Trying to understand valve shims

[Guest]

ok, I know what they are and how they work, but what I am struggling with is what causes us to have to adjust them? I mean the cam lobes stay the same, right? The valve lengths stay pretty much the same. What is wearing or growing that causes a need to adjust the gap between the cam lobe and the valve? Is it a buildup on the head at the valve seat?

This leads me to the next question: Typically, which way do the adjustments go? Do shims typically need to be replaced with thicker shims along the way or is it kind of at random?

Just trying to understand engine theory, I guess.

[tom203]

As the valve hits its seat repeatedly, it sinks deeper into seat causing the clearance at shim/lobe to get smaller over time. Eventually, there is no clearance....valve is held open...bad news since valve needs to sit on it seat briefly to "cool" off. You need to install smaller shim as time goes on.

[Brendan W]

I mean the cam lobes stay the same, right?

That would depend on what oil you use, how often you change it and whether or not the level is kept topped up, how hard you rev it cold and the extent that you use liquid gasket on the base of the cylinders during reassembly.

[fixmybrokecomputer]

That would depend on what oil you use, how often you change it and whether or not the level is kept topped up, how hard you rev it cold and the extent that you use liquid gasket on the base of the cylinders during reassembly.

This is interesting. Could you elaborate on the variables you mentioned?

[Brendan W]

The cam lobe bears on the valve bucket. The only thing preventing metal-to-metal contact is the oil film. If the quality or quantity of oil is defective in any way the rate of wear on the lobe increases. Check the service manuals and you will find service limits for cam lobe height. Loss of height is bad because it reduces the extent and duration of the valve opening.
My first engine rebuild, CB200, was a heroic failure. Used too much liquid gasket on the base and it blocked a pinhole in the riser gallery resulting in a seized camshaft.
As Tom pointed out the main component is valve seat wear, probably an order of magnitude greater than anything else up there and seat wear will let the valve sit higher in the head and close the gap under the lobe.
Valve adjusting was an annual thing for pretty much everything in the 70s and 80s. For comparison our 2001 Fiat has gone past 130k miles and the valves are still in spec.

[fixmybrokecomputer]

Thanks for the explanation!

[bwringer]

It's not really valve seat wear -- the valve itself wears.

In the service manual, there's a diagram and a spec for the edges of the valve. As the valve wears, the edge or lip of the valve gets sharper and sharper, and the valve is considered worn out when the edge thickness falls below a certain point.

[Steve]

It might be a bit valve seat and it might be a bit of valve, but do some simple math and be amazed that you don't have to check clearances more often.

Your 850 engine turns about 5000 rpm at 70 mph. Let's figure that for 60 mph, that will be the same as the revolutions per mile. 5000 x 60 / 70 = 4285.

The valve only opens on every other revolution, so it will open and close 2142 times every mile.

The recommended clearance check interval is 3000 miles. 3000 x 2142 = 6,428,571. Yep, almost 6.5 MILLION times that valve hits 'home' every three thousand miles.

And that is only in top gear. If you ever use gears 1 through 4 or let it idle, it only gets higher.

Depending on your riding style (lower gears, higher revs), it could easily get to 8 MILLION valve closures every three thousand miles. Amazed yet?

The seat might not wear in all that much, the valve might not wear all that much, but I do believe that the machined finish on both surfaces does wear away a bit so the microscopic ridges flatten out. That will let the valve sit just a little deeper in the seat, which requires a thinner shim. That also explains why the clearance tends to stabilize after a few thousand miles. The first service interval recommended by the factory is at 600 miles (1000 km). Several valves might have needed adjustment. A few more might have needed adjustment at 3000. By 20-25,000 miles, another one or two might need it, but maybe not. I know that I have gone about 20,000 miles on my wife's bike and only had to change one shim. (That was from about 45,000 to 65,000 miles.) Does not need you don't need to check them, just that you might not have to change any shims.

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[DKGS850]

Hi All,

I have done a complete engine rebuild of my '79 GS850GN. I have not changed camshafts, valves, shims or anything else, but upon reassembly the valve clearence is now below spec. Some of them less than 0,05 mm.

Can I expect everything to "settle" after running (or handcranking) the engine a couple of times?

Am I risking a total meltdown by letting the engine run, and adjust valves after that?

[Steve]

Hand cranking might not be enough to settle them in. I would at least use the electric starter for 15-12 seconds at a time, let it rest a bit, give it another shot, repeat several times.

The ones you say are less than 0,05mm might still be in spec, the lower limit is 0,03mm. The biggest problem that most will see is that changing a shim will take it to the maximum specified clearance, but many of us actually use a larger limit, we will go up to 0,10mm.

If you are lucky enough to have access to some "X" shims, they are half-way between the standard sizes. For example, if you have near-minimum clearance and a 2,60 shim, you can install a 2,55x shim that should measure 2,575mm.

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