i always hear about it...what exactly is it?...

it's like a timing chain or belt in a car

Quote from: Frosti always hear about it...what exactly is it?...

As davipu implied, a cam chain is the item that keeps your valves in synch with your pistons.

One could approach this topic from umpteen angles, but let's try this sequence....

Without valves, the pistons and cylinders in an engine would be worthless.  The intake valves open to allow a mixture of air and fuel vapor INTO the cylinders, and the exhaust valves open to allow the spent fuel and combustion byproducts OUT OF the cylinders, in preparation for another intake cycle.  (See THIS PAGE from HowStuffWorks.com for a good illustration of valve timing.  Follow the Intro link at the bottom if you'd like to start at the beginning of the article.)

The animation below gives a better view of how the camshaft in the GS engine actuates the valves -- without pushrods or rocker arms.  The lobes (red) on the spinning camshaft (black) push the valves (greenish).  The timing of the push is controlled by the angle at which the lobe is attached to the shaft, and by the shape of the lobe itself:



As you can see in the next diagram, the GS engine actually has 2 camshafts.  The valves are oriented downward, but the principle is the same as in the animation above.  The lobes on the forward camshaft (item 7) move the exhaust valves (item 16) for both cylinders, while the lobes on the rear camshaft (item 1) move the intake valves (item 15).


BTW, while we're here you can see that the lobes don't push directly on the valve stems.  They push against shims (item 22) which sit in tappets or "buckets" (items 13 and 14) which eventually transfer the force to the valve stems.  The shims are what you replace when you adjust your valve clearances -- they come in a whole range of thicknesses.

OK, but how does the camshaft spin at the right time relative to the pistons that are plunging up and down?  Good question!  Before looking at the final diagram, locate the sprockets in the diagram above (items 3 and 9).  Now imagine the cam chain below routed over those sprockets and around a third sprocket on the crankshaft.


Once the engine is running:

1) The pistons push the crankshaft around

2) The crankshaft pulls the cam chain around

3) The cam chain pulls the camshafts around

4) The camshafts move the valves[/list:u]So THAT's how everything stays in synch!  

Whew!  Enough for now....