim and the clearance became 0.229mm. I'm confused. What shim do I need to install?
 

[Johnnylevi]

did you measure the shim with a micro meter? sometimes the shims are worn down a little bit. you might just need to put another 280 in. let us know!

[robs750]

I agree,measure the shims before making changes.

If you bought the bike used,there is a possibilty the PO ground some of the shims.They can be marked,but not be the actual size.

[RodneyCB750K1979]

I checked the shim thickness using a vernier caliper but a micrometer would be better as the center of the shim wears down.

Question 1 - My shop temperature is 60 F.  Should this affect my measurements?
Question 2 - Should the shims be installed with the numbers facing down? And if I grind, which surface?

[Johnnylevi]

My clymer manual says to do them at 95 degrees Fahrenheit 35 degrees Celsius.

[Lucky 1]

It would be a lot easier if you used thousandths instead of mm's.

For instance:
.029mm=.001 thousandths.


The list you have shows the first valve (intake) as....

.085mm=.003 thousandths.

[Lucky 1]

He said all the measurements were in mm.

[Lucky 1]

You cannot grind those. They are precision ground and hardened.

If you grind a shim, it is ruined.
You would need a precision grinder. Thousandths of dollars.
BTW the numbers have to be on the bottom or they would get worn off.

[Lucky 1]

.127mm showing. That would be .005 thousandths. That is too much for an intake valve.

When you had the shim in your hand and it was marked (280) did the number stamped on it match
the thickness of that shim?  Otherwise WAS the shim 280 thousandths thickness????

.280mm=.011 thousandths. Too thin to be a shim.
280mm= 11 inches!!! Not possible.

Measure the actual shim and see how thick it is.

[RodneyCB750K1979]

I read thicknesses in mm since the manual said Honda shims come in mm thicknesses.  The number on the shim was "280"  If there were a decimal point, I couldn't see one. I am ASSUMING 280 means 2.80 mm since tappet shims are available from 2.30 3.50 in .05 increments (mm?).

[RodneyCB750K1979]

I bought a micrometer and new set of feeler gauges. I removed and mic'd each shim.  I discovered the shim thicknesses averaged .05 mm thicker than they were stamped (example: a #285 shim which I thought was 2.85 mm thick mic'd out to be 2.90 mm thick). All measurements were taken at 58 F.  Below is a spreadsheet to help me estimate the new shims needed.  

[RodneyCB750K1979]

I thought I read on some forum where shims could be ground down to proper thickness. You would grind of course on the bottom side of the shim.

[Lucky 1]

You would need a precision grinder to get those flat ,even, and parallel and the grinding stone would need to be very fine. I have never seen a valve shim ground.

They are sold in sets and many times you can trade them for the size you need.

This is a precision table grinder below.

[Lucky 1]

2.85mm=.112 thousandths of an inch.  That makes sense.

You measured it at 2.90mm =.114 thousandths. That is .002 thousandths difference from
what the stamped number printed on the shim.

How would the shim get thicker by .002 thousandths?

Make sure the shim is clean. Even oil can add .001 thousandths.

.05mm is only .001 thousandths of an inch.
That small difference can be oil or dirt or a piece of hair.
OR the micrometer could be off just a little. Was it a digital or analog micrometer?

[RodneyCB750K1979]

I measured some new shims:
shim     measured   converted
250    0.099 inch   2.51 mm
270    0.108 inch   2.74 mm
290    0.116 inch   2.95 mm

I'm using an analog micrometer. Sensitive to amount of pressure applied. Can vary .0005-.001 inch.

[RodneyCB750K1979]

After two trips to my local Honda dealer buying and swapping shims, here is where I'm at with valve lash adjustments.  Almost there. I have three more shims to work on.

[Hoosier Daddy]

Going by your graph, if you are currently the red, "Final Settings" from what I have read the highest one is still a little low. Reportedly the factory specs were tight, the modern thought is  .005" should be your goal.

I do like your chart, having done mine in a similar spreadsheet made it easy for me to compare my final clearances.

[RodneyCB750K1979]

I wished I heard this earlier about the 0.005".  I thought the shop manual was gospel which gives a tolerance of 0.002 to 0.005.  I'm stuck on one exhaust valve with a 292 shim that is .0045" lash and no one seems to have a 293 or 294 shim to bring it down to .0035".  But if I need to target .004 to .005 valve lash, I'm ok.  But now do I need to adjust all the others?  Does anyone have any thoughts?  A know a larger lash is preferred for better valve cooling but performance is compromised.  

[Lucky 1]

Make sure the exhaust valves are loose enough. They tighten up over time.

The intake valve clearances do not change much.

[RodneyCB750K1979]

I'm done adjusting valves. I targeted 0.035 for the intake valves and 0.045 for the exhaust valves.