Valve Clearance Shim Grinding

Can we clarify -
The shim is "Hard" - the cam is "Soft" but has picked up some hard bits.
Let's be generous and say the cam is now 20% hard bits embedded into soft metal. You still have something that is 80% soft rubbing something flat, that is 100% hard. Bathed in oil as well - why does the shim wear ??
Also,
Are you re-establishing the tiny radius at the edge of the shim??

 

Grinding or Machining a Valve Adjusting Shim is NOT recommended.

Fitting a New Shim - OR - using a correct one from a Used Shim Pool is OK.

 

Seems kind of counter intuitive, doesn't it?
Laps are a standard tool in the machining trade. A lap can be a copper plate, and you know how soft copper is, mounted on a face plate, and "charged" with bort. Or it can be an expandable split rod, used to laps bores.
Bort is small bits of abrasive, usually diamond, in a liquid carrier, or dry, depending on the use.
The bort is spread on the lap, and a hardened roller is used to imbed the bits into the copper or other soft metal. The diamond grinder bits you buy at Habba Fleight have an electro deposit on to of the bort, which hold the particles in position longer.
Laps are to smooth off the asperities left by grinding or machining. They are not used for gross stock removal.
Apply all that info to the cam / shim interface. Anything hard enough to get imbedded in the cam, acts as a gouge every time the cam rotates.
What do they use to grind diamond? Diamond.
So anything AS HARD, or harder, than the shim, can become an abrasive particle embedded in the cam, which acts as a lap. Carbon, and diamond is a form of carbon, can be formed and entrained in the oil of engines. Not all of it is hard enough to pose a problem. But the possibility remains that some can find it's way into soft metals, (aluminum pistons?) and become a faux bort.
Every time there is relative motion between the two surfaces, the particle can "plow" through the oil film, and scratch the surface. Just like sandpaper. Enough times, and you have measurable metal removal.


Keep your oil clean!

And any machinist that doesn't "break" the edges of a machined piece, needs to go back to apprentice status. Of more importance is the bevel or radius on the edges of the cam lobes. They prevent a sharp edge from fracturing and depositing a chunk of metal into the oil stream.

Look at the situation with crank shafts with plain bearings. How can a journal become undersize, if it is running on a babbit beraing? And babbit is softer than copper!
Given enough revolutions, running in dirty oil, the steel surface is actually lapped to a smaller diameter.

Hope that explains things.

CZ

 

Goodness, I really stirred up a hornets nest? (lol).
In my original post I did fail to specify what side of the shim I was intent on grinding. My intention was to grind the 'down side" (i.e. the side that faces the bucket). AKA the side with the printing on it. In my case the most I would need to remove is .002" to .003". Case hardness usually exceeds that depth. Moreover, the under surface of the shim is not exposed to the wiping action of the cam lobe and the potential for fretting. Recycling used shims sounds like good economy but I suppose that all depends on the condition of the cam contacting surface. BTW my background is in Tool & Die. I have both shop and design experience in that field. I am a machine designer and have over 30 years behind me in the field.

 

So far, all I've heard is " don't do it because nobody does it, and it shouldn't be done, and the factory says not to do it".
Is there any logical explanation, based on sound physics, that says why it shouldn't be done?
Or just the factory covering it's liability position, and insuring that it sells some high priced parts, and everyone parroting the factory line?
We wouldn't be here talking about this if Gottlieb Daimler and Wilhelm Maybach had not said "Huum, :idea: I betcha if we took old Otto's engine, and put it on this wooden bi-cycle, we could make something cool".
I'm sure that everyone said "You shouldn't do that, nobody else does, and it will scare the cows". :wink:

Kinda sad to see American bikers getting so wimpy. :cry:

CZ

 

All this over a shim that can be replaced for about $8.

For me, personally, "the factory said not to do it" is reason enough. I trust Yamaha to have me out-engineered at every turn.

 

Thanks, that expains a lot.

In this case, it sounds like I'm preaching to the choir.
As you know, even going through the case would not have a deleterious effect on the functioning of the shim.
One other thing, about marking the shim.
I mentioned using acid as a marking agent. Everybody has a battery with acid in it. That was for those who do not have access to an electro marker. And I'm sure that you know what I'm talking about.
Actually, since you can grind what you want, there is no reason to mark them, since the next time they need adjusted, you measure the gap, measure the shim, figure out how much to take off, and go from there. This assumes that you are going to be the next one in there. If you are going to sell the bike, it is nice to let the next mechanic, who doesn't want to grind, know what size it is, and mark it.
Let your conscience be your guide, I've stated my position. :lol:
If you do decide to grind, let us know if you experience any problems, as I can always rethink my position, given valid reason.

And the nay sayers can gloat. :twisted:


CZ

 

Once you've ground down the shim, how well will it react to the bending force being applied by the cam, now that the outer surfaces of the shim are no longer uniformly hardend?

 

My experience with OEM manufacturers indicate that if a process is not absolutely needed, then it won't be done (depends on the level of "quality" being sought, of course). Each few pennies saved over a million+ units of production adds up really quickly. We pretty much know that Yamaha drivetrain engineering standards and performance were quite good (their electrical engineers, not so much), so second-guessing their experience and man-hours of development and access and time on testing may be hazardous to your engine health (perhaps)..............

 

What is the shim setting on?

What percentage of the thickness did you remove by grinding two or three thousandths off?

Will that removal seriously impair the strength of the shim?

CZ

 

I'll asume that you don't know since you only have more questions. I don't have the answers either, but since you actively practice shim grinding I thought that you might know.

I don't want to come off as flippant. I really am interested in why a method, that is counter to everything I've been taught about valvetrain maintainance, is acceptable.

 

I answered the way I did as a method to prompt you to use your powers of observation and analyze the situation.
The shim sets in the bucket, and while I haven't measured the thickness of the bucket top, I assume that it is more than paper thin. The shim sets on that surface, and it provides support for the shim. It is flat, and if the shim bottom is flat, there is no flex of the shim. (Well, actually there is some bending in both pieces, since all metals bend when stressed, but it is so minute that you would be hard pressed to measure it.) The force from the lobe is slightly off center of the bucket, and the valve stem is centered under it, so the force of the cam lobe is pinching the two thick nesses of metal between the lobe and stem. Almost no bending moments there!
I don't have a shim out to measure, but if memory serves, they were about .100 inch thick, depending on what thickness they were ground to at the factory. So a 5% reduction in thickness, that is .005 inches, or .127 mm, would reduce the thickness by 5%
When mechanical devices are engineered for public consumption, there are usually designed with a safety factor as far as ultimate load. Bunglejyme can probably give you a precise number, since he designs machinery, but it is usually more than 200%. So if we reduce the thickness by 5%, even if we say that the total strength was reduced by that amount, (and it won't be. Check beam strength calculation in any engineering handbook), we are still within the safety margin.
What you were taught is what the factory would like you to do. It's reasons are not mine to know, I guessed at two in a previous post.
I do know that there are a lot of people who claim to be good mechanics that employ practices that make me cringe, and if I were to say "yeah, do it this way", and they were not knowledgeable about the fine points of the process, there is a good likelihood that they could omit a vital detail, and the outcome would prove to be a failure. If my reputation was dependant on a good outcome, I would be hesitant to allow the unskilled worker to employ that process.
So I can see why Yamaha advises what it does. They can control what goes into, thus what happens to their product.

Besides, most motorcycle mechanics are not machinists, and few have a grinder.

You are the captain of your own ship, do as you think best.


CZ

 

Mr.Zap here's a thread you might be interested in
http://xjbikes.com/Forums/viewtopic/t=2 ... art=0.html
i know from making the dovetail cutter for this that sometimes we do things just because we can, even though their not practical.
but i mean really, a 8$ shim? and you have other bikes to ride anyhow.
at least use your talent for something a little more valuable

 

that didn't work very well, did it

 

Cute! The only question I have is why didn't you round off the new post to match the other one. :wink:
Don't you just hate people who screw up carbs?

I read the thread, and the only two on it that I would consider worth listening to were Militant_Buddhist and mirco. And even they seem to be ignoring the physics (geometry) of the thing. There are no bending forces involved.
Take a plastic cap from a gallon milk jug. Place it in the table, top up. Press down in the middle.
It bends, doesn't it? This is what you guys seem to be afraid of.
Now take the cap, and put your thumb inside the cap, and your index finger on the top. Imagine that your finger is the cam lobe, and your thumb is the stem. Push down with your finger. Does anything bend?
And yes, I know the answer to that question. :roll:
As far as the chamfered edge, 3M makes a wheel for a bench grinder that is, for lack of a better description, compressed Scotchbright. It takes me longer to type this sentence that it would take to run the freshly ground edge of the shim around and around against the wheel and come up with the prettiest radius on the edge that you will find.

Frankly, I'm getting bored with this thread. You folks seem to have your religion, and it's dogma, down pat, and you are welcome to it. The guy down under, who pays exorbitant rates for parts, might want to be a bit more heretical, and grind shims. Done properly, there is no "down" side. ( That's a pun, son :lol: )

Ride on, CZ

 

From what I've read, I didn't see any reference to what is actually wearing. It's not the shim that is wearing, it's the valve seat. I also would like to point out that there is another option to shim grinding and that is to grind the the end of the valve stem.

 

In case anyone wants to know -
a cylinder will stop firing at 0.0015" of clearance. (cold)
At operating temps, the valve is continuously held open.

 

Is the origin of that little tidbit personal experience?

 

I have to disagree with this statement. my xj750 valve clearances open up as the engine warms up. I know this from personal experience. my bike would barely run till it got warmed up and at ambient temp, there was almost no lash, whereas when the engine was warm I could measure a clearance above 0.015mm

CN

 

Was yours the intake or exhaust??
turns out mine was the intake. I just checked.
- -
While riding at highway speeds, I felt a cylinder "go away" and then babied it home. I saw 100 PSI cold when I was fixing it.