what are rockers and shims?

[00 scrub]

I have heard people talk about rockers, rocker arms, and shims when they discuss the valve trains on cars. What do these parts do and how do they work?

[autxr]

Rockers are a lever. They jus ttransfer force from one place to another.

In the case of an engine, the cam will push one end of the rocker up and the other end will push down on the valve.

The shims go between the valve stem and the rocker. There is an "optimal" clearance between the valve and the rocker, you need to set it to about 0.008 inches (give or take, it is different for every car). This little bit of clearance ensured the valve actually closes and opens at the right times. If the clearance is too small (or not there) then the valve might not close. If the clearance is too large then the valve stays closed too long. Either way power suffers.

So, to set the clearance, you get a piece of metal that is machined to a precice thickness. Place the metal bit on top of the valve to make the space between the valve and rocker just right.

Scott

[00 scrub]

Kk, so the way I see it, the rocker and the shim are almost the same part, but the shim is at the end of the rocker and connects to the valve. Since they are two seperate parts the shim can be set to whatever the optimum size should be.

[dfess1]

If I read the correctly, they are NOT the same part. When setting the tolerance on the rockers, you need the shim to sit between the bottom of the rocker arm and the top of the valve, to insure that you get the .0008 inches of clearance you need to make sure that the valves open and close. Once you have this set, you take the shims out.

Kinda like the shims you'd put under a cabinet to level it, or between a door jam and the rough frame, same idea. Then again, I could be completely wrong.

[autxr]

The shims stay in to set the clearance, much like a matchbook under a wobbly table (like dfess says), the rocker is a mechanical device to change the direction of a force.

The rocker is a teeter-totter (remember the playground), a shim is just a brick setting on the ground at the base of the teeter totter, not the same thing at all.

Scott

[GTS LAID]

rockers were used in old school trains... same idea in changing the direction of the force.. they would attach to the wheels and through smaller angular changes and back and forth thrusting you could get the tip of it to rotate around the whole wheel.